Microsoft Word - NCSTAR 1-1 Building and Fire Codes.doc

Disclaimer No. 1

Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a
procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is
not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or
equipment are necessarily the best available for the purpose. Nor does such identification imply a finding of fault or
negligence by the National Institute of Standards and Technology.

Disclaimer No. 2

The policy of NIST is to use the International System of Units (metric units) in all publications. In this document,
however, units are presented in metric units or the inch-pound system, whichever is prevalent in the discipline.

Disclaimer No. 3

Pursuant to section 7 of the National Construction Safety Team Act, the NIST Director has determined that certain
evidence received by NIST in the course of this Investigation is “voluntarily provided safety-related information” that is
“not directly related to the building failure being investigated” and that “disclosure of that information would inhibit the
voluntary provision of that type of information” (15 USC 7306c).

In addition, a substantial portion of the evidence collected by NIST in the course of the Investigation has been
provided to NIST under nondisclosure agreements.

Disclaimer No. 4

NIST takes no position as to whether the design or construction of a WTC building was compliant with any code
since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the
properties and condition of the materials used, or changes to the original construction made over the life of the
buildings. In addition, NIST could not verify the interpretations of codes used by applicable authorities in determining
compliance when implementing building codes. Where an Investigation report states whether a system was
designed or installed as required by a code

provision

, NIST has documentary or anecdotal evidence indicating

whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the
requirement was met.

Use in Legal Proceedings

No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the
National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter
mentioned in such report (15 USC 281a; as amended by P.L. 107-231).

National Institute of Standards and Technology National Construction Safety Team Act Report 1-1 (Draft)
Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-1 (Draft), 275 pages (September 2005)
CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON: 2005

_________________________________________

For sale by the Superintendent of Documents, U.S. Government Printing Office
Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250
Mail: Stop SSOP, Washington, DC 20402-0001

WTC Investigation, NIST NCSTAR 1-1

ABLE OF

ONTENTS

Abstract ........................................................................................................................................................iii

Table of Contents .......................................................................................................................................... v

List of Figures .............................................................................................................................................. xi

List of Tables .............................................................................................................................................xiii

List of Acronyms and Abbreviations .......................................................................................................... xv

Metric Conversion Table ........................................................................................................................... xix

Preface .....................................................................................................................................................xxiii

Acknowledgments..................................................................................................................................xxxiii

Executive Summary ................................................................................................................................ xxxv

Chapter 1
Introduction ................................................................................................................................. 1

1.1

Background ...................................................................................................................................... 1

1.2

Scope of Report ............................................................................................................................... 1

1.3

Design and Construction Requirements for WTC 1, 2, and 7 ......................................................... 4

1.4

Organization of Report .................................................................................................................... 5

Chapter 2
Description of WTC 1, 2, and 7 .................................................................................................. 7

2.1

Site Plan of WTC Complex ............................................................................................................. 7

2.2

Description of WTC 1 and WTC 2 .................................................................................................. 7

2.2.1

Building Description ............................................................................................................ 7

2.2.2

Structural Description........................................................................................................... 9

2.3

Description of WTC 7.................................................................................................................... 13

2.3.1

Building Description .......................................................................................................... 13

2.3.2

Structural Description......................................................................................................... 14

Chapter 3
Development of Building Codes.............................................................................................. 37

3.1

Building Code Development in the United States ......................................................................... 37

3.2

New York City Building Code ...................................................................................................... 38

3.3

Port Authority Policies for Design and Modifications to Buildings .............................................. 40

3.3.1

Procedures for PANYNJ Owned Projects .......................................................................... 40

Table of Contents

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

3.3.2

Review of Tower Plans by New York City Department of Buildings ............................... 41

3.3.3

Procedures for Tenant Alteration Projects.......................................................................... 42

Chapter 4
Code Provisions for Structural Design ................................................................................... 43

4.1

Contemporaneous Codes ............................................................................................................... 43

4.2

Loads 44

4.2.1

Dead Loads......................................................................................................................... 44

4.2.2

Live Loads.......................................................................................................................... 45

4.2.3

Wind Load.......................................................................................................................... 47

4.2.4

Earthquake Load................................................................................................................. 49

4.2.5

Other Loads ........................................................................................................................ 50

4.2.6

Distribution of Loads.......................................................................................................... 51

4.3

Design standards ............................................................................................................................ 52

4.3.1

Design Standards ................................................................................................................ 52

4.3.2

Load Combinations ............................................................................................................ 53

4.4

Alteration of Existing Buildings .................................................................................................... 55

4.5

Materials and Methods of Construction......................................................................................... 56

4.6

Stability, Bracing, and Secondary Stresses.................................................................................... 58

4.7

Deflection Limitations ................................................................................................................... 59

4.8

Load Tests...................................................................................................................................... 59

Chapter 5
Structural Design of WTC 1, 2, and 7 ...................................................................................... 63

5.1

Design Criteria ............................................................................................................................... 63

5.1.1

Loads .................................................................................................................................. 63

5.1.2

Live Load Reduction .......................................................................................................... 66

5.1.3

Wind Load.......................................................................................................................... 68

5.1.4

Aircraft Impact ................................................................................................................... 70

5.2

Structural Design Requirements .................................................................................................... 71

5.2.1

Concrete Requirements ...................................................................................................... 72

5.2.2

Steel Requirements............................................................................................................. 72

5.2.3

Methods Used to Proportion Structural Members .............................................................. 73

Chapter 6
Innovative Features Incorporated in Structural Design ........................................................ 87

6.1

Innovative features......................................................................................................................... 87

Draft for Public Comment

Table of Contents

NIST NCSTAR 1-1, WTC Investigation

vii

6.2

Lateral-Load-Resisting System...................................................................................................... 87

6.3

Composite Floor System................................................................................................................ 88

6.4

Viscoelastic Damping Units .......................................................................................................... 89

6.5

Wind Tunnel Tests......................................................................................................................... 90

6.5.1

Tests Conducted at CSU..................................................................................................... 91

6.5.2

Tests Conducted at NPL..................................................................................................... 93

Chapter 7
Fabrication and Construction Inspections and Variances................................................... 97

7.1

Introduction.................................................................................................................................... 97

7.2

Fabrication Inspection Requirements for WTC 1 and WTC 2 ...................................................... 97

7.2.1

Floor Trusses ...................................................................................................................... 98

7.2.2

Box Core Columns and Built-Up Beams ........................................................................... 98

7.2.3

Exterior Columns from Elevation 363 ft to the 9th Floor Splice ..................................... 100

7.2.4

Exterior Columns Above the 9th floor Splice .................................................................. 100

7.2.5

Rolled Columns and Beams ............................................................................................. 101

7.2.6

Other Requirements.......................................................................................................... 101

7.3

Fabrication Inspection Requirements for WTC 7 ........................................................................ 102

7.4

Inspection During Construction ................................................................................................... 102

7.4.1

Erection Marks and Marking System WTC 1 and WTC 2............................................... 103

7.4.2

Quality Control and Inspection Program for WTC 1 and WTC 2.................................... 104

7.5

Variances Granted........................................................................................................................ 104

7.5.1

Variances Relating to Fabrication and Erection Tolerances............................................. 105

7.5.2

Variances Relating to Defective Components.................................................................. 105

7.5.3

Variances Relating to Alternate Fabrication and Erection Procedures ............................ 106

7.5.4

Variances Relating to Product Substitutions .................................................................... 106

7.5.5

Variances Relating to Inspection Practice ........................................................................ 107

Chapter 8
Structural Maintenance and Modifications During Occupancy.......................................... 109

8.1

Introduction.................................................................................................................................. 109

8.2

Tenant Construction Review Manuals......................................................................................... 109

8.2.1

1971 Edition ..................................................................................................................... 110

8.2.2

1979 Edition ..................................................................................................................... 110

8.2.3

1984 Edition, Revised 1990 ............................................................................................. 111

8.2.4

1997 Edition ..................................................................................................................... 112

Table of Contents

Draft for Public Comment

viii

NIST NCSTAR 1-1, WTC Investigation

8.3

Standards for Structural Integrity Inspection of the WTC Towers .............................................. 112

8.3.1

Visual Inspections ............................................................................................................ 113

8.3.2

Review of Reports ............................................................................................................ 114

8.3.3

Periodic Measurements .................................................................................................... 115

8.3.4

Recordkeeping.................................................................................................................. 116

8.4

Standards for Architectural and Structural design ....................................................................... 117

8.5

Structural Inspection Programs.................................................................................................... 117

8.5.1

Facility Condition Survey of WTC 2 ............................................................................... 118

8.5.2

Facility Condition Survey of WTC 1 ............................................................................... 121

8.5.3

Facility Condition Survey of WTC 7 ............................................................................... 124

8.5.4

Due Diligence Condition Survey of WTC 1 and WTC 2................................................. 124

8.5.5

Structural Integrity Inspection Program ........................................................................... 125

8.5.6

Summary of Structural Integrity Inspection Programs..................................................... 133

8.5.7

Modifications and Repairs to Structural Framing Systems of WTC 1, 2, and 7 .............. 133

Chapter 9
Comparison of Fire Safety Codes and Practices ................................................................. 141

9.1

Comparison of Fire Provisions in Building Codes ...................................................................... 141

9.1.1

Introduction ...................................................................................................................... 141

9.1.2

Interrelation of Codes, Standards, and Practices .............................................................. 142

9.1.3

Comparison of New York City and Contemporary Building Codes................................ 142

9.1.4

Occupancy Group............................................................................................................. 147

9.1.5

Egress Systems ................................................................................................................. 148

9.2

Summary of Differences between Codes..................................................................................... 149

Chapter 10
Influence of Codes and Standards on the Design and Construction of
WTC 1 and WTC 2 ................................................................................................................... 151

10.1

Egress SYSTEM DESIGN .......................................................................................................... 151

10.1.1

Egress Provisions from Windows on the World .............................................................. 152

10.1.2

Egress Provisions from Top of the World ........................................................................ 156

10.2

Elevators ...................................................................................................................................... 157

10.3

Active Fire Protection Systems.................................................................................................... 158

10.3.1

Fire Alarm Systems .......................................................................................................... 158

10.3.2

Fire Sprinklers .................................................................................................................. 158

10.3.3

Smoke Management ......................................................................................................... 160

Draft for Public Comment

Table of Contents

NIST NCSTAR 1-1, WTC Investigation

10.4

Design and Construction of Fire Safety and Egress Systems ...................................................... 161

10.4.1

Construction Classification .............................................................................................. 161

10.4.2

Occupancy Group............................................................................................................. 161

10.4.3

Compartmentation of WTC 1 and WTC 2 ....................................................................... 161

10.4.4

Construction of Partitions and Shaft Enclosures .............................................................. 162

10.4.5

Tenant Separation Walls................................................................................................... 164

10.4.6

Egress Systems ................................................................................................................. 165

10.4.7

Elevators........................................................................................................................... 165

10.4.8

Active Systems ................................................................................................................. 165

Chapter 11
Maintenance and Modifications to Fire Safety Systems ..................................................... 167

11.1

Local Laws 5 (1973) and 16 (1984)............................................................................................. 167

11.2

Code Compliance Summary Following the 1993 Bombing ........................................................ 168

11.3

WTC Due Diligence Study of November 22, 1996 ..................................................................... 169

Chapter 12
WTC 7 Fuel System................................................................................................................. 171

12.1

Code Requirements...................................................................................................................... 171

12.1.1

Tanks (27-828 and 27-829) .............................................................................................. 171

12.1.2

Piping (27-830)

.............................................................................................................. 171

12.1.3

Power Systems Designs.................................................................................................... 172

12.2

Base Building System .................................................................................................................. 172

12.2.1

Modifications to System................................................................................................... 172

12.2.2

Ambassador Modification ................................................................................................ 175

12.2.3

American Express Modification....................................................................................... 175

12.2.4

Mayor’s Office of Emergency Management (OEM) Modification**.............................. 175

12.2.5

Salomon Brothers Emergency Power System .................................................................. 175

12.3

Possible Failure Modes ................................................................................................................ 176

Chapter 13
Findings ................................................................................................................................... 179

13.1

Findings ....................................................................................................................................... 179

13.1.1

General ............................................................................................................................. 179

13.1.2

Structural Safety ............................................................................................................... 180

13.1.3

Fire Safety ........................................................................................................................ 182

NIST NCSTAR 1-1, WTC Investigation

IST OF

IGURES

Figure P–1. The eight projects in the federal building and fire safety

investigation of the WTC disaster...................................................................................... xxv

Figure 2–1.

WTC site plan. ..................................................................................................................... 17

Figure 2–2.

West elevation of WTC 1..................................................................................................... 18

Figure 2–3.

Elevation of exterior wall from foundation to floor 9.......................................................... 19

Figure 2–4.

Typical WTC tower architectural floor plan........................................................................ 20

Figure 2–5.

Arrangement of express and local elevators. ....................................................................... 21

Figure 2–6.

Framed tube system. ............................................................................................................ 22

Figure 2–7.

Cross section of perimeter columns. .................................................................................... 23

Figure 2–8.

Typical WTC tower exterior wall tree panel........................................................................ 24

Figure 2–9.

Typical WTC tower exterior wall panel............................................................................... 25

Figure 2–10. Elevation of exterior wall frame illustrating staggered panel construction.......................... 26

Figure 2–11. Typical welded box members and rolled shapes between floor 83 and floor 86. ................ 26

Figure 2–12. Core column layout in WTC towers. ................................................................................... 27

Figure 2–13. Typical floor-framing plan................................................................................................... 28

Figure 2–14. Prefabricated floor panel used in WTC 1 and WTC 2. ........................................................ 29

Figure 2–15. Typical WTC floor truss framing zone. ............................................................................... 30

Figure 2–16. Position of viscoelastic damper............................................................................................ 31

Figure 2–17. Perimeter column wall panel and steel truss floor modules................................................. 32

Figure 2–18. Hat truss. .............................................................................................................................. 33

Figure 2–19. Typical floor plan above floor 7. ......................................................................................... 33

Figure 2–20. Perimeter elevations of WTC 7............................................................................................ 34

Figure 2–21. Floor 1 plan of WTC 7......................................................................................................... 34

Figure 2–22. Framing plan for floor 8 through floor 45............................................................................ 35

Figure 2–23. Floor 5 diaphragm plan. ....................................................................................................... 35

Figure 2–24. Typical built-up column details. .......................................................................................... 36

Figure 2–25. Schematic view of transfer trusses and girders between floors 5 and 7............................... 36

Figure 4–1.

Reduced live load as a function of floor location based on the percentage method (for
columns, walls, and piers).................................................................................................... 60

Figure 4–2.

Wind load pressure versus elevation.................................................................................... 61

List of Figures

Draft for Public Comment

xii

NIST NCSTAR 1-1, WTC Investigation

Figure 5–1.

Design dead load criteria for WTC 1 and WTC 2: floor inside of core. .............................. 74

Figure 5–2.

Design partition load criteria for WTC 1 and WTC 2: floor inside of core. ........................ 75

Figure 5–3.

Design dead load criteria for WTC 1 and WTC 2: floor outside of core. ............................ 76

Figure 5–4.

Design load criteria for WTC 7............................................................................................ 77

Figure 5–5.

Design live-load criteria for WTC 1 and WTC 2: floor inside of core. ............................... 78

Figure 5–6.

Design live-load criteria for WTC 1 and WTC 2: floor inside of core. ............................... 79

Figure 5–7.

Design live-load criteria for WTC 1 and WTC 2: column inside of core. ........................... 80

Figure 5–8.

Design live-load criteria for WTC 1 and WTC 2: floor outside of core. ............................. 81

Figure 5–9.

Design live-load criteria for WTC 1 and WTC 2: column outside of core. ......................... 82

Figure 5–10. Live-load reduction criteria for WTC 1 and WTC 2............................................................ 83

Figure 5–11. Live-load reduction criteria for floors inside of core, except for tenant areas. .................... 84

Figure 5–12. Live-load reduction criteria for floors inside of core, tenant areas. ..................................... 85

Figure 5–13. Live-load reduction criteria for floors outside of core. ........................................................ 86

Figure 6–1.

Wide-flange beam member with Type B damping unit....................................................... 94

Figure 6–2.

Wind directions that produced the greatest displacements at the top of the tower
during the wind tunnel tests. ................................................................................................ 95

Figure 6–3.

Definition of grid system and tower configurations for wind tunnel tests at Colorado
State University.................................................................................................................... 96

Figure 10–1. Arrangement of floor 106 egress. ...................................................................................... 154

Figure 10–2. Arrangement of floor 107 egress. ...................................................................................... 155

Figure 10–3. Gypsum plank shaft partition............................................................................................. 162

Figure 10–4. Gypsum plank installation. ................................................................................................ 163

Figure 10–5. Typical finish details.......................................................................................................... 163

Figure 10–6. Stairway detail at 26th floor............................................................................................... 163

Figure 12–1. Section plan showing the final locations of the fuel oil distribution components. ............ 174

NIST NCSTAR 1-1, WTC Investigation

xiii

IST OF

ABLES

Table P–1. Federal building and fire safety investigation of the WTC disaster. .................................... xxiv

Table P–2. Public meetings and briefings of the WTC Investigation.................................................... xxvii

Table 4–1. Examples of dead loads given in NYC Building Code and BOCA Code. .............................. 45

Table 4–2. Comparison of uniform live load values. Examples of minimum uniformly distributed

live loads. ............................................................................................................................. 46

Table 4–3. Reduced live load for beams and girders. ................................................................................ 48

Table 4–4. Base shears and overturning moments from reviewed codes for a building the height of

WTC towers (1,368 ft). ........................................................................................................ 48

Table 4–5. Design standards for concrete and steel. .................................................................................. 53

Table 4–6. Compliance requirements for alterations. ................................................................................ 55

Table 4–7. Excerpts of inspection requirements for materials and assemblies in Article 10 of 1968

NYC Building Code............................................................................................................. 57

Table 4–8. Excerpts of inspection requirements for methods of construction in Article 10 of 1968

NYC Building Code............................................................................................................. 58

Table 5–1. Live loads used in design of WTC 1 and WTC 2. ................................................................... 65

Table 5–2. Percentage of live load per the 1968 NYC Building Code. ..................................................... 66

Table 5–3. Base shears and overturning moments based on the 1968 NYC Building Code and wind

tunnel tests. .......................................................................................................................... 69

Table 8–1. Summary of Structural Integrity Inspections Completed for WTC 1 and WTC 2. ............... 127

Table 8–2. Measured first mode natural frequencies for WTC 1............................................................. 131

Table 8–3. Summary of natural frequency test results for floors of WTC 1, March 1971. ..................... 132

Table 8–4. Summary of natural frequency test results for floors of WTC 1 and WTC 2, March

1995.................................................................................................................................... 132

Table 12–1. Summary of modifications to base emergency power system in WTC 7. ........................... 173

NIST NCSTAR 1-1, WTC Investigation

IST OF

CRONYMS AND

BBREVIATIONS

Acronyms

Minnesota Mining and Manufacturing Company

ACI

American Concrete Institute

AIA

American Institute of Architects

AISC

American Institute of Steel Construction

ASME

American Society of Mechanical Engineers

ASTM

ASTM International

AWCI

Association of Wall and Ceiling Industries

AWS

American Welding Society

BBC

Basic Building Code

BOCA

Building Officials and Code Administrators International

CMU

concrete

masonry

unit

Con Edison

Consolidated Edison

CSU

Colorado State University

FDNY

New York City Fire Department

FSES

Fire Safety Equivalency Systems

Gypsum

Association

HVAC

heating, ventilating, and air conditioning

IBC

International

Building

Code

JB&B

Jaros, Baum & Bolles

KKE

Karl Koch Erecting Company

LERA

Leslie E. Robertson Associates

Local

Law

MER

Mechanical Equipment Room

MCC

Municipal Code of Chicago

MIT

Massachusetts Institute of Technology

NBC

National Building Code

NFPA

National Fire Protection Association

NIST

National Institute of Standards and Technology

List of Acronyms and Abbreviations

Draft for Public Comment

xvi

NIST NCSTAR 1-1, WTC Investigation

NPL

National Physical Laboratory

NYC

New York City

OEM

Office of Emergency Management

PANYNJ

Port Authority of New York and New Jersey

PCF

Pacific Car & Foundry Co.

PDM

Pittsburgh-Des Moines Steel Company

P.L.

Public

Law

PONYA

Port of New York Authority (in 1972 PONYA changed to PANYNJ)

RJA

Rolf Jensen & Associates

Reference

Standard

SBCCI

Southern Building Code Congress International, Inc., published

Southern Standard

Building Code

SFRM

sprayed fire resistive material

SHCR

Skilling, Helle, Christiansen, & Robertson (structural engineers)

SII

Structural Integrity Inspection

SSPC

Steel Structures Painting Council

TRCC

Tishman Realty & Construction Company

UBC

Uniform Building Code

Underwriters’ Laboratories, Inc.

USC

United States Code

WSHJ

Worthington, Skilling, Helle & Jackson

WTC

World

Trade

Center

WTC 1

World Trade Center 1 (North Tower)

WTC 2

World Trade Center 2 (South Tower)

WTC 7

World Trade Center 7

Abbreviations

degrees

Celsius

degrees

Fahrenheit

cfm

cubic foot per minute

cps

cycles per second

yield strength of steel

ft foot

Draft for Public Comment

List of Acronyms and Abbreviations

NIST NCSTAR 1-1, WTC Investigation

xvii

square foot

gal

gallon

gph

gallons per hour

in.

inch

in.

square

inch

kip

a force equal to 1,000 pounds

ksi

1,000 pounds per square inch

kilowatt

L liter

lb pound

m meter

micrometer

min

minute

mph

miles per hour

pcf

pounds per cubic foot

plf

pounds per linear foot

psf

pounds per square foot

psi

pounds-force per square inch

s second

NIST NCSTAR 1-1, WTC Investigation

xix

ETRIC

ONVERSION

ABLE

To convert from

Multiply by

AREA AND SECOND MOMENT OF AREA

square foot (ft

)

square

meter

)

9.290

304

E-02

square inch (in.

)

square

meter

)

6.4516

E-04

square inch (in.

)

square

centimeter

(cm

)

6.4516

E+00

square yard (yd

)

square

meter

)

8.361

274

E-01

ENERGY (includes WORK)

kilowatt hour (kW

⋅

joule

(J)

3.6

E+06

quad (1015 BtuIT)

joule (J)

1.055 056 E+18

therm

(U.S.)

joule

(J)

1.054

804

E+08

ton of TNT (energy equivalent)

joule (J)

4.184 E+09

watt hour (W

⋅

joule

(J)

3.6

E+03

watt second (W

⋅

joule

(J)

1.0

E+00

FORCE

dyne

(dyn)

newton

(N)

1.0

E-05

kilogram-force (kgf)

newton (N)

9.806 65 E+00

kilopond (kilogram-force) (kp)

newton (N)

9.806 65 E+00

kip (1 kip=1,000 lbf)

newton (N)

4.448 222 E+03

kip (1 kip=1,000 lbf)

kilonewton (kN)

4.448 222 E+00

pound-force

(lbf)

newton

(N)

4.448

222

E+00

FORCE DIVIDED BY LENGTH

pound-force per foot (lbf/ft)

newton per meter (N/m)

1.459 390 E+01

pound-force per inch (lbf/in.)

newton per meter (N/m)

1.751 268 E+02

HEAT FLOW RATE

calorieth per minute (calth/min)

watt (W)

6.973 333 E-02

calorieth per second (calth/s)

watt (W)

4.184 E+00

kilocalorieth per minute (kcalth/min)

watt (W)

6.973 333 E+01

kilocalorieth

per

second

(kcalth/s)

watt

(W)

4.184

E+03

Metric Conversion Table

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

To convert from

Multiply by

LENGTH

foot

(ft)

meter

(m)

3.048

E-01

inch

(in)

meter

(m)

2.54

E-02

inch

(in.)

centimeter

(cm)

2.54

E+00

micron

(m)

meter

(m)

1.0

E-06

yard

(yd)

meter

(m)

9.144

E-01

MASS and MOMENT OF INERTIA

kilogram-force second squared

per meter (kgf

⋅

/m)

kilogram

(kg)

9.806

E+00

pound foot squared (lb

⋅

)

kilogram meter squared (kg

⋅

)

4.214 011 E-02

pound inch squared (lb

⋅

in.

)

kilogram meter squared (kg

⋅

)

2.926 397 E-04

ton,

metric

(t)

kilogram

(kg)

1.0

E+03

ton, short (2,000 lb)

kilogram (kg)

9.071 847 E+02

MASS DIVIDED BY AREA

pound per square foot (lb/ft

)

kilogram per square meter (kg/m

)

4.882 428 E+00

pound per square inch

(

not

pound force) (lb/in.

)

kilogram per square meter (kg/m

)

7.030 696 E+02

MASS DIVIDED BY LENGTH

pound per foot (lb/ft)

kilogram per meter (kg/m)

1.488 164 E+00

pound per inch (lb/in.)

kilogram per meter (kg/m)

1.785 797 E+01

pound per yard (lb/yd)

kilogram per meter (kg/m)

4.960 546 E-01

PRESSURE or STRESS (FORCE DIVIDED BY AREA)

kilogram-force per square centimeter (kgf/cm

) pascal (Pa)

9.806 65 E+04

kilogram-force per square meter (kgf/m

)

pascal (Pa)

9.806 65 E+00

kilogram-force per square millimeter (kgf/mm

) pascal (Pa)

9.806 65 E+06

kip per square inch (ksi) (kip/in.

)

pascal

(Pa) 6.894

757

E+06

kip per square inch (ksi) (kip/in.

)

kilopascal

(kPa)

6.894

757

E+03

pound-force per square foot (lbf/ft

)

pascal

(Pa) 4.788

026

E+01

pound-force per square inch (psi) (lbf/in.

)

pascal (Pa)

6.894 757 E+03

pound-force per square inch (psi) (lbf/in.

)

kilopascal (kPa)

6.894 757 E+00

psi (pound-force per square inch) (lbf/in.

)

pascal (Pa)

6.894 757 E+03

psi (pound-force per square inch) (lbf/in.

)

kilopascal (kPa)

6.894 757 E+00

Draft for Public Comment

Metric Conversion Table

NIST NCSTAR 1-1, WTC Investigation

xxi

To convert from

Multiply by

TEMPERATURE

degree Celsius (

kelvin

(K)

T/K

C + 273.15

degree

centigrade

degree

Celsius

(

≈

t /deg. cent.

degree Fahrenheit (

degree

Celsius

(

C = (t/

F -

32)/1.8

degree Fahrenheit (

kelvin

(K)

T/K

(t/

F + 459.67)/1.8

kelvin

(K)

degree

Celsius

(

C = T/K 2

273.15

TEMPERATURE INTERVAL

degree Celsius (

kelvin

(K)

1.0

E+00

degree

centigrade

degree

Celsius

(

1.0

E+00

degree Fahrenheit (

degree

Celsius

(

5.555

556

E-01

degree Fahrenheit (

kelvin

(K)

5.555

556

E-01

degree Rankine (

kelvin

(K)

5.555

556

E-01

VELOCITY (includes SPEED)

foot per second (ft/s)

meter per second (m/s)

3.048 E-01

inch

per

second

(in./s)

meter per second (m/s)

2.54 E-02

kilometer per hour

(km/h)

meter per second (m/s)

2.777 778 E-01

mile per hour (mi/h)

kilometer per hour (km/h)

1.609 344 E+00

mile per minute (mi/min)

meter per second (m/s)

2.682 24 E+01

VOLUME (includes CAPACITY)

cubic foot (ft

)

cubic meter (m

)

2.831

685

E-02

cubic inch (in.

)

cubic meter (m

)

1.638

706

E-05

cubic yard (yd

)

cubic meter (m

)

7.645

549

E-01

gallon

(U.S.)

(gal)

cubic

meter

)

3.785

412

E-03

gallon

(U.S.)

(gal)

liter

(L)

3.785

412

E+00

liter

(L)

cubic

meter

)

1.0

E-03

ounce (U.S. fluid) (fl oz)

cubic meter (m

)

2.957

353

E-05

ounce (U.S. fluid) (fl oz)

milliliter (mL)

2.957 353 E+01

NIST NCSTAR 1-1, WTC Investigation

xxiii

REFACE

Genesis of This Investigation

Immediately following the attack on the World Trade Center (WTC) on September 11, 2001, the Federal
Emergency Management Agency (FEMA) and the American Society of Civil Engineers began planning a
building performance study of the disaster. The week of October 7, as soon as the rescue and search
efforts ceased, the Building Performance Study Team went to the site and began their assessment. This
was to be a brief effort, as the study team consisted of experts who largely volunteered their time away
from their other professional commitments. The Building Performance Study Team issued their report in
May 2002, fulfilling their goal “to determine probable failure mechanisms and to identify areas of future
investigation that could lead to practical measures for improving the damage resistance of buildings
against such unforeseen events.”

On August 21, 2002, with funding from the U.S. Congress through FEMA, the National Institute of
Standards and Technology (NIST) announced its building and fire safety investigation of the WTC
disaster. On October 1, 2002, the National Construction Safety Team Act (Public Law 107-231), was
signed into law. (A copy of the Public Law is included in Appendix A.) The NIST WTC Investigation
was conducted under the authority of the National Construction Safety Team Act.

The goals of the investigation of the WTC disaster were:

•

To investigate the building construction, the materials used, and the technical conditions that
contributed to the outcome of the WTC disaster.

•

To serve as the basis for:

−

Improvements in the way buildings are designed, constructed, maintained, and used;

−

Improved tools and guidance for industry and safety officials;

−

Recommended revisions to current codes, standards, and practices; and

−

Improved public safety.

The specific objectives were:

Determine why and how WTC 1 and WTC 2 collapsed following the initial impacts of the
aircraft and why and how WTC 7 collapsed;

Determine why the injuries and fatalities were so high or low depending on location,
including all technical aspects of fire protection, occupant behavior, evacuation, and
emergency response;

Determine what procedures and practices were used in the design, construction, operation,
and maintenance of WTC 1, 2, and 7; and

Identify, as specifically as possible, areas in current building and fire codes, standards, and
practices that warrant revision.

Preface

Draft for Public Comment

xxiv

NIST NCSTAR 1-1, WTC Investigation

NIST is a nonregulatory agency of the U.S. Department of Commerce’s Technology Administration. The
purposes of NIST investigations under the National Construction Safety Team Act are to improve the
safety and structural integrity of buildings in the United States, and the focus is on fact finding. NIST
investigative teams are required to assess building performance and emergency response and evacuation
procedures in the wake of any building failure that has resulted in substantial loss of life or that posed
significant potential of substantial loss of life. NIST does not have the statutory authority to make
findings of fault or negligence by individuals or organizations. Further, no part of any report resulting
from a NIST investigation into a building failure or from an investigation under the National Construction
Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in
such report (15 USC 281a, as amended by Public Law 107-231).

Organization of the Investigation

The National Construction Safety Team for this Investigation, appointed by the NIST Director, was led
by Dr. S. Shyam Sunder. Dr. William L. Grosshandler served as Associate Lead Investigator,
Mr. Stephen A. Cauffman served as Program Manager for Administration, and Mr. Harold E. Nelson
served on the team as a private sector expert. The Investigation included eight interdependent projects
whose leaders comprised the remainder of the team. A detailed description of each of these eight projects
is available at http://wtc.nist.gov. The purpose of each project is summarized in Table P–1, and the key
interdependencies among the projects are illustrated in Figure P–1.

Table P–1. Federal building and fire safety investigation of the WTC disaster.

Technical Area and Project Leader

Project Purpose

Analysis of Building and Fire Codes and
Practices; Project Leaders: Dr. H. S. Lew
and Mr. Richard W. Bukowski

Document and analyze the code provisions, procedures, and
practices used in the design, construction, operation, and
maintenance of the structural, passive fire protection, and
emergency access and evacuation systems of WTC 1, 2, and 7.

Baseline Structural Performance and
Aircraft Impact Damage Analysis; Project
Leader: Dr. Fahim H. Sadek

Analyze the baseline performance of WTC 1 and WTC 2 under
design, service, and abnormal loads, and aircraft impact damage on
the structural, fire protection, and egress systems.

Mechanical and Metallurgical Analysis of
Structural Steel; Project Leader: Dr. Frank
W. Gayle

Determine and analyze the mechanical and metallurgical properties
and quality of steel, weldments, and connections from steel
recovered from WTC 1, 2, and 7.

Investigation of Active Fire Protection
Systems; Project Leader: Dr. David
D. Evans

Investigate the performance of the active fire protection systems in
WTC 1, 2, and 7 and their role in fire control, emergency response,
and fate of occupants and responders.

Reconstruction of Thermal and Tenability
Environment; Project Leader: Dr. Richard
G. Gann

Reconstruct the time-evolving temperature, thermal environment,
and smoke movement in WTC 1, 2, and 7 for use in evaluating the
structural performance of the buildings and behavior and fate of
occupants and responders.

Structural Fire Response and Collapse
Analysis; Project Leaders: Dr. John
L. Gross and Dr. Therese P. McAllister

Analyze the response of the WTC towers to fires with and without
aircraft damage, the response of WTC 7 in fires, the performance
of composite steel-trussed floor systems, and determine the most
probable structural collapse sequence for WTC 1, 2, and 7.

Occupant Behavior, Egress, and Emergency
Communications; Project Leader: Mr. Jason
D. Averill

Analyze the behavior and fate of occupants and responders, both
those who survived and those who did not, and the performance of
the evacuation system.

Emergency Response Technologies and
Guidelines; Project Leader: Mr. J. Randall
Lawson

Document the activities of the emergency responders from the time
of the attacks on WTC 1 and WTC 2 until the collapse of WTC 7,
including practices followed and technologies used.

Draft for Public Comment

Preface

NIST NCSTAR 1-1, WTC Investigation

xxv

NIST WTC Investigation Projects

Analysis of

Steel

Structural

Collapse

Evacuation

Baseline

Performance

& Impact

Damage

Analysis of
Codes and

Practices

Emergency

Response

Active Fire

Protection

Thermal and

Tenability

Environment

Video/
Photographic
Records

Oral History Data

Emergency
Response
Records

Recovered
Structural Steel

WTC Building
Performance Study
Recommendations

Government,
Industry,
Professional,
Academic Inputs

Public Inputs

Figure P–1. The eight projects in the federal building and fire safety

investigation of the WTC disaster.

National Construction Safety Team Advisory Committee

The NIST Director also established an advisory committee as mandated under the National Construction
Safety Team Act. The initial members of the committee were appointed following a public solicitation.
These were:

•

Paul Fitzgerald, Executive Vice President (retired) FM Global, National Construction Safety
Team Advisory Committee Chair

•

John Barsom, President, Barsom Consulting, Ltd.

•

John Bryan, Professor Emeritus, University of Maryland

•

David Collins, President, The Preview Group, Inc.

•

Glenn Corbett, Professor, John Jay College of Criminal Justice

•

Philip DiNenno, President, Hughes Associates, Inc.

Preface

Draft for Public Comment

xxvi

NIST NCSTAR 1-1, WTC Investigation

•

Robert Hanson, Professor Emeritus, University of Michigan

•

Charles Thornton, Co-Chairman and Managing Principal, The Thornton-Tomasetti Group,
Inc.

•

Kathleen Tierney, Director, Natural Hazards Research and Applications Information Center,
University of Colorado at Boulder

•

Forman Williams, Director, Center for Energy Research, University of California at San
Diego

This National Construction Safety Team Advisory Committee provided technical counsel during the
Investigation and commentary on drafts of the Investigation reports prior to their public release.

Public Outreach

During the course of this Investigation, NIST held public briefings and meetings (listed in Table P–2) to
solicit input from the public, present preliminary findings, and obtain comments on the direction and
progress of the Investigation from the public and the Advisory Committee.

NIST maintained a publicly accessible Web site during this Investigation at http://wtc.nist.gov. The site
contained extensive information on the background and progress of the Investigation.

NIST’s WTC Public-Private Response Plan

The collapse of the WTC buildings has led to broad reexamination of how tall buildings are designed,
constructed, maintained, and used, especially with regard to major events such as fires, natural disasters,
and terrorist attacks. Reflecting the enhanced interest in effecting necessary change, NIST, with support
from Congress and the Administration, has put in place a program, the goal of which is to develop and
implement the standards, technology, and practices needed for cost-effective improvements to the safety
and security of buildings and building occupants, including evacuation, emergency response procedures,
and threat mitigation.

The strategy to meet this goal is a three-part NIST-led public-private response program that includes:

•

A federal building and fire safety investigation to study the most probable factors that
contributed to post-aircraft impact collapse of the WTC towers and the 47-story WTC 7
building, and the associated evacuation and emergency response experience.

•

A research and development (R&D) program to (a) facilitate the implementation of
recommendations resulting from the WTC Investigation, and (b) provide the technical basis
for cost-effective improvements to national building and fire codes, standards, and practices
that enhance the safety of buildings, their occupants, and emergency responders.

Draft for Public Comment

Preface

NIST NCSTAR 1-1, WTC Investigation

xxvii

Table P–2. Public meetings and briefings of the WTC Investigation.

Date Location

Principal

Agenda

June 24, 2002

New York City, NY

Public meeting: Public comments on the

Draft Plan

for the

pending WTC Investigation.

December 9, 2002

Washington, DC

Media briefing on release of the

Public Update

and NIST request

for photographs and videos.

April 8, 2003

New York City, NY

Joint public forum with Columbia University on first-person
interviews.

April 29-30, 2003

Gaithersburg, MD

National Construction Safety Team (NCST) Advisory Committee
meeting on plan for and progress on WTC Investigation with a
public comment session.

May 7, 2003

New York City, NY

Media briefing on release of the

May 2003 Progress Report

August 26-27, 2003

Gaithersburg, MD

NCST Advisory Committee meeting on status of WTC
investigation with a public comment session.

September 17, 2003

New York City, NY

Media briefing and public briefing on initiation of first-person
data collection projects.

December 2-3, 2003

Gaithersburg, MD

NCST Advisory Committee meeting on status and initial results
and the release of the

Public Update

with a public comment

session.

February 12, 2004

New York City, NY

Public meeting: Briefing on progress and preliminary findings
with public comments on issues to be considered in formulating
final recommendations.

June 18, 2004

New York City, NY

Media briefing and public briefing on release of the

June 2004

Progress Report.

June 22-23, 2004

Gaithersburg, MD

NCST Advisory Committee meeting on the status of and
preliminary findings from the WTC Investigation with a public
comment session.

August 24, 2004

Northbrook, IL

Public viewing of standard fire resistance test of WTC floor
system at Underwriters Laboratories, Inc.

October 19-20, 2004

Gaithersburg, MD

NCST Advisory Committee meeting on status and near complete
set of preliminary findings with a public comment session.

November 22, 2004

Gaithersburg, MD

NCST Advisory Committee discussion on draft annual report to
Congress, a public comment session, and a closed session to
discuss pre-draft recommendations for WTC Investigation.

April 5, 2005

New York City, NY

Media briefing and public briefing on release of the probable
collapse sequence for the WTC towers and draft reports for the
projects on codes and practices, evacuation, and emergency
response.

•

A dissemination and technical assistance program (DTAP) to (a) engage leaders of the
construction and building community in ensuring timely adoption and widespread use of
proposed changes to practices, standards, and codes resulting from the WTC Investigation
and the R&D program, and (b) provide practical guidance and tools to better prepare facility
owners, contractors, architects, engineers, emergency responders, and regulatory authorities
to respond to future disasters.

The desired outcomes are to make buildings, occupants, and first responders safer in future disaster
events.

Preface

Draft for Public Comment

xxviii

NIST NCSTAR 1-1, WTC Investigation

National Construction Safety Team Reports on the WTC Investigation

A draft of the final report on the collapses of the WTC towers is being issued as NIST NCSTAR 1. A
companion report on the collapse of WTC 7 is being issued as NIST NCSTAR 1A. The present report is
one of a set that provides more detailed documentation of the Investigation findings and the means by
which these technical results were achieved. As such, it is part of the archival record of this Investigation.
The titles of the full set of Investigation publications are:

NIST (National Institute of Standards and Technology). 2005.

Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team
on the Collapses of the World Trade Center Towers.

NIST NCSTAR 1. Gaithersburg, MD, September.

NIST (National Institute of Standards and Technology). 2005.

Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team
on the Collapse of World Trade Center 7.

NIST NCSTAR 1A. Gaithersburg, MD, December.

Lew, H. S., R. W. Bukowski, and N. J. Carino. 2005.

Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Design, Construction, and Maintenance of Structural and Life Safety
Systems.

NIST NCSTAR 1-1. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005.

Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Design and Construction of Structural Systems.

NIST NCSTAR 1-1A. National Institute of Standards and Technology. Gaithersburg, MD,
September.

Ghosh, S. K., and X. Liang. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Comparison of Building Code Structural Requirements.

NIST

NCSTAR 1-1B. National Institute of Standards and Technology. Gaithersburg, MD, September.

Fanella, D. A., A. T. Derecho, and S. K. Ghosh. 2005.

Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Maintenance and Modifications to Structural
Systems.

NIST NCSTAR 1-1C. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Grill, R. A., and D. A. Johnson. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Fire Protection and Life Safety Provisions Applied to the Design and
Construction of World Trade Center 1, 2, and 7 and Post-Construction Provisions Applied after
Occupancy

. NIST NCSTAR 1-1D. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Razza, J. C., and R. A. Grill. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Comparison of Codes, Standards, and Practices in Use at the Time of the
Design and Construction of World Trade Center 1, 2, and 7

. NIST NCSTAR 1-1E. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005.

Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Comparison of the 1968 and Current (2003) New

Draft for Public Comment

Preface

NIST NCSTAR 1-1, WTC Investigation

xxix

York City Building Code Provisions

. NIST NCSTAR 1-1F. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Grill, R. A., and D. A. Johnson. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Amendments to the Fire Protection and Life Safety Provisions of the New
York City Building Code by Local Laws Adopted While World Trade Center 1, 2, and 7 Were in
Use

. NIST NCSTAR 1-1G. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Grill, R. A., and D. A. Johnson. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Post-Construction Modifications to Fire Protection and Life Safety Systems
of World Trade Center 1 and 2

. NIST NCSTAR 1-1H. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Grill, R. A., D. A. Johnson, and D. A. Fanella. 2005.

Federal Building and Fire Safety Investigation

of the World Trade Center Disaster: Post-Construction Modifications to Fire Protection, Life
Safety, and Structural Systems of World Trade Center 7

. NIST NCSTAR 1-1I. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Grill, R. A., and D. A. Johnson. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Design, Installation, and Operation of Fuel System for Emergency Power in
World Trade Center 7

. NIST NCSTAR 1-1J. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Sadek, F. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center Disaster:

Baseline Structural Performance and Aircraft Impact Damage Analysis of the World Trade Center
Towers.

NIST NCSTAR 1-2. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Faschan, W. J., and R. B. Garlock. 2005.

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster: Reference Structural Models and Baseline Performance Analysis of
the World Trade Center Towers.

NIST NCSTAR 1-2A. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Kirkpatrick, S. W., R. T. Bocchieri, F. Sadek, R. A. MacNeill, S. Holmes, B. D. Peterson,
R. W. Cilke, C. Navarro.

2005.

Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Analysis of Aircraft Impacts into the World Trade Center Towers,

NIST

NCSTAR 1-2B. National Institute of Standards and Technology. Gaithersburg, MD, September.

Gayle, F. W., R. J. Fields, W. E. Luecke, S. W. Banovic, T. Foecke, C. McCowan, T. A. Siewert, and
J. D. McColskey. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Mechanical and Metallurgical Analysis of Structural Steel.

NIST NCSTAR 1-3. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Luecke, W. E., T. A. Siewert, and F. W. Gayle. 2005.

Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Contemporaneous Structural Steel
Specifications.

NIST Special Publication 1-3A. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Preface

Draft for Public Comment

xxx

NIST NCSTAR 1-1, WTC Investigation

Banovic, S. W. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Steel Inventory and Identification.

NIST NCSTAR 1-3B. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Banovic, S. W., and T. Foecke. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Damage and Failure Modes of Structural Steel Components.

NIST

NCSTAR 1-3C. National Institute of Standards and Technology. Gaithersburg, MD, September.

Luecke, W. E., J. D. McColskey, C. McCowan, S. W. Banovic, R. J. Fields, T. Foecke,
T. A. Siewert, and F. W. Gayle. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Mechanical Properties of Structural Steels.

NIST NCSTAR 1-3D.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Banovic, S. W., C. McCowan, and W. E. Luecke. 2005.

Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Physical Properties of Structural Steels.

NIST

NCSTAR 1 3E. National Institute of Standards and Technology. Gaithersburg, MD, September.

Evans, D. D., E. D. Kuligowski, W. S. Dols, and W. L. Grosshandler. 2005.

Federal Building and Fire

Safety Investigation of the World Trade Center Disaster: Active Fire Protection Systems.

NIST

NCSTAR 1-4. National Institute of Standards and Technology. Gaithersburg, MD, September.

Kuligowski, E. D., and D. D. Evans. 2005.

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster: Post-Construction Fires Prior to September 11, 2001.

NIST

NCSTAR 1-4A. National Institute of Standards and Technology. Gaithersburg, MD, September.

Hopkins, M., J. Schoenrock, and E. Budnick. 2005.

Federal Building and Fire Safety Investigation

of the World Trade Center Disaster: Fire Suppression Systems.

NIST NCSTAR 1-4B. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Keough, R. J., and R. A. Grill. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Fire Alarm Systems.

NIST NCSTAR 1-4C. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Ferreira, M. J., and S. M. Strege. 2005.

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster: Smoke Management Systems.

NIST NCSTAR 1-4D. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Gann, R. G., A. Hamins, H. E. Nelson, K. B. McGrattan, G. W. Mulholland, T. J. Ohlemiller,
W. M. Pitts, and K. R. Prasad. 2005.

Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Reconstruction of the Fires in the World Trade Center Towers.

NIST NCSTAR 1-5.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Pitts, W. M., and K. M. Butler. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Visual Evidence, Damage Estimates, and Timeline Analysis.

NIST

NCSTAR 1-5A. National Institute of Standards and Technology. Gaithersburg, MD, September.

Hamins, A., A. Maranghides, K. B. McGrattan, E. Johnsson, T. J. Ohlemiller, M. Donnelly,
J. Yang, G. Mulholland, K. R. Prasad, S. Kukuck, R. Anleitner and T. McAllister. 2005.

Federal

Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and

Draft for Public Comment

Preface

NIST NCSTAR 1-1, WTC Investigation

xxxi

Modeling of Structural Steel Elements Exposed to Fire.

NIST NCSTAR 1-5B. National Institute of

Standards and Technology. Gaithersburg, MD, September.

Ohlemiller, T. J., G. W. Mulholland, A. Maranghides, J. J. Filliben, and R. G. Gann. 2005.

Federal

Building and Fire Safety Investigation of the World Trade Center Disaster: Fire Tests of Single
Office Workstations.

NIST NCSTAR 1-5C. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Gann, R. G., M. A. Riley, J. M. Repp, A. S. Whittaker, A. M. Reinhorn, and P. A. Hough. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Reaction of
Ceiling Tile Systems to Shocks.

NIST NCSTAR 1-5D. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Hamins, A., A Maranghides, K. B. McGrattan, T. J. Ohlemiller, and R. Anleitner. 2005.

Federal

Building and Fire Safety Investigation of the World Trade Center Disaster: Experiments and
Modeling of Multiple Workstations Burning in a Compartment.

NIST NCSTAR 1-5E. National

Institute of Standards and Technology. Gaithersburg, MD, September.

McGrattan, K. B., C. Bouldin, and G. Forney. 2005.

Federal Building and Fire Safety

Investigation of the World Trade Center Disaster: Computer Simulation of the Fires in the World
Trade Center Towers.

NIST NCSTAR 1-5F. National Institute of Standards and Technology.

Gaithersburg, MD, September.

Prasad, K. R., and H. R. Baum. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: Fire Structure Interface and Thermal Response of the World Trade Center
Towers.

NIST NCSTAR 1-5G. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Gross, J. L., and T. McAllister. 2005.

Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Structural Fire Response and Probable Collapse Sequence of the World Trade Center
Towers.

NIST NCSTAR 1-6. National Institute of Standards and Technology. Gaithersburg, MD,

September.

Carino, N. J., D. P. Bentz, R. W. Bukowski, J. L. Gross, S. Kukuck, K. R. Prasad, and
M. A. Starnes. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Passive Fire Protection.

NIST NCSTAR 1-6A. National Institute of Standards and

Technology. Gaithersburg, MD, September.

Gross, J., F. Hervey, M. Izydorek, J. Mammoser, and J. Treadway. 2005.

Federal Building and

Fire Safety Investigation of the World Trade Center Disaster: Fire Resistance Tests of Floor Truss
Systems.

NIST NCSTAR 1-6B. National Institute of Standards and Technology. Gaithersburg,

MD, September.

Zarghamee, M. S., A. A. Liepins, F. W. Kan, M. Mudlock, O. O. Erbay, Y. Kitane, W. I. Naguib,
A. T. Sarawit. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Component, Connection, and Subsystem Structural Analysis.

NIST NCSTAR 1-6C.

National Institute of Standards and Technology. Gaithersburg, MD, September.

Preface

Draft for Public Comment

xxxii

NIST NCSTAR 1-1, WTC Investigation

Zarghamee, M. S., O. O. Erbay, Y. Kitane. 2005.

Federal Building and Fire Safety Investigation of

the World Trade Center Disaster: Global Structural Analysis of the Response of the World Trade
Center Towers to Impact Damage and Fire.

NIST NCSTAR 1-6D. National Institute of Standards

and Technology. Gaithersburg, MD, September.

McAllister, T., R. G. Gann, J. L. Gross, K. B. McGrattan, H. E. Nelson, W. M. Pitts, K. R. Prasad. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Structural Fire
Response and Probable Collapse Sequence of World Trade Center 7.

2005. NIST NCSTAR 1-6E.

National Institute of Standards and Technology. Gaithersburg, MD, December.

Gilsanz, R., V. Arbitrio, C. Anders, D. Chlebus, K. Ezzeldin, W. Guo, P. Moloney, A. Montalva,
J. Oh, K. Rubenacker. 2005.

Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Structural Analysis of the Response of World Trade Center 7 to Debris Damage
and Fire

. NIST NCSTAR 1-6F. National Institute of Standards and Technology. Gaithersburg,

MD, December.

Kim, W. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Analysis of September 11, 2001, Seismogram Data

, NIST NCSTAR 1-6G. National

Institute of Standards and Technology. Gaithersburg, MD, December.

Nelson, K. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: The ConEd Substation in World Trade Center 7

, NIST NCSTAR 1-6H. National

Institute of Standards and Technology. Gaithersburg, MD, December.

Averill, J. D., D. S. Mileti, R. D. Peacock, E. D. Kuligowski, N. Groner, G. Proulx, and P. A. Reneke.
2005.

Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Occupant

Behavior, Egress, and Emergency Communication.

NIST NCSTAR 1-7. National Institute of Standards

and Technology. Gaithersburg, MD, September.

Fahy, R., and G. Proulx. 2005.

Federal Building and Fire Safety Investigation of the World Trade

Center Disaster: Analysis of Published Accounts of the World Trade Center Evacuation.

NIST

NCSTAR 1-7A. National Institute of Standards and Technology. Gaithersburg, MD, September.

Zmud, J. 2005.

Federal Building and Fire Safety Investigation of the World Trade Center

Disaster: Technical Documentation for Survey Administration.

NIST NCSTAR 1-7B. National

Institute of Standards and Technology. Gaithersburg, MD, September.

Lawson, J. R., and R. L. Vettori. 2005.

Federal Building and Fire Safety Investigation of the World

Trade Center Disaster: The Emergency Response Operations.

NIST NCSTAR 1-8. National Institute of

Standards and Technology. Gaithersburg, MD, September.

NIST NCSTAR 1-1, WTC Investigation

xxxv

XECUTIVE

UMMARY

E.1 INTRODUCTION

One of the four primary objectives of the National Institute of Standards and Technology (NIST)
Investigation of the World Trade Center (WTC) disaster is to determine the procedures and practices that
were used in the design, construction, operation, and maintenance of the structural, passive and active fire
protection, and emergency and evacuation systems of WTC 1, 2, and 7 and the impacts these had on the
buildings over their life, up to the attacks of September 11, 2001.

To accomplish this objective, relevant information was collected by reviewing design and construction
documents, correspondence, and memoranda related to the building projects; and tenant alterations;
interviewing individuals involved in the design, construction, and maintenance of the buildings; obtaining
information from regulatory and emergency services agencies of New York City; and reviewing books
and published journal and magazine articles related to the WTC building projects. Information obtained
from various sources was synthesized and summarized in this report. Specifically, this report presents:

Provisions used to design and construct the structural, fire protection, and egress systems of
the buildings;

Tests performed to support the design of these systems;

Criteria that governed the design of the structural and fire protection systems;

Methods used to proportion structural members and other components of the buildings;

Innovative features, technologies, and materials that were incorporated in the design and
construction of the structural and fire protection systems;

Details of variances to the contract documents granted by the Port Authority of New York
and New Jersey (PANYNJ or Port Authority);

Fabrication and inspection requirements at the fabrication yard; and

Inspection protocol during construction.

Alterations made to the buildings to accommodate specific needs of tenants or to respond to
changes to the Building Code of New York City as implemented in Local Laws (LL) and
interpreted in rules.

This report also addresses the fuel systems for the diesel generators that supplied emergency power to
many of the tenants in WTC 7.

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NIST NCSTAR 1-1, WTC Investigation

E.2

DESCRIPTION OF WTC 1, 2, AND 7

The WTC complex was located at lower west side of Manhattan, New York City, near the Hudson River.
The complex was composed of seven buildings (referred to in this report as WTC 1 through WTC 7).
The two towers, WTC 1 (North Tower) and WTC 2 (South Tower), were each 110 stories high. WTC 3
(Marriott Hotel) was 22 stories. WTC 4 (South Plaza Building) and WTC 5 (North Plaza Building) were
both nine-story office buildings. WTC 6 (U.S. Customs House) was an eight-story office building. These
six buildings were built around a 5 acre WTC Plaza. WTC 7 was a 47-story office building that was built
just north of the six-building WTC site.

The first six buildings on the sixteen-acre site were developed by the Port Authority. Groundbreaking for
WTC 1 and WTC 2 was in 1966, and the first tenant began to occupy WTC 1 in December 1970 and
WTC 2 in January 1972. Construction of the other buildings continued during the 1970s and the 1980s.
WTC 7 was developed by a consortium comprising the Seven World Trade Company, and Silverstein
Development Corporation, and was completed in 1987.

The NIST Investigation is focused only on WTC 1, 2, and 7.

WTC 1 and WTC 2

Although the WTC towers were similar, they were not identical. The height of WTC 1 at the roof level
was 1,368 ft above the Concourse level, was 6 ft taller than WTC 2, and supported a 360 ft tall antenna
for television and radio transmission. Each tower had a square plan with the side dimension of
approximately 207 ft. The corners of the tower were chamfered 6 ft 11 in. Each tower had a core service
area of approximately 135 ft by 87 ft. All elevators and three egress stairs were located within the core,
although on any given floor the arrangements of the elevators and the location of the stairs varied.
Placing all service systems within the core provided a nearly column-free floor space of approximately
31,000 ft

per floor outside the core. The two towers had about 10 million ft

of rentable floor area.

The towers were designed as a “framed-tube” structural system with closely spaced exterior perimeter
columns connected by spandrel beams around the perimeter at each floor level. The core was designed as
a conventional frame with a grid of columns interconnected with beams.

The exterior walls were composed of box-shaped welded steel columns and spandrel beams comprised of
a steel plate

Each building face consisted of 59 columns spaced at 3 ft 4 in. on center. As part of the

framed-tube system, the exterior columns were designed structurally such that they resisted the total
lateral loads and about 50 percent of gravity loads. Below floor 7, the columns were combined in groups
of three to form single base columns which were spaced 10 ft on center and extended to the footings. An
important architectural feature of the towers was the uniform look of the exterior walls, presented by the
uniform width of the exterior columns up the height of the buildings. This was produced by maintaining
a constant exterior dimension the columns and changing the strength of the steel with height. Thus,
twelve different grades of steel, with yield strengths ranging from 36 ksi to 100 ksi, were used for the
exterior columns. The external cladding, which covered the columns and spandrel beams, consisted of
aluminum sheets. The window openings were infilled with glass fitted into aluminum covers and sealed
with neoprene gaskets.

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xxxvii

The core columns were of two types: welded box columns for the lower floors and rolled wide flange
shapes for the upper floors. They were designed to support about 50 percent of gravity loads. Below
floor 7 to the foundation, where there were fewer perimeter columns in the outer walls, bracings were
used in the outer perimeter of the core area to increase lateral stiffness. In the lower part of the towers,
the outer core columns were designed to resist a portion of the lateral forces. Hidden within the building,
the core columns were thicker and larger on the lower floors. Thus, core columns used fewer grades of
steel. The box columns were either 36 ksi or 42 ksi. Core wide flange columns were one of four grades,
yield strengths ranging from 36 ksi to 50 ksi, but most (approximately 90 percent) were primarily 36 ksi
or 42 ksi steel.

The floor system of WTC 1 and WTC 2 was composed of concrete-steel composite members. The area
inside the cores and on the mechanical floors was framed with rolled structural steel shapes with welded
shear studs acting compositely with normal-weight concrete slabs. The thickness of the slabs varied from
4.5 in. to 8 in. depending on design loads. The area outside the core, typically on tenant floors, was
framed with steel trusses acting compositely with 4 in. thick lightweight concrete slabs cast on 1½ in.,
22 gauge fluted metal deck. The trusses consisted of double angle top and bottom chords with round bar
webs. Some floors, immediately adjacent to the mechanical floors, used a hybrid of beam and truss
framing acting compositely with the concrete slab.

Fire protection of exposed structural steel members in the WTC towers was provided by applied fire
resistive materials. They were either sprayed fire resistive materials (SFRMs), gypsum wallboards, or a
combination of the two, depending upon the type of structural members, to meet the requirements of
Construction Classification of 1B of the 1968 New York City (NYC) Building Code. All floor trusses
and beams were protected with SFRM. The columns inside the core were either covered with gypsum
wall board or a combination of gypsum wall board and SFRM. For the exterior columns, vermiculite
plaster was applied to the side of the column facing the interior of the building, whereas SFRM was
applied to other three faces. No fire resistive material was specified for the underside of the metal deck,
which was in contact with the concrete slab above. For typical tenant floors, the ceiling was suspended
from the steel trusses. The space between the ceiling and the floor above was used for the mechanical and
electrical systems.

Elevators were the primary mode of routine ingress and egress from the towers for tens of thousands of
people daily. In order to minimize the total floor space needed for elevators, each tower was divided
vertically into three zones by skylobbies, which served to distribute passengers among express and local
elevators. In this way, the local elevators within a zone were placed on top of one another within a
common shaft. Local elevators serving the lower portion of a zone were terminated to return to the space
occupied by those shafts to leasable tenant space. People transferred from express elevators to local
elevators at the skylobbies which were located on the 44th and 78th floors in both towers. Each tower
had 99 passenger and 7 freight elevators, all located within the core of the building.

WTC 7

WTC 7 was a 47-story commercial office building constructed by Silverstein Properties as a tenant
alteration on land owned by the Port Authority. The overall dimensions of WTC 7 were approximately
330 ft long, 140 ft wide, and 610 ft high. It contained about 2 million ft

of rentable floor area. The

building was constructed over a pre-existing electrical substation owned by Consolidated Edison

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NIST NCSTAR 1-1, WTC Investigation

(Con Edison). The original plans for the Con Edison Substation included supporting a high-rise building,
and the foundation was sized for the planned structure. However, the final design for WTC 7 had a larger
footprint than originally planned.

Above floor 7, the building had typical steel framing for high-rise construction. The floor systems had
composite construction with steel beams of 50 ksi yield strength supporting concrete slabs on metal deck,
with a floor thickness of 5.5 in. The core and perimeter columns supported the floor system and carried
their loads to the foundation. Above floor 7, the perimeter moment frame resisted wind forces. Below
floor 7, a combination of moment and braced frames around the perimeter and a series of braced frames
in the core resisted the wind load.

Columns above floor 7 did not align with the foundation columns, so braced frames, transfer trusses, and
transfer girders were used to transfer loads between these column systems, primarily between floors 5
and 7. Floors 5 and 7 were heavily reinforced concrete slabs on metal decks, with thicknesses of 14 in.
and 8 in., respectively.

Core columns were primarily rolled wide-flange shapes with a yield strength of either 36 ksi or 50 ksi,
while the exterior columns were typically rolled W14 shapes with a yield strength of 36 ksi.

E.3 CODE

PROVISIONS

FOR STRUCTURAL DESIGN

The design of WTC 1, 2, and 7 was based on the 1968 edition of the NYC Building Code. As an
interstate compact under the U.S. Constitution, the Port Authority was not subjected to any state or local
building codes. In May 1963, the Port of New York Authority (PONYA or Port Authority) instructed the
architect and structural engineer to prepare their designs for WTC 1 and WTC 2 in accordance with the
NYC Building Code. At that time, the 1938 edition of that Code was in effect. In September of 1965, the
PONYA instructed the architect and structural engineer to revise their designs for WTC 1 and WTC 2 to
comply with the second and third drafts of the new NYC Building Code that was under development.
Prior to issuance of this instruction, the Port Authority recognized that the draft version of the new
NYC Building Code had incorporated advanced techniques and the Port Authority favored the use of
advanced techniques in the design of the WTC towers. By adopting the draft versions of the new NYC
Building Code, WTC 1 and WTC 2 could be classified as Type 1-B Construction, and several features
related to egress such as the elimination of the fire tower and the reduction of the number of egress stairs
required from six to three with narrower doors were incorporated into the final design.

The new Code was adopted on December 6, 1968. Subsequently, the NYC Building Code was amended
by numerous Local Laws to improve safety requirements or to incorporate technological advances, some
of which had impacts on the towers. When WTC 7 was designed, the 1968 Building Code was in effect
and the Local Laws impacting fire, life safety, and structural arrangements were in place, so these were
incorporated into the original design of that building.

To put the design of WTC 1, 2, and 7 into the context of building codes and practices of the time, the
structural provisions of the 1968 edition of the NYC Building Code were compared with the structural
provisions in a number of contemporaneous codes, as well as in the 2001 edition of the NYC Building
Code, which is currently in effect. Specifically, the following codes were selected for comparison of the
structural provisions: the 1964 New York State Building Construction Code (NYSBC 1964); the 1965
Building Officials and Code Administrators (BOCA) Basic Building Code (BOCA/BBC 1965); the 1967

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xxxix

Municipal Code of Chicago Relating to Buildings (MCC 1967); and the 2001 edition of the NYC
Building Code (NYCBC 2001). The 1964 New York State Building Construction Code was selected for
comparison, as it would have been a governing building code outside New York City limits. The 1965
BOCA Basic Building Code was selected, as it was typically adopted by local jurisdictions in the
northeastern region of the United States. The 1968 NYC Building Code is compared with the 1967
Municipal Code of Chicago to see whether there are any substantial differences in the structural and fire
safety requirements of the two codes. In the late 1960s and early 1970s, several tall buildings were built in
Chicago, including the Sears Tower (110 stories) and the John Hancock Tower (100 stories). The 2001
edition of the NYC Building Code is compared with the 1968 version to examine the extent to which
Local Laws have modified the code provisions.

Structural provisions include those concerning design loads, such as dead loads, live loads (including live
load reduction), wind loads, earthquake loads, and other loads. They also include provisions concerning
what is called “structural work” in the NYC Building Codes (this term is not used in the other codes). The
scope of “structural work” includes, but is not limited to, materials and methods of construction, design
methods including design load combinations, and the materials of construction including concrete,
masonry, steel, and wood. Structural provisions also include those for foundation design and construction.

With respect to structural design provisions, the major changes from the 1968 to the 2001 edition of the
NYC Building Code are the inclusion of seismic design requirements and updating of standards. Of the
codes contemporaneous with the 1968 NYC Building Code examined for this investigation, only the
BOCA Basic Building Code had seismic design requirements, which were adopted from the 1962 edition
of the Uniform Building Code (UBC). Taller buildings have longer periods of vibration, which means
lower seismic design forces. Also, since New York City is in an area of moderate seismicity (UBC Zone
2A), additional seismic detailing requirements are minimal to non-existent.

The alternate live load reduction provisions for columns, walls, and piers of the 1968 and 2001 NYC
Building Codes are the same as in the Chicago Municipal Code. The New York State Building Code has
more liberal live load reduction provisions for upper portions of buildings. The NYC Building Codes
also have live load reduction provisions based on contributory floor area and live-to-dead load ratio. For
live-to-dead load ratios of 0.625 or less, the New York City code provisions may yield higher live load
reduction for columns, walls, and piers than allowed by the other codes. For beams and girders, the live
load reduction provisions of the NYC Building Code are comparable to those of the New York State
Building Code and the BOCA Basic Building Code. The Chicago Municipal Code has more conservative
requirements. The maximum live load reduction allowed for beams and girders in the Chicago Municipal
Code is 15 percent, compared with 40 percent in the other codes.

When the wind load provisions in the codes are compared, the largest shear force at the base of a building
is obtained from the BOCA Basic Building Code when the height of the building is taken equal to
1,368 ft (i.e., the height of WTC 1). Similarly, the largest overturning moment at the base of a building
with the height of the WTC towers is also obtained from the BOCA Basic Building Code. Thus, the NYC
Building Code does not have the most stringent wind load provisions.

The 1968 NYC Building Code requires that weights of partitions be considered in two ways: (1) using
line loads at locations shown on plans or (2) using the equivalent uniform load. Equivalent uniform loads
must be used in areas where the locations of partitions are not shown on plans, or in areas where partitions
can be relocated. The 1964 New York State Building Construction Code did not have a specific

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NIST NCSTAR 1-1, WTC Investigation

provision in this regard. The 1967 Municipal Code of Chicago prescribed a minimum partition load of
20 psf. The BOCA Basic Building Code required consideration of the actual weight of the partitions or
an equivalent uniform load of at least 20 psf.

The primary materials design standards referenced by the 1968 NYC Building Code, the Chicago
Municipal Code, and the BOCA Basic Building Code are the 1963 edition of the American Concrete
Institute’s (ACI’s),

Building Code Requirements for Reinforced Concrete (ACI 318)

, and the American

Institute of Steel Construction’s,

Specifications for the Design, Fabrication and Erection of Structural

Steel for Buildings (AISC 1963)

. The New York State Building Code, being a performance code, does

not adopt any specific standards by reference. The 2001 NYC Building Code adopts the 1989 edition of
ACI 318, AISC 1989,

Specifications for Structural Steel Buildings – ASD and Plastic Design

, and AISC-

LRFD 1993,

Load and Resistance Factor Design Specifications for Structural Steel Buildings

The NYC Building Codes have extensive and quite rigorous foundation design and construction
requirements. The foundation related provisions of the other codes are less extensive and typically less
rigorous.

The NYC Building Codes prescribe testing and inspection requirements for all materials, assemblies,
forms and methods of construction. The other three codes require that materials and methods of
construction meet the criteria of generally accepted standards. With respect to foundations, only the NYC
Building Codes have specific requirements for foundation inspection.

E.4

STRUCTURAL DESIGN OF WTC 1, 2, AND 7

For WTC 1 and WTC 2 the design criteria were established referencing provisions of the 1968 NYC
Building Code as minimum. The design dead loads and live loads specified in the design criteria were
greater than or equal to corresponding design loads in the Building Code. Live load reduction
requirements given in the design criteria were equal to or more stringent than Code requirements.

Wind forces on the towers were determined based on a series of wind tunnel tests that were conducted at
the Colorado State University and the National Physical Laboratory, Teddington, Middlesex, United
Kingdom. Such tests were permitted by the Code to determine wind pressures in lieu of those tabulated
in the Code. The code prescribed base shear and overturning moment occur simultaneously on the same
face of the tower, and these values are the same for all four faces. The base shear and the overturning
moment obtained from the wind tunnel tests represent the largest values related to most unfavorable wind
direction; so, they may not occur simultaneously on the same face of the tower. Thus, the base shear
value obtained from the wind tunnel tests is about 42 percent greater than that obtained using the code
prescribed wind pressure values, whereas the overturning moments obtained from the wind tunnel tests is
about 65 percent greater than that obtained using the code prescribed wind pressure values.

The allowable stress method in the 1963 American Institute of Steel Construction (AISC)

Specification

for the Design, Fabrication, and Erection of Structural Steel for Buildings

was used to proportion the

exterior columns and spandrels for the combined effects of axial compression, bending moment, and
shear due to gravity and wind forces. Composite floor trusses were designed based on the AISC
Specification. The allowable stress method was also used to proportion the members in the hat trusses
that were located between the 107th floor and the roof in WTC 1 and WTC 2. In the core area, composite
steel beams, columns, and their connections were designed by the appropriate requirements in the 1963

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xli

AISC Specification. The ultimate strength method in the 1963 edition of the ACI

Building Code

Requirements for Reinforced Concrete

was used to design the concrete floor slabs in WTC 1 and WTC 2.

For WTC 7, the project specifications required that the structural steel be designed in accordance with the
1968 edition of the NYC Building Code, edited and amended through January 1, 1985, and the 1978
edition of the AISC

Specification for the Design, Fabrication, and Erection of Structural Steel for

Buildings.

Design load criteria for WTC 7, listed on one of the structural drawings, show that the design

values for the superimposed dead loads could not be ascertained, since the actual materials used for
partitions, flooring, and ductwork were not specified. The live loads in the design criteria were equal to
those in the 1968 NYC Building Code at the floors where the type of occupancy was noted. No
documents were found that indicated what live load reduction was used.

No design criteria or calculations including wind load analysis of WTC 7 were available for this
investigation. However, a wind tunnel study of WTC 7 was carried out in 1983 by the University of
Western Ontario at the request of the structural engineer of record.

E.5

INNOVATIVE FEATURES INCORPORATED IN STRUCTURAL DESIGN

A number of innovative features were incorporated in the structural design of WTC 1 and WTC 2. They
were incorporated in both the lateral-load-resisting system and the gravity-load-carrying system.

These features include the following:

•

Application of the framed-tube system to resist lateral loads.

•

Uniform exterior column geometry (14 in. by 14 in. cross-section) was maintained over most
of the height of the 110-story buildings by using twelve different grades of steel.

•

Use of deep spandrel plates as beam elements connecting perimeter columns.

•

Use of long-span composite steel trusses for the floor systems. Composite action was
achieved between the steel trusses and the concrete floor slab by extending the truss
diagonals above the top chord into the slab.

•

Application of viscoelastic dampers connecting the floor trusses to the perimeter framed tube
system to control dynamic response.

•

Use of wind tunnel test data to establish the wind loads used in the design of the towers.

E.6

FABRICATION AND CONSTRUCTION INSPECTIONS AND VARIANCES

The contract documents for WTC 1 and WTC 2 between the Port of New York Authority and the steel
fabricators and erector, and the construction contract specifications for WTC 7, indicate that inspection
programs were instituted at the steel fabrication sites. The inspection requirements were listed in the
contract documents. However, the records of inspections for both the WTC 1 and WTC 2 and the WTC 7
projects were not available to the investigation. The records for WTC 1 and WTC 2, which were kept in
WTC 1 were destroyed, and the records for WTC 7 were discarded by the general contractor after
retaining them for 7 years.

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NIST NCSTAR 1-1, WTC Investigation

WTC 1 and WTC 2

Fabrication and inspection requirements were contained in the contracts for the floor trusses, box core
columns and built-up beams, members of the exterior wall, and rolled columns and beams. In general, the
inspection requirements from the specifications for the various contracts were at a minimum equivalent to
those in the 1968 NYC Building Code. The Code contains provisions that govern the fabrication and
inspection of materials used in buildings. However, in a number of cases, the contract requirements were
more comprehensive and stringent than the corresponding provisions in the Code The Code refers to the
requirements in the 1963 AISC

Specification for the Design, Fabrication, and Erection of Structural Steel

for Buildings

(AISC 1963). The AISC Specification contained minimum fabrication requirements for the

following:

•

Straightening of materials

•

Gas cutting

•

Planing of edges

•

Riveted and bolted construction – holes

•

Riveted and high strength bolted construction – assembling

•

Welded construction

•

Finishing

•

Tolerances

Specific inspection requirements during fabrication of various structural members were covered in the
contract documents between PONYA and individual fabricators.

WTC 7

The contract specification for WTC 7 required that structural steel for WTC 7 was to be fabricated in
accordance with the applicable requirements in the 1968 NYC Building Code, the 1963 AISC

Specification for the Design

Fabrication

and Erection of Structural Steel for Buildings

, and other

specifications related to bolts, welds, and painting. The specification also notes that there was a separate
contract for testing and inspection. This contract was not found. However, specific requirements for
inspection of shop and field welds by a testing agency were included in the specification.

E.7

INSPECTION PROTOCOL DURING CONSTRUCTION

WTC 1 and WTC 2

Karl Koch Erecting Co., the company that performed the structural steel erection work for WTC 1 and
WTC 2, developed a quality control and safety program. This program included information on ten
different key areas that were to be addressed during construction, including:

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xliii

•

Survey control

•

Control of construction and erection loads

•

Field welding

•

Bolting of structural steel

•

Control of stud welding operations

•

Erection procedures

•

Control of workmanship

•

Control of erection tolerances

•

As-built drawings

•

Safety programs

WTC 7

The WTC 7 specifications contained general erection requirements for fasteners, anchor bolts, column
bases, installation, and bracing. The specification did not include any requirements for inspection.

E.8

VARIANCES GRANTED BY PANYNJ

The Port Authority approved numerous variances in the fabrication and erection of structural members in
WTC 1 and WTC 2. The Office of the Construction Manager at the Port Authority approved variances to
the contract documents after the structural engineer of record; Skilling, Helle, Christiansen, and
Robertson (SHCR), reviewed the details of the variances and recommended approval. In many cases,
SHCR submitted alternative methods, which were incorporated into the variance.

The variances that were granted for the structural members and their materials may be categorized into
the following groups:

•

Variances relating to fabrication/erection tolerances (box columns, box beams, and floor
trusses)

•

Variances relating to defective components (column trees and floor trusses)

•

Variances relating to alternative fabrication/erection procedures (core columns, floor trusses,
exterior wall columns, and beam seats)

•

Variances relating to product substitutions (exterior wall)

•

Variances relating to inspection practice (exterior wall and welds).

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Fabrication and erection inspections identified many deviations from the contract drawings and
specifications. Many variance requests were based on inspection results.

E.9 STRUCTURAL

MAINTENANCE AND MODIFICATIONS DURING

OCCUPANCY

Both architectural and structural modifications were made to meet the occupancy needs of individual
tenants throughout the history of occupancy of WTC 1, 2, and 7. PONYA, later PANYNJ, reviewed all
modifications to maintain the structural integrity of the buildings and to ensure that modifications were
compatible with existing building conditions. In order to guide tenants in their modification process, the
PONYA issued

Tenant Alteration Review Manual

in 1971 and updated the manual periodically

through 1997.

In anticipation of structural degradation, the PANYNJ issued in 1986 the

Standard for Structural Integrity

Inspection of the World Trade Center Towers A & B

to guide periodic inspection of structural members.

Deteriorated and damaged members were identified for repair. The standard was used by consultants who
were retained by PANYNJ for systematic examination of WTC 1 and WTC 2.

In 1998, the PANYNJ issued the

Standards for Architectural and Structural Design

for modification

works. The standards included not only the design guide, but also included specifications and standard
details to be used in modification works. Tenants proposing any modifications were required to follow the
specified standards.

Apart from the repairs following the 1993 bombing of WTC 1, most of the structural modifications in
WTC 1 and WTC 2 were performed to accommodate tenant requirements. Openings were cut in existing
floors to construct new stairways linking two or more floors, and floor systems were reconstructed over
previously cut openings. In a number of cases, floor trusses outside of the core area and steel beams in
the core area had to be reinforced due to heavy loads imposed by tenant requirements. All such
modifications were reviewed and approved by the structural engineer of record (SHCR).

Similar to WTC 1 and WTC 2, most of the structural modifications in WTC 7 were done to accommodate
tenant requirements. Horizontal members of the floor framing system were strengthened due to increased
loading from high-density files. Strengthening of these beams and girders was achieved by welding cover
plates to the bottom flanges, the underside of the top flanges, or both. In some cases, new beams were
introduced to carry a portion of the new load.

Structural Integrity Inspection Program

In 1986, PANYNJ implemented an inspection program to detect, record, and correct any signs of distress,
deterioration, or deformation that could signal structural problems. This structural integrity inspection
program contained detailed guidelines on inspection, record-keeping, and follow-up procedures.

Inspection findings were to be categorized as “Immediate,” “Priority,” or “Routine.” Repairs falling into
the “Immediate” category included possible closure of the area and/or structure affected until interim
remedial action could be implemented. The “Priority” category was for those conditions where no
immediate action was required, or for which immediate action had been completed, but for which further

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investigation, design, and implementation of interim or long-term repairs were to be undertaken on a
priority basis (i.e., taking precedence over all other scheduled work). Repairs falling into the “Routine” or
“non-priority” category were to be undertaken as part of a scheduled major work program or other
scheduled project, or when routine facility maintenance was to be performed, depending on the type of
repair that was required. An important requirement in the inspection program was that where inspection
procedures involved the removal of fireproofing, such fireproofing was to be properly replaced on
completion of inspection.

In general, the structural integrity inspections findings indicated that the structural systems of WTC 1, 2,
and 7 were in good condition. The inspections resulted in numerous routine and some priority
recommendations for repairs, as outlined in the inspection standard. According to the PANYNJ, all of the
construction records on repairs following the inspections were lost on September 11, 2001. Thus, it
cannot be determined whether all of the recommended repairs were performed.

Repair Work Following the 1993 Explosion

The explosion of February 26, 1993, occurred on Level B2 near the center of the south wall of WTC 1
and adjacent to WTC 3 (Vista Hotel). Structural steel columns, diagonal braces, and spandrel beams in
the vicinity of the blast were damaged. Concrete floor slabs at Levels B1 and B2 and unreinforced
masonry walls were also damaged over a large area.

The explosion severely bent and tore out the diagonal brace between columns. Spandrel beams at level B1
were also damaged by the blast. A crack developed along the field splice in a column. Ultrasonic testing
determined that the crack extended across the full width of the weld on the south face of the column and
at each end of the weld on the north face. Magnetic-particle testing procedure determined that the crack
extended across the east face of the column. The explosion also damaged floor beams at levels B1 and
B2. Concrete spandrel beams at level B3 also sustained damage. Masonry walls in WTC 1 were breached
over distances of approximately 50 ft to the east and 120 ft to the west of the blast origin.

The diagonal bracing members between levels B1 and B2 that were damaged by the explosion were
removed and replaced with new members. New plates were added to the damaged spandrel beam at level
B1. Also, the cracked weld on the south face of the spandrel beam at level B1 was removed and replaced.

Six different inspections were performed before and after repairs were made to WTC 1. No anomalies
were detected in the welds used to repair structural members.

E.10

CODE PROVISIONS FOR DESIGN OF THE FIRE SAFETY AND EGRESS
SYSTEMS

The fire safety provisions of the 1968 NYC Building Code (NYCBC 1968) were compared with four
other building codes: the 1964 New York State Building Construction Code (NYSBC 1964), the 1965
BOCA Basic Building Code (BOCA/BBC 1965), the 1967 Municipal Code of Chicago Relating to
Buildings (MCC 1967), and the 2001 edition of the NYC Building Code (NYCBC 2001). In addition,
comparisons were made to the 1966 edition of National Fire Protection Association (NFPA) 101, Code
for Safety to Life in Buildings and Structures. While not a building code, NFPA 101 is widely adopted
for its requirements for life safety in fires.

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NIST NCSTAR 1-1, WTC Investigation

The NYC Building Code was regularly amended by local laws, two of which, Local Law 5 (1973) and
Local Law 16 (1984), had a significant influence on WTC 1 and WTC 2, even though the buildings were
completed and occupied at the time of adoption. It is normal practice not to apply building code changes
to existing buildings, but the Port Authority chose to follow the revised provisions and to retrofit the
buildings as required under the new provisions. The resulting changes to WTC 1 and WTC 2 are
discussed primarily in the sections on modifications to the building systems.

While New York City developed its own building code, their code development committees were
influenced by the same forces that bore on the model codes. Thus, there were relatively few differences
between the NYC Building Code and the others.

Construction Classification

In Construction Classifications, the 1968 Building Code, the New York State Building Code, and the
1965 BOCA all recognized Class 1A or Class 1B (with the same fire resistance ratings for building
elements) for most unsprinklered buildings of unlimited height, while the 1967 Chicago Code recognized
only 1A. New York City imposed a 75 ft height limit on unsprinklered buildings with the adoption of
Local Law 16 (1984).

Active Systems

At the time of construction, sprinklers were primarily for property protection and were rare even in
high-rise buildings (except for underground spaces). Fire alarm systems were mostly manually initiated
but there was concern about smoke being recirculated through the heating, ventilating, and air
conditioning (HVAC) systems, so smoke detectors controlled dampers at return shafts to prevent this.
This is the arrangement of the fire alarm system originally installed in the towers. Voice communication
systems were a response to phased evacuation with the recognition that it was necessary to provide
instructions to occupants who were relocated or held within the building at least until they were told to
leave. Requirements for voice systems first appeared in national standards in the mid-1980s, at the same
time as NYC adopted LL 16 (1984).

Technical Standards

All building codes rely on referenced technical standards to provide the details of design, installation,
operation, and maintenance of required systems. Most building codes reference national (consensus)
standards as published, but New York City cites their own reference standards that are based on these
national standards but are often highly modified. For example, fire alarm systems and fire sprinkler
systems are addressed in Reference Standard (RS) 17, with Class E fire alarm systems (required in office
occupancies) covered in RS 17-3A and general fire alarm system requirements in RS 17-5. The former is
entirely written by a NYC code committee, and the latter is based on NFPA 72 (National Fire Alarm
Code) but highly modified by the deletion of many sections and modification of many others. One major
modification is that RS 17 does not include the “Survivability” section for high-rise voice communication
systems that requires duplicate communication trunks so that loss on one trunk does not result in loss of
communication with a floor. However the voice communication system installed in WTC 1 and WTC 2
was consistent with the National Fire Alarm Code (NFPA 72) in addition to RS 17 and had redundant
trunks run in Stairways A and C.

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Egress Systems

Prior to 1988, all building codes determined egress capacity by the (22 in.) Units of Exit Width method,
which New York City still uses. In 1988, other codes changed to a method involving an allowance of
width per person, which provides credit for non-standard widths of corridors and doors, but for standard
dimensioned components yields the same results. Another difference in egress design is that New York
City applies the occupant load factor for business occupancies (100 ft

per person) to the net floor area

while other codes use the gross floor area. Other codes use net for some and gross for others. The NYC
Building Code allows doubling stair capacity allowances with one or tripling of the stair capacity on
floors with two or more horizontal exits where other codes only allow doubling for one horizontal exit
(see discussion of Windows on the World).

Miscellaneous Details

There are a number of detail differences between the NYC Building Code and the other building codes.
The NYC Building Code has no requirements for fire extinguishers since they require occupant hose
reels. The 1968 NYC Building Code was the first code to include smoke developed ratings for finish
materials in addition to flame spread. Now, all of the codes have similar requirements.

Specifications for the Original Buildings

As stated above the primary occupancy group was Group B (Business) with the Windows on the World
space in WTC 1 being Group F (Assembly). While there was a Port Authority cafeteria on the 44th floor,
employee cafeterias not open to the public are specifically exempted from assembly classification because
they do not increase occupant load and are only used intermittently. Incidental mercantile spaces such as
news stands and coffee bars at the concourse level are also exempt from reclassification in most building
codes.

The NYC Building Code and Port Authority practice required partitions to separate tenant spaces from
each other and from common spaces such as the corridors that served the elevators, stairs, and other
common spaces in the building core. Fire rated partitions are intended to limit fire spread on a floor, to
prevent spread of fire in one tenant space to that of another, Partitions separating tenant space from exit
access corridors were permitted to be 1 h, although the Port Authority specified them to be 2 h, allowing
dead ends to extend to 100 ft (rather than 50 ft with 1 h partitions), which permitted more flexibility in
tenant layouts. Partitions separating tenant spaces (so-called demising walls) were required to be 1 h (see
Sec. 10.4.5). Enclosures for vertical shafts, including stairways and transfer corridors, elevator hoistways,
and mechanical or utility shafts were required to be of 2 h fire rated construction. Protection of vertical
shafts is intended to limit the spread of fire and smoke from floor to floor.

The primary egress system for the office spaces was the three stairways located in the building core.
These included two 44 in. (designated A and C) and one 56 in. wide (designated B) stairs which provided
exactly the code required capacity for an occupant load of 390 per floor (39,000 ft

net at 100 ft

per

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person). The layout within the building core was consistent with the Building Code requirements for
maximum travel distance (200 ft unsprinklered, 300 ft sprinklered) and, while the separation was
consistent with New York City requirements (15 ft and later 30 ft), it was short of the more common
requirements found in all current building codes (one-half the diagonal of the space served if
unsprinklered, or one-third the diagonal if sprinklered) on some of the floors where the transfer corridors
brought the stair access closer together.

There were 99 passenger elevators in each tower, arranged in three vertical zones to move occupants in
stages to skylobbies on the 44th and 78th floors. These were arranged as express (generally larger cars
that moved at higher speeds) and local elevators in an innovative system first introduced in WTC 1 and
WTC 2. There were 8 express elevators from the concourse to the 44th floor and 10 express elevators
from the concourse to the 78th floor as well as 24 local elevators per zone, which served groups of floors
in those zones. There were seven freight elevators, only one of which served all floors. All elevators had
been upgraded to incorporate firefighter emergency operation per American Society of Mechanical
Engineers (ASME) A17.1 and Local Law 5 (1973).

Consistent with practice at the time, the original fire alarm system in WTC 1 and WTC 2 was a manual
system with four smoke detectors on each tenant floor, positioned to monitor for smoke entering the
HVAC returns and arranged to stop the fans to prevent smoke circulation to non-fire areas. Local Law 5
(1973) included retroactive requirements for fire alarm systems and emergency voice communication
systems in business occupancies over 100 ft in height. Subsequently, such systems were installed in
WTC 1 and WTC 2 with the required fire command center located in the underground parking garage,
where it was destroyed by the blast in the 1993 bombing, rendering most fire safety features inoperable.
Following the 1993 bombing, the fire command stations were relocated to the tower building lobbies,
with a third monitoring location in the Port Authority offices. The lobby location (within sight of the
elevators) is specified in the NYC Building Code for fire command centers required in high-rise
buildings. There are no code requirements for off-site monitoring of fire alarm systems in this occupancy.

Modifications to the Fire and Life Safety Systems

The general practice is that buildings are governed by the building code in force at the time the building
permits are issued except in the rare case of the adoption of retroactive requirements. Local Laws 5
(1973) and 16 (1984) were adopted after completion of WTC 1 and WTC 2 but did contain some
retroactive provisions. However, the Port Authority chose to implement virtually all of the provisions of
LL 5/73 and LL 16/84, which drove most of the modifications to the fire and life safety systems that
occurred over the life of the buildings. These modifications included the complete sprinklering of the
buildings and several upgrades to the fire alarm system.

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was adopted, which required sprinklers in new high-rise buildings, including offices. Under Local
Law 16 (1984) all floor spaces had to either be subdivided in accordance with the compartmentation
requirement or sprinklered by February 8, 1988. A 1997 report states that there were four floors and the
skylobbies (all in WTC 1) left to be sprinklered and that the installation of sprinklers at these floors was
underway (Coty 1997). An October 1999 report states that sprinklering of the tenant floors was
completed and sprinklering of the skylobbies was “currently underway” (PANYNJ 1999).

Issues identified after completion of the buildings that were not related to amendments to the NYC
Building Code that were addressed during the occupancy included the extension of the tenant separation
walls to run slab to slab, upgrading of the fireproofing to 1½ in. on the floor trusses, and correction of the
egress deficiencies for Windows on the World by creating three areas of refuge on each floor with 2 h
separations, each including a stair. These issues were identified through various independent reviews
conducted by PANYNJ and contractors hired by PANYNJ to conduct “due diligence” surveys. One
example was the surveys conducted in 1996 by Rolf Jensen and Associates and Jaros, Gaum & Bolles
which identified inconsistencies with the code and programs to address them, which are discussed in this
report in detail.

Innovations in Fire and Life Safety Features

Little about the towers’ fire and life safety features would be considered novel or innovative. The fire
alarm systems as originally provided and as upgraded over the life of the buildings were of high quality
and state-of-the-art, but followed accepted practice as it evolved in those years. Similarly, the fire
sprinkler system was high quality and state-of-the-art, following accepted practice with a few features
following New York City practice that differed from the rest of the nation. This included manually
operated fire pumps with a so called “standpipe telephone system” to communicate with the pump
operator. Most codes and standards specify automatic fire pumps.

Two features that were novel (and thus innovative) were the use of lightweight trusses in the floor system
with fire protection of spray applied material on steel bars (rather than angles). Another was the shaft
enclosure system of reinforced gypsum planks with applied steel channels that formed the framing.
While gypsum shaft enclosure systems are now common, this particular arrangement was not used before
or since.

Fuel System for Emergency Generators in WTC 7

Several of the tenants in WTC 7 installed generators to supply critical operations with continuous power.
These generators were installed on several floors within the building (5, 7, 8, and 9) and fed from small
(275 gal) “day tanks” near the generators. These day tanks were kept full by an automatic system of
piping running to primary storage tanks (24,000 gal) located under the loading dock or a 6,000 gal tank in
a 1st floor storage room associated with the generators for the Mayor’s Office of Emergency Management
on the 7th floor. Details of the system design and installation are found in NIST NCSTAR 1-1J.

This reference is to one of the companion documents from this Investigation. A list of these documents appears in the Preface

to this report.

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E.11 FINDINGS

The findings of this report are grouped into three categories: (1) general; (2) factors related to structural
safety; and (3) factors related to fire safety.

E.11.1 General

Finding 1:

The NYC Department of Buildings reviewed the WTC tower drawings in 1968 and provided

comments to the PANYNJ concerning the plans in relation to the 1938 NYC Building Code. The
architect-of-record submitted to the PANYNJ responses to those comments, noting how the drawings
conformed to the 1968 NYC Building Code. All six comments made by the NYC Department of
Buildings dealt with egress issues, but none questioned the large occupant loads for Windows on the
World in WTC 1 or Top of the World in WTC 2.

Finding 2:

In 1993, the PANYNJ and the NYC Department of Buildings entered into a memorandum of

understanding that restated the PANYNJ’s longstanding stated policy to ensure that its facilities in the
City of New York meet and, where appropriate, exceed the requirements of the NYC Building Code. The
agreement also provided specific commitments to the NYC Department of Buildings regarding
procedures to be undertaken by the PANYNJ to ensure that buildings owned or operated by the PANYNJ
are in conformance with the Building Standards contained in the NYC Building Code. Some salient
points included in this agreement and the 1995 enhancement to the agreement are:

•

Each project would be reviewed and examined for compliance with the Code.

•

All plans would be prepared, sealed, and reviewed by New York State licensed professional
engineers or architects.

•

The PANYNJ engineer or architect approving the plans would be licensed in the State of
New York and would not have assisted in the preparation of the plans.

•

The person or firm performing the review and certification of plans for WTC tenants may be
the same person or firm providing certification that the project had been constructed in
accordance with the plans and specifications unless the proposed alteration would “change
the character of the occupancy group under paragraph 27-237 of the NYC Building Code
which would have been applicable to such space had such space been located in a privately
owned building.”

•

Variances from the Code, acceptable to the PANYNJ, would be submitted to the
NYC Department of Buildings for review and concurrence. Disagreements between the
PANYNJ and the NYC Department of Buildings over such variances from the Code would
be referred to the Port Authority Board of Commissioners for resolution.

Finding 3:

While the PANYNJ entered into agreements with the NYC Department of Buildings in the

1990s with regard to conformance of PANYNJ buildings constructed in New York City to the
NYC Building Code and sought review and concurrence as required by the agreements, the PANYNJ was
not required to yield, and appears not have yielded, approval authority to New York City. The PANYNJ
was created as an interstate entity “body corporate and politic,” under its charter, pursuant to Article 1

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Section 10 of the U.S. Constitution permitting compacts between states, and like many other
nongovernmental and quasi-governmental entities in the United States is not subject to building and fire
safety code requirements of any governmental jurisdiction.

Finding 4:

State and local jurisdictions do not require retention of documents related to the design,

construction, operation, maintenance, and modifications of buildings, with few exceptions. These
documents are in the possession of building owners, contractors, architects, engineers, and consultants.
Such documents are not archived for more than about 6 years to 7 years, and there are no requirements
that they be kept in safe custody physically remote from the building throughout its service life. In the
case of the WTC towers, the PANYNJ and its contractors and consultants maintained an unusually
comprehensive set of documents, a significant portion of which had not been destroyed in the collapse of
the buildings but could be assembled and provided to the investigation. In the case of WTC 7, several
key documents could not be reviewed since they were lost in the collapse of the building.

Finding 5:

Consistent with the practice at the time the (code) architect was responsible for specifying the

fire protection and designing the egress system in accordance with the prescriptive provisions of the
Building Code. The architect and owner engaged the services of structural engineers to perform the
structural design and to ensure that his/her design was properly implemented. At that time the fire
protection engineering profession was not sufficiently mature to require the same standard of care
employed with the structural design. There is no reason to believe that the involvement of a fire
protection engineer at that time would have resulted in any differences in the design or performance of the
fire protection systems. However, the technical base and sophistication of the practice of fire protection
engineering today is well advanced of where it was then. Today, particularly when designing a building
employing innovative features, the involvement of a fire protection engineer in a role similar to the
structural engineer, and under the overall coordination of the Design Professional in Responsible Charge
is central to the standard of care. Further, when designing the structure of selected tall buildings or
selected other buildings to resist fires, or evaluating the fire resistance of such structures, it is essential for
the structural engineer and the fire protection engineer to jointly provide the needed standard of care.

E.11.2 Structural

Safety

Applicable Building Codes

Finding 6:

Although not required to conform to New York City codes, the PANYNJ adopted the

provisions of the proposed 1968 edition of the NYC Building Code, more than 3 years before it went into
effect. The proposed 1968 edition allowed the PANYNJ to take advantage of less restrictive provisions
and of technological advances compared with the 1938 edition, which was in effect when design began
for the WTC towers in 1962. The 1968 code:

•

Changed partition loads from 20 psf to one based on weight of partitions per unit length (that
reduced such loads for many buildings including the WTC buildings); and

•

Permitted wind tunnel tests using models to establish design values for the wind load.

Many of these newer requirements, instituted in the 1968 NYC Building Code, are contained in current
model codes and building regulations.

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NIST NCSTAR 1-1, WTC Investigation

Structural Integrity

Finding 7:

Building codes lack

explicit structural integrity provisions to mitigate progressive collapse.

Federal agencies have developed guidelines to mitigate progressive collapse and routinely incorporate
such requirements in the construction of new federal buildings. The United Kingdom incorporates such
code requirements for all buildings. New York City adopted by rule in 1973 a requirement for buildings
to resist progressive collapse under extreme local loads. The rules, which were adopted after the
WTC towers were built but before WTC 7 was built, applied specifically to buildings that used precast
concrete wall panels and not to other types of buildings.

Finding 8:

Building codes lack minimum structural integrity provisions for the means of egress

(stairwells and elevator shafts) in the building core that are critical to life safety. In most tall buildings the
core is designed to be part of the vertical gravity load carrying system of the structure. However, in many
of those buildings, especially in regions where earthquakes are not dominant, the core may not be part of
the lateral load carrying system of the structure. Thus, the core may be designed to carry only vertical
gravity loads with no capacity to resist lateral loads, i.e., overturning moment and shear loads. In such
situations, the structural designer may prefer the use of partition walls over structural walls in the core
area to reduce building weight. The decision to have the core carry a specified fraction of the lateral
design loads or be made part of a dual system to carry lateral loads, each of which would enhance the
structural integrity of the core if structural walls were used, is left to the discretion of the structural
engineer. Alternatively, stairway/elevator cores built with concrete or reinforced concrete block, which
are not part of the lateral load carrying system, may be able to provide sufficient structural integrity if
they meet, for example, ASTM E1996-03, or other more appropriate test for impact resistance. In the
case of the WTC towers, the core had 2 h fire-rated partition walls with little structural integrity and the
core framing was required to carry only gravity loads. Had there been a minimum structural integrity
requirement to satisfy normal building and fire safety considerations, it is conceivable that the damage to
stairways, especially above the floors of impact, may have been less extensive.

Finding 9:

Standards and code provisions for conducting wind tunnel tests and for the methods used in

practice to estimate design wind loads from test results do not exist. Building codes allow the
determination of wind pressures from wind tunnel tests for use in design. Such tests are frequently used
in the design of tall buildings. Results of two sets of wind tunnel tests conducted for the WTC towers in
2002 by independent commercial laboratories as part of insurance litigation, and voluntarily provided to
NIST by the parties to the litigation, show large differences, of as much as about 40 percent, in resultant
forces on the structures, i.e., overturning moments and base shears. Independent reviews by a NIST
expert on wind effects on structures and a leading engineering design firm contracted by NIST indicated
that the documentation of the test results did not provide sufficient basis to reconcile the differences.
Wind loads were a major governing factor in the design of structural components that made up the frame-
tube steel framing system.

E.11.3 Fire

Safety

Applicable Building Codes

Finding 10:

Although not required to conform to New York City codes, the PANYNJ adopted the

provisions of the proposed 1968 edition of the NYC Building Code, more than 3 years before it went into

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effect. The 1968 edition allowed the PANYNJ to take advantage of less restrictive provisions compared
with the 1938 edition that was in effect when design began for the WTC towers in 1962. The 1968 code:

•

Eliminated a fire tower

as a required means of fire department access;

•

Reduced the number of required stairwells from 6 to 3 and the size of doors leading to the
stairs from 44 in. to 36 in.(by increasing stairway and door capacity allowances);

•

Reduced the required fire rating of the shaft walls in the building core from 3 h to 2 h; and

•

Permitted a 1 h reduction in fire rating for all structural components (columns from 4 h to 3 h
and floor framing members from 3 h to 2 h) by allowing the owner/architect to select Class
1B construction for business occupancy and unlimited building height.

Many of these newer requirements, instituted in the 1968 NYC Building Code, are contained in current
codes.

Finding 11:

In 1993, the PANYNJ adopted a policy providing for implementation of fire safety

recommendations made by local government fire departments after a fire safety inspection of a PANYNJ
facility and for the prior review by local fire safety agencies of fire safety systems to be introduced or
added to a facility. Later that year, the PANYNJ entered into an agreement with the New York City Fire
Department (FDNY), which reiterated the policy adopted by the PANYNJ, recognized the right of FDNY
to conduct fire safety inspections of PANYNJ properties in the City of New York, provided guidelines for
FDNY to communicate needed corrective actions to the PANYNJ, ensured that new or modified fire
safety systems are in compliance with local codes and regulations, and required third-party review of such
systems by a New York State licensed architect or engineer.

Standard Fire-Resistance Tests

Finding 12:

Code provisions with detailed procedures to analyze and evaluate data from fire resistance

tests of other building components and assemblies to qualify an untested building element do not exist.
Based on available data and records, no technical basis has been found for selecting the SFRM used (two
competing materials were under evaluation) or its thickness for the large-span open-web floor trusses of
the WTC towers. The assessment of the fireproofing thickness needed to meet the 2 h fire rating
requirement for the untested WTC floor system evolved over time:

•

In October 1969, the PANYNJ directed the fireproofing contractor to apply ½ in. of
fireproofing to the floor trusses.

•

In 1999, the PANYNJ issued guidelines requiring that fireproofing be upgraded to 1½ in. for
full floors undergoing alterations.

A fire tower (also called a smoke-proof stair) is a stairway that is accessed through an enclosed vestibule that is open to the

outside or to an open ventilation shaft providing natural ventilation that prevents any accumulation of smoke without the need
for mechanical pressurization.

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NIST NCSTAR 1-1, WTC Investigation

•

Unrelated to the WTC buildings, an International Conference of Building Officials
Evaluation Service report (ER-1244), re-issued June 1, 2001, using the same SFRM
recommends a minimum thickness of 2 in. for “unrestrained steel joists” with “lightweight
concrete” slab.

Finding 13:

Code provisions that require the conduct of a fire resistance test if adequate data do not exist

from other building components and assemblies to qualify an untested building element are needed.
Instead, several alternate methods based on other fire-resistance designs or calculations or alternative
protection methods are permitted with limited guidance on detailed procedures to be followed. Both the
architect-of-record (in 1966) and the structural-engineer-of-record (in 1975) stated that the fire rating of
the floor system of the WTC towers could not be determined without testing. NIST has not found
evidence indicating that such a test was conducted to determine the fire rating of the WTC floor system.
The PANYNJ has informed NIST that there are no such test records in its files.

Finding 14:

Use of the “structural frame” approach, in conjunction with the prescriptive fire rating,

would have required the floor trusses, the core floor framing, and perimeter spandrels in the WTC towers
to be 3 h fire-rated, like the columns for Class 1B construction in the 1968 NYC Building Code. Neither
the 1968 edition of the NYC Building Code which was used in the design of the WTC towers, nor the
2001 edition of the code, adopted the “structural frame” requirement. The “structural frame” approach to
fire resistance ratings requires structural members, other than columns, that are essential to the stability of
the building as a whole to be fire protected to the same rating as columns. This approach, which appeared
in the Uniform Building Code (a model building code) as early as 1953, was carried into the
2000 International Building Code (one of two current model codes) which states: “The structural frame
shall be considered to be the columns and the girders, beams, trusses and spandrels having direct
connections to the columns and bracing members designed to carry gravity loads.” The WTC floor
system was essential to the stability of the building as a whole since it provided lateral stability to the
columns and diaphragm action to distribute wind loads to the columns of the frame-tube system.

Finding 15:

A technical basis to establish whether the construction classification and fire rating

requirements in modern building codes are risk-consistent with respect to the design-basis hazard and the
consequences of that hazard is needed. The fire rating requirements, which were originally developed
based on experience with buildings less than about 20 stories in height, have generally decreased over the
past 80 years since historical fire data for buildings suggested considerable conservatism in those
requirements. However, for tall buildings, the likely consequences of a given threat to an occupant on the
upper floors are more severe than the consequences to an occupant, say, on the first floor. It is not
apparent how the current height and area tables in building codes consider the technical basis for the
progressively increasing risk to an occupant on the upper floors of tall buildings that are much greater
than about 20 stories in height where access by firefighters without the availability of firefighter elevators
is limited by physiological factors. The maximum required fire rating in current codes applies to any
building more than about 12 stories in height. There are no additional categories for buildings above, for
example, 40 stories and 80 stories, where different building classification and fire ratings requirements
may be appropriate, recognizing factors such as the time required for stairwell evacuation without
functioning elevators (e.g., due to power failure or major water leakage), the time required for first
responder access without functioning elevators, the presence of skylobbies and/or refuge floors, and
limitations on the height of elevator shafts. The 110-story WTC towers, initially classified as Class IA
based on the 1938 NYC Building Code, were classified as Class 1B before being built to take advantage

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of the provisions in the 1968 edition of the code. This re-classification permitted a reduction of 1 h in the
fire rating of the components (columns from 4 h to 3 h and floor framing members from 3 h to 2 h).

Fire Performance of Structures

Finding 16:

Rigorous field application and inspection provisions and regulatory requirements to ensure

that the as-built condition of the passive fire protection, such as SFRM, conforms to conditions found in
fire resistance tests of building components and assemblies is needed. For example, provisions are not
available to ensure that the as-applied average fireproofing thickness and variability (reflecting the quality
of application) is thermally equivalent to the specified minimum fireproofing thickness. In addition,
requirements are not available for in-service inspections of passive fire protection during the life of the
building. The adequacy of the fireproofing of the WTC towers posed an issue of some concern to the
PANYNJ over the life of the buildings, and the availability of accepted requirements and procedures for
conducting in-service inspections would have provided useful guidance

Finding 17:

Structural design does not consider fire as a design condition, as it does the effects of dead

loads, live loads, wind loads, and earthquake loads. Current prescriptive code provisions for determining
fire resistance of structures—used in the design of the WTC towers and WTC 7—are based on tests using
a standard fire that may be adequate for many simple structures and for comparing the relative
performance of structural components in more complex structures. A building system with 3 h rated
columns and 2 h rated girders and floors could last longer than 3 h or shorter than 2 h depending upon the
performance of the structure as a 3-dimensional system in a real fire. The standard tests cannot be used to
evaluate the actual performance (i.e., load carrying capacity) in a real fire of the structural component, or
the structure as a whole system, including the connections between components. Performance-based code
provisions and standards are not available for use by engineers, as an alternative to the current
prescriptive fire rating approach, to (1) evaluate the system performance of tall-building structures under
real fire scenarios, and (2) enable risk consistent design with appropriate thickness of passive protection
being provided where it is needed on the structure. Standards development organizations, including the
American Institute of Steel Construction, have initiated development of performance-based provisions to
consider fire effects in structural design.

Finding 18:

Detailed procedures to select appropriate design-basis fire scenarios to be considered in the

performance-based design of the sprinkler system, compartmentation, and passive protection of the
structure are needed. The standard fire in current prescriptive fire resistance tests is not adequate for use
in performance-based design. While the NFPA 5000 model building code contains general guidance on
design fire scenarios (the IBC Performance Code contains no such guidance), the details of the scenarios
are left to the fire engineer and regulatory official. The three major scenarios that are not considered
adequately are: frequent but low severity events (for design of sprinkler system), moderate but less
frequent events (for design of compartmentation), and a maximum credible fire (for design of passive fire
protection on the structure). The maximum credible fire scenario for passive protection of structures
would assume that the sprinkler system is compromised or overwhelmed and that there is no active
firefighting, as is explicitly considered for U.S. Department of Energy facilities. These building-specific
representative fire scenarios are similar in concept, though not identical, to the approach used in building
design where the performance objectives and design-basis of the hazard are better defined (e.g., a two-
level design that includes an operational event with a 10 percent probability of occurrence in 50 years and
a life safety event with a 2 percent probability of occurrence in 50 years). The design-basis fire hazards

Executive Summary

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NIST NCSTAR 1-1, WTC Investigation

for the WTC towers and WTC 7 are unknown, and it is difficult to evaluate the performance of the fire
protection systems in these buildings under specific fire scenarios.

Finding 19:

Code provisions to ensure that structural connections are provided the same degree of fire

protection as the more restrictive protection of the connected elements are needed. The provisions that
were used for the WTC towers and WTC 7 did not require specification of a fire-rating requirement for
connections separate from those for the connected elements. It is not clear what the fire rating of the
connections were when the connecting elements had different fire ratings and whether the applied
fireproofing achieved that rating.

Finding 20:

A technical basis to establish whether the minimum mechanical and durability related

properties of SFRM are sufficient to ensure acceptable in-service performance in buildings is needed.
While minimum bond strength requirements exist, there are no serviceability requirements for such
materials to withstand typical shock, impact, vibration, or abrasion effects over the life of a building.
There are existing testing standards for determining many of these properties, but the technical basis is
insufficient to establish serviceability requirements. Knowledge of such serviceability requirements is
relevant to determine the post-impact fireproofing condition of the WTC towers.

Finding 21:

Validated and verified tools for use in performance-based design practice to analyze the

dynamics of building fires and their effects on the structural system that would allow engineers to
evaluate structural performance under alternative fire scenarios and fire protection strategies are needed.
Existing tools are either too simplified to adequately capture the performance of interest or too complex
and computationally demanding and lack adequate validation. While considerable progress has been
made in recent years, significant work remains to be done before adequate tools are available for use in
routine practice. NIST has had to further develop and validate existing tools to investigate the fire
performance of the WTC towers and WTC 7.

Compartmentation and Sprinklers

Finding 22

: Building fire protection is based on a four-level hierarchical strategy comprising detection,

suppression (sprinklers and firefighting), compartmentation, and passive protection of the structure.

•

Detectors are typically used to activate fire alarms and notify building occupants and
emergency services.

•

Sprinklers are designed to control small and medium fires and to prevent fire spread beyond
the typical water supply design area of about 1,500 ft

•

Compartmentation mitigates the horizontal spread of more severe but less frequent fires and
typically requires fire-rated partitions for areas of about 7,500 ft

. Active firefighting

measures also cover up to about 5,000 ft

to 7,500 ft

•

Passive protection of the structure seeks to ensure that a maximum credible fire scenario, with
sprinklers compromised or overwhelmed and no active firefighting, results in burnout, not
overall building collapse. The intent of building codes is also for the building to withstand
local structural collapse until occupants can escape and the fire service can complete search
and rescue operations.

Draft for Public Comment

Executive Summary

NIST NCSTAR 1-1, WTC Investigation

lvii

Compartmentation of spaces is a key building fire safety requirement to limit fire spread. The WTC
towers initially had 1 h fire-rated partitions separating tenants (demising walls) that extended from the
floor to the suspended ceiling, not the floor above (the ceiling tiles were not fire rated). Over the years,
these partitions were replaced with partitions that were continuous from floor to floor (separation wall) as
required by the 1968 NYC Building Code. Some partitions had not been upgraded by 1997, and a
consultant recommended to the PANYNJ that it develop and implement a survey program to ensure that
the remediation process occurred as quickly as possible. It appears that with few exceptions, nearly all of
the floors not upgraded were occupied by a single tenant, and it is not clear whether separation walls
would have mattered in terms of meeting the 1968 code. The PANYNJ adopted guidelines in 1998 that
required such partitions to provide a continuous fire barrier from top of floor to underside of slab.

Finding 23:

Building codes typically require 1 h fire-rated tenant separations but do not impose minimum

compartmentation requirements (e.g., 13,000 ft

) for buildings with large open floor plans to mitigate the

horizontal spread of fire. This is the case with both the 1968 NYC Building Code, which did not require
sprinklers in occupied spaces on or above the ground floor, and the 2001 NYC Building Code, which
requires sprinklers in Group E (Business) buildings over 100 ft in height. The sprinkler option was
chosen for the WTC towers in preference to the compartmentation option in meeting the subsequent
requirements of Local Law 5 adopted by New York City in 1973. Thus, if there was only one tenant on a
WTC floor there would be no horizontal compartmentation requirement. Conversely, if there were a
large number of tenants on a WTC floor, it would be highly compartmented with separation walls. The
affected floors in the WTC towers were mostly open—with a modest number of perimeter offices and
conference rooms and an occasional special purpose area. Some floors had two tenants and those spaces,
like the core areas, were partitioned (slab to slab). Photographic and videographic evidence confirms that
even non-tenant space partitions (such as those that divided spaces to provide corner conference rooms)
provided substantial resistance to fire spread in the affected floors. For the duration of about 50 to 100
min prior to collapse of the WTC towers that the fires were active, the presence of undamaged 1 h fire-
rated compartments may have assisted in mitigating fire spread and consequent thermal weakening of
structural components.

Finding 24:

State and local building regulations are needed that require installation of sprinklers in

existing buildings on a reasonable time schedule, not as an option in lieu of compartmentation.
Functioning sprinklers can provide significant improvement in safety for most common building fires and
prevent them from becoming large fires. NYC promulgated local laws in 1973 and 1984 to encourage
installation of sprinklers in new buildings and is now considering a law to require sprinklers in existing
buildings. The WTC towers were fully sprinklered by 2001, about 30 years after their construction.
Sprinklering of the tenant floors in the WTC towers was completed by October 1999, while sprinklering
of the skylobbies was still underway at that time. The sprinkler system was installed in three phases.
Phase 1 was completed during initial building construction and included the sub-grade areas. Phase 2 was
completed in 1976, in compliance with Local Law 5, and included sprinklering the corridors, storage
rooms, lobbies, and certain tenant spaces. Phase 3 was begun in 1983 and completed in 2001 and resulted
in fully sprinklering the buildings.

Finding 25:

Modern building codes allow a lower fire rating for structural elements when a building is

sprinklered. This trade-off provides an economic incentive to encourage installation of sprinklers.
Sprinklers provide better intervention against small and medium fires, fires which are more likely to occur
than a WTC disaster, as long as the water supply is not compromised and there is redundant technology in

Executive Summary

Draft for Public Comment

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NIST NCSTAR 1-1, WTC Investigation

place. The required technical basis is not available to establish whether the “sprinkler trade-off” in
current codes adequately considers fire safety risk factors such as: (1) the complementary functions of
sprinklers and fire-protected structural elements, (2) the different fire scenarios for which each system is
designed to provide protection, and (3) the need for redundancy should one system fail. It is noteworthy
that the British Standards Institution has established a group to review all the sprinkler trade-offs
contained in their standards. No such formal review has yet been initiated in the United States. Although
the classification and fire rating of the WTC towers did not take advantage of the sprinkler-tradeoff since
such provisions were not contained in the 1968 NYC Building Code, had such provisions existed, they
would have permitted a lower fire rating for many WTC building elements.

Use of Elevators in Emergencies

Finding 26:

With a few special exceptions, building codes in the United States do not permit the use of

fire-protected elevators for routine emergency access by first responders or as a secondary method (after
stairwells) for emergency evacuation of building occupants. The use of elevators by first responders
would additionally mitigate counterflow problems in stairwells. While the United States conducted
research on specially protected elevators in the late 1970s, the United Kingdom along with several other
countries that typically utilize British standards have required such “firefighter lifts,” located in protected
shafts, for a number of years. Without functioning elevators (e.g., due to a power failure or major water
leakage), first responders carrying gear typically require about a minute per floor to reach an incident
using the stairs. While it is difficult to maintain this pace for more than about the first 20 stories, it would
take a first responder about an hour to reach, for example, the 60th floor of a tall building if that pace
could be maintained. Such a delay, combined with the resulting fatigue and physical effects on first
responders that were reported on September 11, 2001, would make firefighting and rescue efforts difficult
even in tall building emergencies not involving a terrorist attack. Each of the WTC towers had 106
elevators, and WTC 7 had 38 elevators. By code, the elevators could not be used for fire service access or
occupant egress during an emergency since they were not fire-protected, nor were they located in
protected shafts. The elevators were equipped through normal modernization with fire service recall.
Most were damaged by the aircraft impacts; though prior to the impact in WTC 2 the elevators were
functioning and contributed greatly to the much faster initial evacuation rate in WTC 2.

NIST NCSTAR 1-1, WTC Investigation

Chapter 1

NTRODUCTION

1.1 BACKGROUND

On September 11, 2001, the 110-story twin towers of the World Trade Center (WTC) complex

(WTC 1

and WTC 2) were each attacked by a hijacked Boeing 767 airplane. The first airplane struck WTC 1 at
8:46 a.m. Eastern Daylight Time, and the second airplane struck WTC 2 at 9:03 a.m. Eastern Daylight
Time. The impact of the airplanes caused severe damage to the buildings and significant fire. WTC 1
collapsed at 10:29 a.m. and WTC 2 at 9:59 a.m. Debris from the collapse of the towers caused severe
damage to surrounding buildings of the WTC complex (WTC 3 through WTC 7). WTC 7, a 47-story
office building, burned unattended for about 7 h before collapsing at 5:20 p.m.

As stated in the Preface, the National Institute of Standards and Technology (NIST) Investigation is
comprised of eight interdependent projects (refer to Table P–1). This report presents the results of
Project 1 “Analysis of Building and Fire Codes and Practices.” The project was carried out to support
one of the four primary objectives of the NIST Investigation, which is to determine the procedures and
practices that were used in the design, construction, operation, and maintenance of the WTC 1, 2, and 7
(for other objectives, see the Preface). This report documents criteria used to design and construct the
buildings and maintenance of the structural and fire safety systems. It also addresses innovative systems
and materials that were incorporated into the design and construction process. Based on this information,
NIST has identified procedures and practices for which improvements are recommended.

1.2

SCOPE OF REPORT

The assessment of the criteria, procedures, and practices that were used in the design, construction,
operation, and maintenance of WTC 1, 2, and 7 involved reviewing the design and construction
documents of these buildings, including design drawings, specifications, and design calculations. In
addition, since the 1968 New York City (NYC) Building Code was adopted by the Port of New York
Authority (whose name was changed to the Port Authority of New York and New Jersey [PANYNJ or
Port Authority] in 1972, and is subsequently referred to as the Port Authority herein) for design and
construction of the WTC 1, 2, and 7, review of relevant provisions of that code and similar provisions of
other contemporaneous codes was necessary to place in context the design and construction practices that
were used for WTC 1, 2, and 7.

Traditionally, owners and designers of major construction projects maintain the design and construction
documents. In the case of the WTC buildings, the design and construction documents that were kept at
the Port Authority office in WTC 1 were destroyed when the tower collapsed. Thus, available copies of
design and construction documents of WTC 1, 2, and 7 had to be assembled from various sources that
were associated with the WTC projects.

The WTC complex was composed of seven buildings. They are referred to as WTC 1 through WTC 7 in this report. For

specific details, see Sec. 2.1.

Chapter 1

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

NIST obtained a considerable amount of information (design drawings, shop drawings, specifications,
project correspondence, and inspection reports) related to WTC 1 and WTC 2 from the structural
engineering firm involved in the original design and subsequent modifications to WTC 1 and WTC 2.
The Port Authority provided construction related files for WTC 1, 2, and 7, mostly pertaining to tenant
alteration projects, wherein tenants modified parts of the buildings to meet their needs. No document was
obtained from the general contractor of WTC 1, 2, and 7 who had discarded the construction documents
after retaining them for about 7 years. As a result, records were not available from the general contractor
pertaining to changes to the structural and fire safety systems that were made during construction.

The information collected enabled NIST to document the following:

•

Factors related to the design and construction of structural systems:

−

Provisions used to design and construct the buildings.

−

Criteria used to proportion structural members and other components of the buildings,
including structural connections.

−

Innovative systems, technologies, and materials that were incorporated in the design and
construction.

−

Tests performed to support the design, such as wind tunnel tests and tests of structural
assemblies.

−

Variances granted by the Port Authority, including the justification for those variances.

−

Special fabrication and inspection requirements.

−

Inspection protocols used during construction.

−

Technical problems that occurred during construction of the buildings and their
resolution.

•

Comparison of the structural provisions in the 1968 New York City (NYC) Building Code
with other contemporaneous code provisions:

−

Differences between the 1968 NYC Building Code and the contemporaneous building
codes of New York State, Chicago, and Building Officials Conference of America
(BOCA), and the 2001 NYC Building Code.

•

Maintenance of and modifications to the structural system:

−

Guidelines used by the Port Authority for inspection, repair, and modifications.

−

Structural integrity inspection programs during the occupancy of the buildings.

−

Any significant modifications to and/or repairs of the original structural framing system
by the owner or tenants during original construction and occupancy.

Draft for Public Comment

Introduction

NIST NCSTAR 1-1, WTC Investigation

•

Factors related to the design and construction of the fire protection and egress systems:

−

Provisions used to design and construct the fire protection and egress systems of the
buildings.

−

Building regulations adopted after the issuance of the certificates of occupancy that were
applied to the buildings retroactively, including any provisions of New York City Local
Laws, and any permits issued or special inspections required resulting from the
installation of special hazards or equipment in the buildings.

•

Comparison of the fire safety provisions in the 1968 NYC Building Code with other
contemporaneous code provisions:

−

Differences between the 1968 NYC Building Code and the contemporaneous building
codes of New York State, Chicago, and Building Officials Conference of America
(BOCA), and the 2001 NYC Building Code.

−

Evolution of the life safety provisions in the NYC Building Code since the design of
WTC 1 and WTC 2.

•

Maintenance of and modifications to the fire protection and egress systems:

−

Guidelines used by the Port Authority for inspection, repair, and modifications to fire
protection and egress systems.

−

Any repairs and modifications made to the passive and active fire protection systems
from initial occupancy to September 11, 2001.

•

The fuel system for emergency power in WTC 7 to determine:

−

Locations of emergency power generating systems.

−

Size and locations of the fuel storage tanks and distribution systems.

−

Specific fire protection systems used for the fuel storage and distribution systems.

−

Normal and emergency operating procedures.

−

Maintenance history.

This report provides an overview and comparison of building codes in use at the time when WTC 1, 2,
and 7 were designed and constructed. It includes a description of the buildings as designed and relates
features of the buildings to the code requirements and accepted practices of the time. Also presented is
the evolution of codes during the time the buildings were in use and a description of how the buildings
were modified and upgraded over the same period. Even though many of the new code requirements did
not apply to existing buildings, in several instances these new approaches were incorporated and systems
upgraded accordingly. Also identified were some issues that were not consistent with code requirements,
such as the spray applied fireproofing and tenant separation walls that were eventually addressed by

Chapter 1

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

upgrade projects. The upgrades were performed on change of tenancy over many years. The reader
should note that the documentation of certain systems and their condition and arrangement on
September 11, 2001 are included in other reports. Specifically, the elevators and egress stairs are
discussed in NIST NCSTAR 1-7, and the fire alarm, sprinkler, and smoke management systems in NIST
NCSTAR 1-4. This reference is to one of the companion documents from this Investigation. A list of
these documents appears in the Preface to this report.

1.3

DESIGN AND CONSTRUCTION REQUIREMENTS FOR WTC 1, 2, AND 7

For most buildings constructed in the United States, building codes adopted by local jurisdictions
establish minimum requirements for design and construction. However, because the Port Authority is an
interstate entity, which was established in 1921 under a clause in the U.S. Constitution, its construction
projects are not required to comply with any state or local building code. For the design of the WTC
towers, which began in 1962, the Port Authority in May 1963 instructed the architect and engineers to
prepare their designs of WTC 1 and WTC 2 to comply with the NYC Building Code.

While not

specifically stated in the 1963 letter to the architect, the 1938 edition of the Code was in effect at that
time. In areas where the Code was not explicit or where technological advances made portions of the
1938 Code obsolete, the Port Authority also directed the architect and engineers to propose designs
“based on acceptable engineering practice.” When such situations occurred, the Port Authority required
the architect and engineers to inform the Planning Division of the WTC. The Port Authority established a
special WTC office that reviewed and approved plans and specifications, issued variances, and conducted
inspections during construction instead of the city agencies that would normally perform these duties.

In September 1965, the Port Authority instructed the architect and engineers to revise their designs for
WTC 1 and WTC 2 to comply with the second and third drafts of the NYC Building Code that was under
development and to undertake any design modifications necessary to comply with the new code
provisions.

Prior to issuance of this instruction, the Port Authority recognized that the draft version of

the new NYC Building Code had incorporated advanced techniques, and the Port Authority favored the
use of advanced techniques in the design of the WTC towers.

By adopting the draft versions of the new

NYC Building Code, the Port Authority had an option of classifying WTC 1 and WTC 2 as Type 1-B
Construction instead of Type 1-A Construction (see Sec. 9.1.3 for definition and fire protection
requirements of Construction Type), and several architectural features related to egress were modified in
the final design (see Sec. 10.1). This relaxation of code requirements allowed the Port Authority to gain
economic advantage.

The new NYC Building Code (NYC BC 1968) was enacted by the City Council on

October 22, 1968, approved by the Mayor on November 6, 1968, and became effective on December 6,
1968.

Letter dated May 15, 1963 from Malcolm P. Levy (Chief, Planning Division, World Trade Department, PANYNJ) to Minoru

Yamasaki (architect, Minoru Yamasaki & Associates) (See Appendix A).

Letter dated September 29, 1965 from Malcolm P. Levy (Chief, Planning Division, World Trade Department, PANYNJ) to

Minoru Yamasaki (architect, Minoru Yamasaki & Associates) (See Appendix A).

Memorandum dated June 22, 1965 from John M. Kyle (Chief Engineer, PANYNJ) to Malcolm P. Levy (Chief, Planning

Division, World Trade Department, PANYNJ) (See Appendix A).

Memorandum dated January 15, 1987 fromLester S. Feld (Chief Structural Engineer, World Trade Department) to Robert J.

Linn (Deputy Director, Physical Facilities, World Trade Department), (See Appendix A).

Draft for Public Comment

Introduction

NIST NCSTAR 1-1, WTC Investigation

The Port Authority also required that all design concepts were to be reviewed before the final design by
the Chief Engineer of the Port Authority and by the appropriate New York City agencies. A letter in 1975
from the architect-of-record for the WTC project to the Port Authority indicates that the New York City
Building Department reviewed the design drawings of WTC 1 and WTC 2 in February 1968.

Unlike WTC 1 and WTC 2, which were developed and owned by the Port Authority, WTC 7 was
developed on land owned by the Port Authority, but the building was owned by Seven World Trade
Company and Silverstein Development Corporation, General Partners. It was designed and constructed as
a “Tenant Alteration Project” of the Port Authority. When WTC 7 was designed in the mid-1980s, the
1968 NYC Building Code with amendments was in effect. The Project Specifications for WTC 7, issued
in 1984, required that the structural steel be designed in accordance with the then current NYC Building
Code.

The Port Authority developed a tenant alteration process for any modifications to leased spaces in WTC 1
and WTC 2 to maintain structural integrity and fire safety. In 1971, the Port Authority issued the first
edition of a set of requirements,

Tenant Construction Review Manual

(See NIST NCSTAR 1-1C,

Appendix A), shortly after the first tenants occupied WTC 1 in December 1970 and before initial
occupancy of WTC 2 in 1972. The manual contained the technical criteria to be used in planning
alterations (architectural, structural, mechanical, electrical, and fire protection) to suit the needs of
tenants. The manual included applicable standards to be used by tenants and their agents and review
criteria to be used by the Engineering Department of the Port Authority. Alteration designs were to be
completed by registered design professionals, and at the completion of the work, as-built drawings were
to be submitted to the Port Authority. The 1968 NYC Building Code was referenced, and specific code
provisions were referenced in various checklists. The review manual was updated in 1979, 1984, 1990,
and 1997, at which times changes that had been made to the NYC Building Code were incorporated. In
1998, the manual was replaced by

Architectural and Structural Design Guidelines, Specifications, and

Standard Details

(see NIST NCSTAR 1-1C, Appendix F), which dealt specifically with alterations to

WTC 1 and WTC 2. Since WTC 7 was built as a “tenant alteration project,” its design and construction
followed the requirements in the 1984 edition of the

Tenant Construction Review Manual

. Any

modifications to the building after initial occupancy were carried out in accordance with the manual.

1.4 ORGANIZATION

REPORT

This report is organized in fourteen chapters:

•

Chapter 1 covers the background and the scope of the report.

•

Chapter 2 presents architectural and structural descriptions of WTC 1, 2, and 7.

•

Chapter 3 presents the evolution of building codes in the United States, the development of the
building code of New York City, and design requirements and policies of the Port Authority
of New York and New Jersey.

Letter dated February 18, 1975 from Joseph H. Solomon (Architect, Emory Roth & Sons) to Malcolm P. Levy (General

Manager, World Trade Center Operations), (See Appendix A).

Chapter 1

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

•

Chapter 4 provides an overview and comparison of building codes in use at the time WTC 1,
2, and 7 were designed and constructed. Also presented is the evolution of codes during the
time the buildings were in use and how the buildings were modified and upgraded over the
same period. The structural code provisions compared include the 1964 New York State
Building Construction Code, the 1965 BOCA model building code, and the 1967 Municipal
Code of Chicago. A comparison was also made between the 1968 NYC Building Code and
the current (2001) NYC Building Code.

•

Chapter 5 presents the criteria for structures used to design WTC 1, 2, and 7.

•

Chapter 6 presents innovative features incorporated in the structural design of WTC 1 and
WTC 2.

•

Chapter 7 presents the protocols for inspection of steel members during fabrication and
erection, and variances that were requested by fabricators and the erector and granted by the
Port Authority.

•

Chapter 8 covers structural maintenance and modifications to WTC 1, 2, and 7 during
occupancy.

•

Chapter 9 compares the fire safety provisions in the 1964 New York State Building
Construction Code (NYSBC 1964), the 1965 BOCA model building code (Basic Building
Code), and the 1967 Municipal Code of Chicago (MCC 1967). A comparison was also made
between the 1968 NYC Building Code and the current (2001) NYC Building Code. This
chapter also describes various construction classifications of buildings.

•

Chapter 10 describes passive and active fire protection systems used in WTC 1, 2, and 7, and
egress provisions in the WTC towers.

•

Chapter 11 presents maintenance of and modifications to fire safety systems in WTC 1, 2, and
7 during occupancy.

•

Chapter 12 presents the fuel system distribution for emergency power generators in WTC 7.

•

Chapter 13 presents the findings of this report.

•

Chapter 14 covers the reference cited in this report.

NIST NCSTAR 1-1, WTC Investigation

Chapter 2

ESCRIPTION OF

WTC

AND

2.1

SITE PLAN OF WTC COMPLEX

The World Trade Center (WTC) complex was located in Manhattan, New York City, near the Hudson
River. The complex was comprised of seven buildings (referred to in this report as WTC 1 through
WTC 7). Figure 2–1 depicts the locations of these buildings relative to the surrounding streets. The two
towers, WTC 1 (North Tower) and WTC 2 (South Tower), were each 110 stories high. WTC 3 (Marriott
Hotel) was 22 stories. WTC 4 (South Plaza Building) and WTC 5 (South Plaza Building) were both nine-
story office buildings. WTC 6 (U.S. Customs House) was an eight-story office building. These six
buildings were built around a 5-acre WTC Plaza. WTC 7 was a 47-story office building which was built
just north of the six-building WTC site. There was a six-story subterranean structure below a large
portion of the WTC Plaza and WTC 1, 2, 3, and 6. In order to build this subterranean structure, a
bentonite slurry wall was built surrounding the perimeter of the subterranean structure prior to excavation.
The slurry wall was replaced section by section with reinforced concrete wall which served as a
continuous foundation wall for the subterranean structure. The reinforced concrete wall was temporarily
supported by rock anchors to provide lateral stability. The permanent lateral support was provided by the
subterranean floor slabs. The application of slurry wall technology was considered to be an innovative
idea (ENR 1964).

The first six buildings on the site were developed by the Port Authority. Groundbreaking for WTC 1 and
WTC 2 was in 1966, and the first tenant began to occupy WTC 1 in December 1970 and WTC 2 in
January 1972. Construction of other buildings continued during the 1970s and the 1980s.

Construction

of the last building, WTC 7, was completed in 1987. It was developed by a consortium of Seven World
Trade Company and Silverstein Development Corporation.

2.2

DESCRIPTION OF WTC 1 AND WTC 2

2.2.1 Building

Description

WTC 1 and WTC 2 (also known as North Tower and South Tower) each consisted of a 110-story
structure above the Concourse level (109-story above the Plaza level) and 6-story structure below the
Concourse level.

Although the towers were similar, they were not identical. The height of WTC 1 at the

roof level was 1,368 ft above the Concourse level, 6 ft taller than WTC 2, and WTC 1 supported a 360 ft
tall antenna for television and radio transmission. Figure 2–2 shows the west elevation of WTC 1, and
Fig. 2–3 shows a typical exterior wall from the foundation to floor 9.

A brochure entitled “The World Trade Center” published by the Port of New York Authority, New York, NY and “World

Trade Center Fact Sheet” published by the Port Authority of New York and New Jersey, New York, NY, April 1994.

The architectural and structural descriptions and dimensions of the WTC buildings in this report are based on the design

drawings of these buildings obtained from the Port Authority of New york and New Jersey.

Chapter 2

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

Each tower had a square plan with the side dimension of approximately 207 ft. The exterior columns
(perimeter columns) were placed with respect to the column reference lines, wherein the geometric
centers of the exterior columns were offset from the column reference lines (See Sec. A-A, Fig. 2–9).
The four reference lines surrounding the base of the tower established the footprint of the building. The
column reference lines were spaced at 207 ft 2 in. The corners of the tower were chamfered 6 ft 11 in.

Each tower had a core service area of approximately 135 ft by 87 ft. All elevators were located within the
core. Three stairs were also located within the core except at the mechanical floors where the stairs were
located outside the structural core area (the area enclosed by the four corner columns of the core). For
detailed descriptions of the stair locations, see Table 2–1, NIST NCSTAR 1-7. A typical architectural
floor plan in the tower is shown in Fig. 2–4. As can be seen in this figure, placing all service systems
within the core provided nearly a column-free floor space of approximately 31,000 ft

per floor outside

the core. The long axis of the core in WTC 1 was oriented in the east-west direction, while the long axis
of the core in WTC 2 was oriented in the north-south direction. Design wind forces

were different for the

two towers (the presence of one tower had an effect on the wind pressures on the other tower, see NIST
NCSTAR 1-2), and that resulted in somewhat different lateral-force resisting system design. Thus, the
two towers appear similar, but they were structurally different.

The exterior walls were composed of steel columns and spandrel beams

Above the 7th floor level, the

columns were welded steel plate box columns of an approximately 14 in. square section. Each building
face consisted of 59 columns spaced at 3 ft 4 in. on center. Adjacent columns were interconnected at each
floor level by deep spandrel plates, typically 52 in. deep. As seen in Fig. 2–3, below floor 7, the columns
are combined in groups of three to form single base columns which are spaced 10 ft on center. The
external cladding, which covers the columns and spandrel beams, consisted of aluminum sheets. The
window openings were infilled with glass fitted into aluminum frames and sealed with neoprene gaskets.

Fire protection of structural steel members in the WTC towers was provided by fire resistive materials,
either sprayed fire resistive materials (SFRMs), gypsum wallboards, or a combination of the two,
depending upon the type of structural members. All floor trusses and beams were protected with SFRM.
The columns inside the core were either covered with gypsum wall board or a combination of gypsum
wall board and SFRM. For the exterior columns, vermiculite plaster was applied to the side of the
column facing the interior of the building, whereas SFRM was applied to the other three faces. No fire
resistive material was applied to the underside of the metal deck, which was in contact with the concrete
slab above. For a detailed discussion of the passive fire protection of steel members, see NIST
NCSTAR 1-6A.

For typical tenant floors, the ceiling was suspended from the steel trusses. The space between the ceiling
and the floor above was used for the mechanical and electrical systems.

Elevators were the primary mode of routine ingress and egress from the towers for tens of thousands of
people on a daily basis. In order to minimize the total floor space needed for elevators, each tower was
divided into three zones by the skylobbies, which served to distribute passengers among express and local
elevators (for details, see NIST NCSTAR 1-7). In this way, the local elevators within a zone were placed
on top of one another within a common shaft. Figure 2–5 shows the elevator riser diagram for WTC 1
and WTC 2. People transferred from express elevators to local elevators at the skylobbies which were
located on the 44th and 78th floors in the both towers. Each tower had 99 elevators within the core of the
building, including seven freight elevators, most serving a particular zone, and dedicated express

Draft for Public Comment

Description of WTC 1, 2, and 7

NIST NCSTAR 1-1, WTC Investigation

elevators that served the restaurant, bars, and meeting rooms on floors 106 and 107 of WTC 1, as well as
the observation deck in WTC 2. The concept of multiple elevators in a common shaft was first used in the
WTC towers and has since become the norm for buildings taller than about 50 stories. This approach
allowed an increase of useable space in WTC 1 and WTC 2 from 62 percent to 75 percent per floor
(Sullivan 1964).

The architectural design was performed by Minoru Yamasaki & Associates, with Emory Roth &
Sons, P.C. serving as the architect of record. The structural engineer of record was the firm of Skilling,
Helle, Christiansen, Robertson (SHCR). Jaros, Baum & Bollers were the mechanical engineers, and
Joseph R. Loring & Associates were the electrical engineers. Tishman Construction Corporation was the
general contractor. The foundation of the towers was designed by the Engineering Department of the Port
of New York Authority (see footnote 6).

2.2.2 Structural

Description

As described above (Sec. 2.2.1), both WTC 1 and WTC 2 were 116 stories above the foundation
(110 stories above grade and 6 stories below grade). The buildings were square in plan, 207 ft 2 in. by
207 ft 2 in. (based on column reference lines), and with story heights of typically 12 ft. The core area was
approximately 135 ft by 87 ft in plan. (Approximate dimensions are stated as the dimensions on the
architectural and structural drawings are given in reference to “column reference lines,” and they do not
necessarily coincide with the centroid of the column cross section.)

Each tower was comprised of five structural systems: a framed tube for the exterior walls above grade,
simple frames (beams and columns with simple connections) for the core, braced frames for the exterior
walls below grade, composite floor framing, and hat trusses at the roof level. As a framed-tube system,
the exterior walls of each tower, comprised of closely spaced columns that were connected by spandrel
beams around the perimeter at each floor level, were designed to resist all lateral loads. The resistance to
lateral load was provided by the caltilever action of the tube. All columns of a frame-tube building
experience mainly axial forces. For a square framed-tube building, the exterior columns on the faces
normal to the wind direction are either in tension or in compression. The columns in the windward-side
wall are in tension, and the columns in the leeward-side wall are in compression. The side walls are
analogous to the web of a beam, mainly in shearing action. Thus, the axial forces in columns of the side
walls vary from the windward side in tension to the leeward side in compression. Figure 2–6 illustrates
the axial force distribution in columns. The figure also shows the shear-lag effect due to the in-plane
flexibility of the spandrel beams. The shear lag effect increases the column loads near the corners and
decreases in the center region of the walls that are perpendicular to the direction of wind. Analyses of the
towers under wind loads indicate that the patterns of the axial force distribution in the columns due to
wind loads are similar to those shown in Fig. 2–6 (NIST NCSTAR 1-2). Typical cross sections of the
exterior walls are shown in Fig. 2–7. As seen in the figure, they were constructed of steel built-up
columns and spandrel beams comprised of plates.

Since the lateral loads are resisted mainly by the exterior walls in a framed tube system, the interior core
columns do not contribute to the over-all lateral stiffness of the building. For the WTC towers, both the
exterior columns and the core columns were designed to support an approximately equal amount of the
total gravity loads (see NIST NCSTAR 1-2). In the typical WTC tower floor plan, the area inside the
core was framed with structural steel shapes acting compositely with formed concrete slabs. Most of the

Chapter 2

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

floor beam members were connected to columns by simple connection. The columns in the interior core
of the towers were designed to carry mainly the gravity (vertical) loads, except in the atrium area (below
floor 7 to the foundation), where there were fewer perimeter columns in the outer walls; bracings were
used in the outer perimeter of the core area to increase lateral stiffness. In the lower part of the towers,
the outer core columns were designed to resist a portion of the lateral forces.

Exterior Walls

The exterior wall columns , built-up of steels plates, from the foundation level up to Elevation 363 ft
(column splice point below floor 7, see Fig. 2–3) were spaced 10 ft on center, and they were connected by
spandrels. Between the Concourse Level and the foundation, these columns were braced diagonally to
form braced frames in the plane of the exterior walls (Fig. 2–3). Between Elevation 363 ft and floor 7,
single exterior wall columns spaced 10 ft on center transitioned to three columns spaced 3 ft 4 in. on
center (Fig. 2–8) to form “tree” assemblies.

Between floors 9 and 107, the perimeter structure consisted of closely spaced, built-up box columns.
Each building face consisted of 59 columns. The columns were fabricated by welding plates of steel to
form an approximately 14 in. square section (Fig. 2–7). The columns were interconnected at each floor
level by 52 in. deep spandrel plates to form a 10 ft wide and 36 ft tall panel (Fig. 2–9). Heavy end, or
“butt” plates of 1.375 in. to 3 in. thick were welded to the top and bottom of each column. Fillet welds
were used inside the columns along three edges, with a groove weld on the fourth, outside edge. The
exterior walls were erected by connecting the prefabricated panels. The panels were field-bolted to
adjacent panels with dual splice plates (see Fig. 2–7), and columns were bolted to the adjacent columns,
using ASTM International (ASTM) A 325 bolts except for the heaviest butt plates, where ASTM A 490
bolts were used. Other than at the mechanical floors, panels were staggered so that only one third of the
columns were spliced (i.e., connected) in any one story (Fig. 2–10). At the mechanical floors, the
perimeter columns were spliced at the same level (i.e. floors 74 and 77). These splices were both welded
and bolted.

At each corner of the building, the spandrel plate connected the column on one face of the building to the
column on the other face at each floor level. The corner spandrel plates between two floors were
interconnected by a box-shape vertical member. The vertical members were attached to the corner
spandrels at alternate stories and thus, they are not continuous from the top of the building to the
foundation. The corner vertical members were attached to the building during the construction period to
aid hoisting of construction material.

Fourteen grades of steel were specified in the design documents for the perimeter columns, with
minimum yield strengths of (36, 42, 45, 46, 50, 55, 60, 65, 70, 75, 80, 85, 90, and 100) ksi. Twelve grades
of steels were actually used (See NIST NCSTAR 1-3). Twelve grades of steel were specified for the
spandrels, with the same strength levels as the columns, but without the two highest strength steels. The
structural engineering plans indicate that the flanges and webs of a given column section consist of a
single grade (i.e., minimum yield strength) of steel, but each column and spandrel within a single
prefabricated panel could be fabricated from different grades of steel. The use of different grades of steel
facilitated in maintaining uniform exterior dimensions of the exterior columns throughout the building as
well as equalize the dead load stresses and shortening of very tall steel columns.

Draft for Public Comment

Description of WTC 1, 2, and 7

NIST NCSTAR 1-1, WTC Investigation

Columns in the upper stories were typically fabricated of thinner steel plates, as thin as 0.25 in., with the
grade of steel dictated by the calculated gravity and wind loads. In this manner, the gravity load on the
lower stories was minimized. In the lower stories the perimeter column webs were often more than 2 in.
thick.

The spandrels formed an integral part of the columns; there was no inner web plate at spandrel locations.
Spandrels were generally specified with a yield strength lower than that of the column webs and flanges,
as well as a heavier gauge than the adjacent inner webs.

Core Columns

As stated above, the core columns were designed to support approximately 50 percent of the gravity
loads. The core columns were of two types: welded box columns and rolled wide flange shapes
(Fig. 2–11). The columns in the lower floors were primarily very large box columns, as large as 12 in.
by 52 in., comprised of welded plates up to 7 in. thick. In the upper floors, the columns shifted to the
rolled wide-flange shapes. The transition floors are indicated in Fig. 2–12 for each of the core columns.
Core columns were typically spliced at three-story intervals. Diagonal bracing was used at the
mechanical floors and in the area of the hat truss. Steel used for core box columns was either 36 ksi or
42 ksi. Core wide flange columns were specified to be one of four grades, but were primarily 36 ksi and
42 ksi steel; only about 1 percent of all the core columns were made of 45 ksi or 50 ksi steel.

Foundation

For the core columns, a column base plate distributed the column load to a steel grillage comprised of
two-layers of steel beams. The steel grillage, in turn, distributed the column load to the reinforced
concrete spread footings, which were directly in contact with the bedrock.

For the exterior columns, a large steel base plate, ranging from 7 to 9 ft

, was used to transfer the

individual column load to the reinforced concrete wall footing. The wall footing was placed around the
perimeter of the tower. The concrete footing was in direct contact with the bedrock.

Floor Framing System

The floor system of a framed-tube structure is designed for four main functions. First, it supports the
vertical gravity loads on the floor and transfers these loads to the external and core columns. Second, as a
diaphragm it distributes wind loads to the side walls of the framed tube structure. Third, it, together with
the external frame, provides the stiffness to resist torsional motion of the building. Fourth, it provides
lateral support to the columns, thereby, keeping the columns stable. The effectiveness of the framed-tube
action is dependent on the inplane stiffness of the floor framing system. If the floor inplane stiffness is
low (flexible diaphragm), the framed tube action cannot be developed effectively, and the structure
behaves like a moment-resisting frame. On the other hand, if the inplane floor stiffness is high (rigid
diaphragm), wind loads are distributed to the columns in the side frames, and the structure behaves like a
framed tube. For WTC 1 and WTC 2, the floor system was comprised of concrete-steel composite
members as described below. A typical trussed-framed floor framing is shown in Fig. 2–13. Analyses of
the inplane stiffness showed that the typical floor system of WTC towers behaved as a stiff diaphragm

Chapter 2

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

(see NIST NCSTAR 1-2A), and the WTC towers behaved more like a framed tube than a moment-
resisting frame.

The floor inside the core and the mechanical floors were framed with structural steel shapes with welded
shear studs, acting compositely with normal-weight concrete slabs. The thickness of concrete slab in
these floors varied from 4.5 in. to 8 in. depending upon the design load requirements. The area outside the
core (typically tenant floors) was framed with steel trusses, acting compositely with 4 in. thick
lightweight concrete slabs cast on 1½ in., 22 gauge fluted metal deck. The trusses consisted of double
angle top and bottom chords with round bar webs.  The composite action was achieved by the shear
connection provided by the web bar extending above the top chord and into the slab in the form of a
“knuckle” (see Fig. 2–14).  Pared trusses, spaced at 6 ft 8 in. on center, were supported at every other
exterior column. The metal deck which spanned parallel to the main trusses was directly supported by
transverse bridging trusses spaced at 13 ft 4 in. and intermediate deck support angles spaced at 6 ft 8 in.
from the transverse bridging trusses. The typical floor consisted of three truss zones; a long-span zone, a
short-span zone, and a two-way zone (see Fig. 2–15).  The span of the trusses was about 35 ft in the short-
span zone and 60 ft in the long-span zone.

The floor trusses were pre-assembled into floor panels. The prefabricated floor panels were typically
20 ft wide, containing two sets of double trusses in the interior and a single truss along each edge. In
addition, the bottom chord of each pair of trusses was attached to perimeter spandrels with viscoelastic
dampers (see Fig. 2–16). The main purpose of these dampers was to supplement the steel frame in
limiting wind-induced building oscillations.

Pairs of flat bars (straps) extended diagonally from the top chord of the floor trusses to the perimeter
columns (see Fig. 2–13). Once in place, 4 in. of lightweight concrete was placed on the steel deck.
Figure 2–17 shows an assembled floor panel before the concrete was placed.

The minimum yield strengths of the steel for the design of the floor trusses were specified to be 36 ksi and
50 ksi for different parts of the trusses. According to the fabrication drawings prepared by Laclede Steel
Company, both 36 ksi and 50 ksi steels were specified.

Hat Trusses

At the top of each tower (floor 107 to the roof), an assembly of hat trusses interconnected the core
columns and the exterior wall panels (see Fig. 2–18). Diagonals of the hat truss were typically W12 or
W14 wide flange members. In addition, four diagonal braces (18 in. by 26 in. box beams spanning the
35 ft gap, and 18 in. by 30 in. box beams spanning the 60 ft gap) and four horizontal floor beams
connected the hat truss to each perimeter wall at the floor 108 spandrel. The hat truss was designed
primarily to provide a base for antennae atop both towers, although only the WTC 1 antenna was actually
built. The hat truss also controlled the expansion and contraction of the tower due to unequal column
temperatures, although not specifically designed for this purpose.

Draft for Public Comment

Description of WTC 1, 2, and 7

NIST NCSTAR 1-1, WTC Investigation

2.3

DESCRIPTION OF WTC 7

2.3.1 Building

Description

WTC 7 was a 47-story commercial office building, completed in 1987. Its location relative to the WTC
Plaza is shown in Fig. 2–1. It contained approximately 2 million ft

of floor area. A typical floor plan

above floor 7 is shown in Fig. 2–19. WTC 7 was connected to the WTC complex with a 120 ft wide
elevated plaza at floor 3, and a 22 ft wide pedestrian bridge, also at floor 3.

The overall dimensions of WTC 7 were approximately 330 ft long, 140 ft wide, and 610 ft high. The
building was constructed over a pre-existing electrical substation owned by Consolidated Edison
(Con Edison). The original plans for the Con Ed Substation included supporting a high-rise building, and
the foundation was sized for the planned structure. However, the final design for WTC 7 had a larger
footprint than originally planned. The building elevations are shown in Fig. 2–20. Over the years,
numerous structural modifications were made throughout the building, mainly to suit its largest tenant,
Salomon Brothers Inc. (later to become Salomon Smith Barney, now CitiGroup), who leased 25 of the
47 floors. One of the more substantial modifications was the addition of a penthouse that was used to
house the chiller plant and the cooling towers for Salomon Brothers. Also, large portions of the 41st and
43rd floor slabs and the floor framing were removed on the east side of the building to accommodate
trading floors for Salomon Brothers. The removed floor areas were subsequently restored after the
trading activity was moved to another venue.

Above floor 7, the building had typical steel framing for high-rise construction. The floor systems had
composite construction with steel beams supporting concrete slabs on metal decks, with a floor thickness
of 5.5 in. The core and perimeter columns supported the floor system and carried their loads to the
foundation. The perimeter moment frame also resisted wind forces. Columns above floor 7 did not align
with the foundation columns, so braced frames, transfer trusses, and transfer girders were used to transfer
loads between these column systems, primarily between floors 5 and 7. Floors 5 and 7 were heavily
reinforced concrete slabs on metal decks, with thicknesses of 14 in. and 8 in., respectively.

The architectural design was performed by Emory Roth & Sons, P.C. The structural engineer of record
was the Office of Irwin G. Cantor, P.C. Syska & Hennessy, P.C. was the mechanical engineer. Tishman
Construction Corporation was the general contractor.

Consolidated Edison Substation

The Con Edison Substation was constructed in 1967 and consisted of a steel framed structure with cast-
in-place concrete floors and walls. It was placed on the northern portion of the site and extended
approximately 40 ft north of the north facade of WTC 7, as shown in Fig. 2–21. Its southern boundary
was irregular, but extended approximately one-third to two-thirds of the width of WTC 7. The
Con Edison Substation was three stories high.

The substation’s lateral system consisted of a moment frame along the northern row of interior columns.
Along the south edge of the substation there was a braced frame. This braced frame was coincident with
the north side of the WTC 7 core. Lateral loads from WTC 7 were passed directly from the core above to
the Con Edison braced frame below. There were also two moment frames within the substation, oriented
in the north-south direction, one on each end of the WTC 7 core.

Chapter 2

Draft for Public Comment

NIST NCSTAR 1-1, WTC Investigation

The WTC 7 columns

which were within the perimeter of the substation

were supported by substation

columns. During the construction

of WTC 7, heavy plates were welded to the tops of the existing

substation columns, which then supported the new building columns.

The exterior columns above the Con Edison structure that did not align with the columns of the Con
Edison structure were supported by a series of transfer girders. The arrangements of the transfer girders
are described in detail in Sec. 2.3.2

2.3.2 Structural

Description

Typical Floor Systems above Floor 7

The typical floor framing system, shown in Fig. 2–22, was composed of rolled steel wide-flange beams
with composite metal decking and concrete slabs. Floors 8 through 45 had essentially the same framing
plan, but the core layout varied over the height of the building.

Floors 8 through 45 had floor slabs with a total thickness of 5.5 in. that were composed of 3 in., 20 gauge
metal deck with 2.5 in., normal weight concrete of 3,500 psi. There was one layer of 6x6 W1.4xW1.4
welded wire fabric within the concrete. The structural design drawings show a second layer of welded
wire fabric placed over girders at the slab edges. The fastening requirements for the metal deck are not
shown on the drawings. The drawings contain a note calling for 1.5 in., 20 gauge deck with 4 in. concrete
topping (5.5 in. total) in the elevator lobbies, where there was a 3 in. floor finish specified by the
architect.

Typical floor framing for floors 8 through 20 and floors 24 through 45 consisted of 50 ksi wide-flange
beams and girders. A grid of beams and girders spanned between the core columns. Core girders ranged
in size from W16x31 to W36x135, depending on the span and load. (W16x31 describes a steel wide-
flange beam, sometimes referred to as an ‘I’ beam; the nomenclature indicates the cross section is
nominally 16 in. deep and weighs 31 lb per lineal foot.) Beams spanned directly between the core and the
exterior of the building, at approximately 9 ft on center. On the north and east sides, the typical beam was
a W24x55 with 28 shear studs, spanning 53 ft. On the south side, the typical beam was a W16x26 with
24 shear studs spanning 36 ft. Between the exterior columns were moment connected girders that formed
part of the lateral-load-resisting system of the building. On floors 10, 19, and 20, a portion of the floor
framing was reinforced with plates attached to the bottom flange. Certain connections at these floors
were also reinforced.

Floors 21 to 23 had slightly heavier steel framing than the typical floors. Core girders were generally one
size class larger than the typical floor; the beams between the core and the south facade were W16x31
instead of W16x26. There were additional studs on the W24x55 beams on the north and west sides.

Most of the beams and girders were made composite with the slabs through the use of shear studs.
Typically, the shear studs were 0.75 in. in diameter by 5 in. long, spaced 1 ft to 2 ft on center.

Structural descriptions are determined from the design drawings.

Draft for Public Comment

Description of WTC 1, 2, and 7

NIST NCSTAR 1-1, WTC Investigation

Floor Framing of Other Floors

The remaining floors, floors 1 to 7 and floors 46 to 47, were atypical. Floor 1 was built adjacent to the
Con Edison substation and included the truck ramp for the WTC complex (see Fig. 2–21). The floor was
framed with steel beams that were encased in a formed concrete slab. The floor slab was 14 in. The
southeast portion of the floor above the WTC truck ramp had a 6 in. formed concrete slab. The floor
slabs for floors 2, 3, 4, and 6 had a 3 in., 20 gauge metal deck with 3 in. normal weight concrete, for a
total thickness of 6 in. Floors 2 and 3 were also partial floors adjacent to the substation. In addition, they
had a floor opening on the south side to form the atrium above the ground level lobby. Floor 4 was above
the substation and had a large opening over most of the south side of the building to form a double-height
space above the 3rd floor lobby. Floor 5 had an 11 in. thick slab of normal weight concrete on top of
3 in., 18 gauge steel deck for a total slab thickness of 14 in. The slab was heavily reinforced with #7
reinforcing bars spaced at 12 in. on center in both directions on top and #9 reinforcing bars spaced at
12 in. on center on bottom. This floor also had steel WT sections embedded in the 11 in. concrete slab
above the steel deck. The WT sections were designed to act as a horizontal truss within the plane of the
floor between the perimeter and core columns (see Fig. 2–23). Floor 6 had two openings on the floor to
form a double-height mechanical space, one at the east side and the other at the southeast corner. Floor 7
had 5 in. normal weight concrete on top of 3 in., 18 gauge metal deck, which made a total thickness of
8 in. The slab was reinforced with #5 reinforcing bars spaced at 6 in. on center in both directions.

Columns

Core columns were primarily rolled wide-flange shapes of grade 36 or 50 steel. As the loads increased
toward the base of the building, many of these column sizes were increased through the use of built-up
shapes. These built-up columns had a W14x730 core with cover plates welded to the flanges (to form a
box) or web plates welded between the flanges as shown in Fig. 2–24. The reinforcing plate welds were
specified to be continuous 0.5 in. fillet welds at the cover plates and 0.313 in. minimum at the web plates.
Plate thickness ranged from 1.5 in to 8 in. Steel used for reinforcing plates were specified as follows:

Plate thickness t (in.):

2 < t < 4

ASTM A 588 Grade 50

4 < t < 6

ASTM A 572 Grade 42

t > 6

ASTM A 588 Grade 42

Typical core column splices were specified to be milled. The splice plates were welded or bolted to the
outsides of the column web and flanges. Built-up columns were also milled at their bearing ends, but the
splice plates were fillet welded to the cover plates.

Perimeter columns were nominally 14 in. wide-flange shapes (W14) of ASTM A 36 steel. Perimeter
column splices were similar to the core column splices.

Column Transfer Trusses and Girders

Because the layout of the substructure and Con Edison columns did not align with the column layout in
the upper portion of WTC 7, a series of column transfers were constructed. These transfers occurred

NIST NCSTAR 1-1, WTC Investigation

Chapter 3

EVELOPMENT OF

UILDING

ODES

Since World Trade Center (WTC) 1, 2, and 7 were designed according to the New York City (NYC)
Building Code, it is important to understand the evolution of this building code. This chapter presents the
historical background of the development of the NYC Building Code. This chapter also summarizes the
Port Authority policies for design and construction of its buildings.

3.1

BUILDING CODE DEVELOPMENT IN THE UNITED STATES

In the United States, building codes were introduced to minimize losses from fire. Following large fires in
major cities such as Boston, New York, Chicago, and Baltimore in the late 1800s, the first model building
code was developed by the fire insurance industry. The National Board of Fire Underwriters (predecessor
of the American Insurance Association) published the

National Building Code

in 1905. Subsequently, the

Pacific Coast Building Officials Conference (predecessor of the International Conference of Building
Officials) issued the

Uniform Building Code

(UBC) in 1927, the Southern Building Code Congress

International Inc. (SBCCI) published its

Southern Standard Building Code

in 1946, and the Building

Officials and Code Administrators, Inc. (BOCA) published the

Basic Building Code

in 1950. In the mid-

1980s, the Basic Building Code was changed to the

BOCA National Building Code

(NBC). The three

model building codes, namely the BOCA National Building Code, the Southern Standard Building Code,
and the Uniform Building Code, were revised annually to incorporate developments in new materials,
construction methods, and practices, and new editions were published every three years.

Before the issuance of the

International Building Code

(IBC) in 2000, which was published by the

International Code Council (an amalgamation of the three model code organizations), most local and state
building codes in the United States were patterned after one of the three model building codes. The
model codes were sometimes adopted by these jurisdictions in their entirety and other times with
significant modifications. The version adopted is law in that jurisdiction. In early 1900s the National
Fire Protection Association (NFPA) initiated the development of a “life safety code” for safety of
building occupants. This code, while not a building code, is frequently used as a supplement to the
building codes. In 2002, NFPA also published a model building code known as the

NFPA Building

Construction and Safety Code

(NFPA 5000). A number of major cities in the United States have

developed their own building codes to meet their specific needs, such as San Francisco for earthquake
resistant design and New York City for high-rise buildings. At the present time, 44 states have adopted
IBC with some modifications

and it is being considered for adoption by New York City.

These model building codes establish minimum requirements to safeguard life, health, property, and
public welfare through provisions pertaining to the design, construction, and quality of materials, use and
occupancy, and maintenance of buildings. When buildings are designed, constructed, and maintained
according to building code requirements, they are considered to have met minimum requirements. While

The International Code Council updates the number of local jurisdictions that have adopted IBC (www.iccsafe.org).

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NIST NCSTAR 1-1, WTC Investigation

building code regulations address a number of objectives demanded by society, the primary objectives of
building codes are structural stability and fire safety.

3.2

NEW YORK CITY BUILDING CODE

The New York City (NYC) Building Code is part of the Administrative Code of New York City.

Although New York City had laws governing construction as early as 1674, after a tenement fire in 1860
took 20 lives, New York City modified and strengthened building safety laws extensively. New York
City building laws are amended from time to time by Local Laws to improve safety requirements or to
incorporate technological advances.

Local Laws are enacted by the NYC Council. Any member can introduce a bill to the Council for the
purpose of amending the Building Code requirements. When passed by the Council and approved by the
Mayor, the bill becomes a Local Law. The current Building Code was enacted on December 6, 1968.
Through 2002, 79 Local Laws were adopted that modified the 1968 Building Code.

To aid the implementation of and to clarify Building Code requirements, New York City issues “rules.”
Typically these rules are initiated by City Government offices such as the Department of Buildings and
the Department of Environment, and issued by the Building Commissioner. The rules do not require
enactment by the City Council, and new rules issued by the Building Commissioner can be put into effect
expeditiously. The rules, although are not part of the Building code, are required to be complied with for
design, construction, and maintenance of buildings.

The 1968 NYC Building Code includes “Reference Standards,” including standard test methods
published by the American Society for Testing and Materials (ASTM), and design standards published by
other organizations such as the American Concrete Institute and the American Institute of Steel
Construction. These reference standards may include modifications to the provisions in the published
standards, or they may be stand-alone requirements developed by New York City.

At the time the WTC project was begun (early 1960s), the 1938 NYC Building Code, which was first
adopted on January 1, 1938, was in effect and enforced throughout the five boroughs. In the late 1950s, it
was noted that “great changes have occurred in all facets of the building industry” and that “As a result of
these developments, and the failure in many instances, of the Code to keep pace, there had been a
growing dissatisfaction with it” (Schaffner 1964). Thus, in 1960, the Building Commissioner requested
the New York Building Congress to form a working committee to study the problem. The committee
recommended that the Code should not be rewritten by a group of volunteers and that a local educational
institution should conduct a study to develop an approach to solve the problem. The Polytechnic Institute
of Brooklyn conducted the study, and in July 1961, the Institute made the following recommendations
(Schaffner 1964):

The NYC Building Code should be completely rewritten. The new Code should provide for
frequent periodic revision through a committee or board appointed solely for this purpose.

The historical information about the development the New York City Building Code may be found at the New York City/the

Buildings Department web site (www.nyc.gov).

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NIST NCSTAR 1-1, WTC Investigation

The new Code should be a combination of performance and specification types with heavy
emphasis on performance, wherever possible, and with liberal reference to accepted national
standards.

The BOCA Basic Building Code should be used as a guide for the development of the NYC
Building Code.

The Code should be rewritten by a private professional group such as an engineering
company, architectural firm, educational institution, or any combination of the three. Those
rewriting the Code should work closely with the NYC Building Department. They should be
supported, for review purposes, by volunteer committees composed of representatives of
professional, trade, and industry associations.

In April 1962, New York City signed an agreement with the Polytechnic Institute of Brooklyn for the
writing of a new Code to be completed in 3 years. The first draft was completed in 1964. A public
relations document highlighted the “major advantages to be gained from recommendations in the
proposed new Building Code” (Bell and Stanton 1964). One of these related to “area and height
limitations,” and it was stated that:

Area and height limitations will be liberalized and present unrealistically
high construction requirements for fire protection in structures of low
combustible content such as auditoriums, halls, schools, institutions and
residences will be significantly reduced and considerable economy will
result.

On December 6, 1968, Local Law 76 repealed the 1938 code and replaced it with the 1968 Code, which
itself has been subsequently amended by Local Laws. As is the general custom with changes to building
codes, the new provisions generally are not applied to existing buildings (those approved under the prior
code) provided they do not represent a danger to public safety and welfare.

Between 1969 and 2002, there were 79 Local Laws adopted that modified the 1968 code. Of particular
importance with regard to fire protection and life safety are Local Law 5, adopted in 1973, and Local
Law 16, adopted in 1984 (see NIST NCSTAR 1-1D). Local Law 5, among other things, added
requirements on compartmentation of large floor areas, and Local Law 16 added requirements for
sprinklers in high-rise buildings (greater than 100 ft). Local Law 5 is particularly significant because its
provisions, which are reviewed in Sec. 11.1, applied retroactively to existing office buildings taller than
100 ft in height. Local Law 84, which was passed in 1979, revised the compliance dates of Local Law 5
so that full compliance was required by February 7, 1988.

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NIST NCSTAR 1-1, WTC Investigation

3.3

PORT AUTHORITY POLICIES FOR DESIGN AND MODIFICATIONS TO
BUILDINGS

3.3.1

Procedures for PANYNJ Owned Projects

Established in 1921, the Port of New York Authority (PONYA)

was a self-supporting, public interstate

agency and is not subject to the local laws of jurisdictions where its properties are constructed. This
means that for the construction of the WTC buildings, the PONYA was not bound by the NYC Building
Code or any regulations requiring inspection or approval of the building construction or operation. The
PONYA could establish its own requirements, conduct its own inspections, and enforce its own rules
without independent oversight.

The PONYA established an office to act as the Authority Having Jurisdiction for their facilities generally,
and there was a special office for the towers. The PONYA staff reviewed and approved plans, monitored
construction, and developed specifications. They developed a series of manuals that described the
building infrastructure (sprinkler systems, fire alarm systems, smoke control systems) and how tenants
would interface systems in their space to the building. Large tenants were generally permitted to contract
for their own systems as long as they were compatible and complied with the manuals. Smaller tenants
could use the PONYA office for this purpose. In either case approvals and inspections were performed by
the PONYA and did not involve the City services (Department of Buildings or Fire Department).

To reaffirm and formally state the Port Authority’s “long standing policy” that its facilities meet or
exceed New York Building Code requirements, a memorandum of understanding between the Port
Authority and the New York City Department of Buildings was established in 1993.

Specific

commitments were made by the Port Authority to the Department of that would ensure that any building
construction project undertaken by the Port Authority or by any of its tenants at the buildings owned and
operated by the Port Authority that were located within the Department of Buildings’ jurisdiction conform
to the NYC Building Code.

A summary of the 1993 agreement follows:

•

The Port Authority was to thoroughly review and examine all plans for conformance with the
requirements of the then current NYC Building Code. Such reviews were to be conducted by
New York State licensed professional engineers or architects retained or employed by the Port
Authority. Plans for projects undertaken by Port Authority tenants were to be prepared and
sealed by a New York State licensed professional engineer or architect retained or employed
by the tenant. Similarly, for projects undertaken by the Port Authority, plans were to be
prepared and sealed by a New York State licensed professional engineer or architect retained
or employed by the Port Authority.

•

The Port Authority was to maintain a file containing the most recent drawings, plans, and
other documents required in connection with the review of the project for code conformance.

In 1972, PONYA’s name was changed to the Port Authority of New York and New Jersey (PANYNJ).

Memorandum of Understanding between the New York City Department of Buildings and PANYNJ, 1993 (WTCI-160-P, see

Appendix A).

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NIST NCSTAR 1-1, WTC Investigation

•

The Port Authority was required to obtain the certification of a New York State licensed
professional engineer or architect that any tenant projects undertaken at any of its facilities
was constructed in accordance with the approved plans and specifications for the project. Such
certification was to be kept in the project file described in item 2 above.

•

The Port Authority was required to provide copies of any project files to the Department of
Buildings at any time.

•

The Port Authority was to promptly advise the Department of Buildings of any variances from
code requirements that were proposed on a project. In cases where the Department of
Buildings believed that such variances were unacceptable, further review by the Port
Authority Board of Commissioners was required.

•

The Port Authority was required to perform building inspections and structural integrity
inspections on a cyclical basis for all of its structures located in New York City.

•

The Port Authority was responsible for life safety in buildings at its facilities. The Department
of Buildings was not responsible for any type of inspection or review.

•

Personnel from the Port Authority and the Department of Buildings were not to be held
personally responsible under any provision of this agreement.

A supplement to the 1993 agreement was executed in 1995.

The supplement added that the design

professional responsible for performing the review and certification of plans for WTC tenants must not be
the same design professional providing certification that the project had been constructed in accordance
with the plans and specifications. But the plans were to be approved by the Port Authority and held for
possible inspection by the City if the Port Authority so chose.

3.3.2

Review of Tower Plans by New York City Department of Buildings

While the Port Authority facilities, including the WTC buildings, were not required to undergo review or
approval by the NYC Department of Buildings, a letter dated February 18, 1975, from Joseph Solomon of
Emory Roth & Sons (the code architect for the towers) to Malcolm Levy, General Manager, World Trade
Center Operations states, “The Building Department reviewed the tower drawings in 1968 and made six
comments concerning the plans in relation to the old code. Specific answers noting how the drawings
conformed to the new code with regard to these points were submitted to the Port Authority on March 21,
1968.”

NIST has attempted to locate the March 21, 1968, letter without success. NIST hoped to gain information
about the six points and the level of review provided by the NYC Department of Buildings because they
were under no obligation to conduct any review. However, NIST located a letter dated January 25, 1968,
from Mr. Solomon to Mr. Levy that appears to list the six items questioned in the NYC Department of
Buildings’ review (note that the letter states five points and contains five numbered paragraphs, which are

Supplement to Memorandum of Understanding between the New York City Department of Buildings and PANYNJ (1995)

(WTCI-113-P; see Appendix A).

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NIST NCSTAR 1-1, WTC Investigation

followed by an additional point in an unnumbered paragraph).

The copy of this letter provided by

PANYNJ is illegible. Both the original and the NIST reconstructed copies are shown in Appendix B.

It is interesting to note that all six points raised deal with egress issues. They do not address innovative
features of the building, and egress from Windows on the World is not mentioned even though the
restaurant was a part of the design from the beginning.

3.3.3

Procedures for Tenant Alteration Projects

To maintain structural integrity and fire safety, the Port Authority developed a tenant alteration process
for any modifications to leased spaces in WTC 1 and WTC 2 for tenants who would adapt their spaces to
their own needs. In 1971, shortly after the first tenants occupied WTC 1 in December 1970 and before
initial occupancy of WTC 2 in 1972, the Port Authority issued the first edition of a set of requirements the

Tenant Construction Review Manual

. The manual contained the technical criteria to be used in planning

alterations (architectural, structural, mechanical, electrical, fire protection, and others). Applicable
standards to be used by tenants and their agents and review criteria to be used by the Engineering
Department of the Port Authority were included. Registered design professionals were to complete
alteration design, and at the completion of the work, as-built drawings were to be submitted to the Port
Authority. The manual referenced the 1968 NYC Building Code, and specific code provisions were
referenced in various checklists. The review manual was updated in 1979, 1984, 1990, and 1997, at
which times changes that had been made to the NYC Building Code were incorporated. In 1998, the
manual was replaced by the

Architectural and Structural Design Guidelines, Specifications, and Standard

Details

, which dealt specifically with alterations to WTC 1 and WTC 2.

Since WTC 7 was built as a “tenant alteration project,” its design and construction followed the
requirements of the 1984 edition of the

Tenant Construction Review Manual

. Any modifications to the

building after initial occupancy were carried out in accordance with the Manual.

Letter dated January 25, 1968 from Joseph H. Solomon (Emery Roth & Son) to Malcolm P. Levy (General Manager,

World Trade Center Operations) (see Appendix B).

NIST NCSTAR 1-1, WTC Investigation

Chapter 4

ODE

ROVISIONS FOR

TRUCTURAL

ESIGN

This chapter presents a summary of the provisions for structural design in the 1968 edition of the New
York City (NYC) Building Code, and comparison of structural provisions of this code with similar
provisions of other contemporaneous codes. As previously noted in Chapter 1, the design of the World
Trade Center (WTC) towers was based on the 1968 Code, and so was the design for WTC 7. The
contemporaneous codes compared include the 1964 New York State Building Construction Code
(NYSBC 1964), the 1965 Building Officials and Code Administrators (BOCA) model building code
(Basic Building Code [BBC]), and the 1967 Municipal Code of Chicago (MCC 1967). A comparison
was also made between the 1968 NYC Building Code and the current (2001) NYC Building Code. The
current NYC Building Code (NYCBC 2001) consists of the code adopted in 1968 with modifications
made over the years by adoption of Local Laws.

This chapter also provides a summary of the criteria used for the design of WTC 1, 2, and 7. Only those
provisions that relate to the design of WTC 1, 2, and 7 are discussed here. Unless otherwise noted,
referenced article and section numbers are from the 1968 edition of the NYC Building Code.

4.1 CONTEMPORANEOUS

CODES

Three contemporaneous codes were selected for code comparison. The 1964 New York State Building
Construction Code was selected, as it would have been a governing building code outside the New York
City limits. The 1965 BOCA Basic Building Code was selected as it was typically adopted by local
jurisdictions in the northeastern region of the United States. The 1968 NYC Building Code is compared
with the 1967 Municipal Code of Chicago to note any substantial differences in the structural and fire
safety requirements of the two codes. In the late 1960s and early 1970s, several tall buildings were built
in Chicago including the Sears Tower (110 stories) and the John Hancock Tower (100 stories). In
addition, the 2001 edition of the NYC Building Code is compared with the 1968 version to examine the
extent to which Local Laws have modified the code provisions, and in most cases, is only addressed in
areas where changes have occurred between the two versions.

A provision by provision comparison was made between the 1968 NYC Building Code and these four
codes and documented in NIST NCSTAR 1-1B,

Comparison of Building Regulatory and Code

Requirements for WTC 1, 2 and 7

. The only code provisions compared were the requirements related to

structural stability. This chapter presents a summary of substantial differences noted in the comparison.
This summary focuses on the following topics:

•

Loads to be considered in the design of buildings.

•

Requirements for materials, design, and construction.

With respect to structural stability, no Local Law other than Local Law 17 (seismic provisions for new
construction) has been adopted that modified the structural requirements of the 1968 NYC Building Code.

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NIST NCSTAR 1-1, WTC Investigation

Hence, comparison between the structural requirements of the 1968 and 2001 NYC Building Code is not
discussed here, with the exception of earthquake loads.

4.2 LOADS

A key aspect of any structural design is the loading that the structure is intended to support. Building
codes provide minimum values for the different types of loads that are considered in typical building
designs. The designer is permitted to use larger values for these loads but is not permitted to use smaller
values without approval by the building official. This section compares the specified loads in the selected
codes. Similarities and differences are noted.

4.2.1 Dead

Loads

Dead loads refer to loads that are permanently present in a building. They include, for example, the
weight of the structural components, the weights of permanent partitions, the weights of floor and wall
finishes, and the weights of service equipment that is part of the building (elevator equipment, plumbing,
electrical, heating, air conditioning, and ventilation systems). Weights of the structural components are
computed from the sizes of the members and the densities of the materials, and codes typically provide
default density values for different materials. The dead loads of partitions and walls are typically
prescribed in terms of weight per unit area of wall, and the weight per unit length of wall or partition is
determined from these prescribed values and the heights of the partitions or walls. Floor finishes and
ceilings are typically specified in terms of a uniform load per unit area of floor or ceiling. Table 4–1
gives examples of the minimum values of dead load prescribed in Reference Standard RS 9-1 in the 1968
New York City (NYC) Building Code and in Appendix J of the 1965 BOCA Basic Building Code. There
are no corresponding provisions in the 1964 New York State Building Construction Code or the
1967 Municipal Code of Chicago. All building codes permit the designer to use weights based on
available data that are greater than the specified minimum values in the code, but the designer is not
permitted to use lower values without approval of the Code Official.

According to the 1968 NYC Building Code, weights from service equipment (plumbing stacks, piping,
heating, ventilating, and air conditioning [HVAC], etc.) are to be included in the dead load (C26-901.2).

The weight of equipment that is part of the occupancy of a given area is to be considered as live load (see
next section). The 1964 New York State Building Construction Code and the 1967 Municipal Code of
Chicago do not have a provision in this regard. The 1965 BOCA Basic Building Code has a similar
provision but does not cite specific types of service equipment as the NYC Building Code.

The 1968 NYC Building Code requires that weights of partitions be considered in two ways: (1) using
line loads at locations shown on plans or (2) using the equivalent uniform load given in Reference
Standard RS 9-1. The stipulated equivalent uniform load depends on the partition weight, for example, if
a partition weighs 201 plf to 350 plf, it may be taken into account by designing for a uniform load of
20 psf. The uniform loading approach, however, is not permitted in certain situations for which actual
partition weights must be used. Equivalent uniform loads must be used in areas where the locations of
partitions are not shown on plans, or in areas where partitions can be relocated. The 1964 New York

Refers to section number in the 1968 New York City Building Code.

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NIST NCSTAR 1-1, WTC Investigation

Table 4–1. Examples of dead loads given in NYC Building Code and BOCA Code.

NYC

BOCA

Walls and Partitions

Hollow concrete block – 8 in. thick
Clay tile, nonload bearing – 8 in. thick
Plaster partition, metal studs and lath, gypsum plaster both sides

53 psf
34 psf
18 psf

50 psf
36 psf
18 psf

Floor Finishes

Resilient flooring
Hardwood flooring 7/8 in. thick (1 in. for BOCA)
Cement, 1 in. thick

2 psf
4 psf

12 psf

2 psf
4 psf

12 psf

Ceilings

Suspended acoustical tile
Suspended metal lath and gypsum plaster

2 psf
9 psf

–

10 psf

Miscellaneous Materials

Marble
Concrete (normal density stone or gravel)
Reinforced concrete (normal density)

168 pcf

144 pcf
150 pcf

168 pcf
144 pcf
150 pcf

a. Note that the units in the 1968 NYC Building Code are given incorrectly as “psf.”

State Building Construction Code does not have a specific provision in this regard. The 1967 Municipal
Code of Chicago prescribes a minimum partition load of 20 psf. The BOCA Basic Building Code
requires consideration of the actual weight of the partitions or an equivalent uniform load of at least
20 psf.

4.2.2 Live

Loads

Live loads are those resulting from the use and occupancy of the building, and include loads such as
weights of occupants, furniture, filing cabinets, safes, mechanical equipment, and other items that the
structure is called upon to support. Live loads are specified in terms of weight per unit of floor (or roof)
area or in terms of concentrated loads. The values specified in codes are based largely on load survey
data, experience, and judgment.

Floor Live Loads

In general, values of minimum uniformly distributed live loads specified in codes are organized on the
basis of use or occupancy of spaces, and there is no consistency in the names of these use categories.
Thus, comparison between codes is not straightforward. Table 4–2 gives some examples of minimum
uniformly distributed live loads for floors. It is seen that there is general agreement in the values of these
selected minimum uniform live loads specified by the four codes.

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NIST NCSTAR 1-1, WTC Investigation

Table 4–2. Comparison of uniform live load values. Examples of minimum uniformly

distributed live loads.

1968 NYC

1964 NYS

1967 Chicago

1965 BOCA

Office space

50 psf

Restaurant

100 psf

–

100 psf

Lobbies

100 psf

Stairways

100 psf

75–100 psf

100

psf

Rest rooms

40 psf

60 psf

–

Hospital operating room

60 psf

40 psf

60 psf

School classroom

40 psf

60 psf

40 psf

–

a. Depends on occupancy, for example, 75 psf for business, 100 psf for schools.

The codes also specify concentrated live loads placed so as to result in maximum stresses.

Live-Load Reduction

There is a low likelihood that the full design floor live loads will be present on all floors of a building at
the same time. In addition, the likelihood that the complete area any one floor is loaded with the design
live load decreases as the floor area increases. To account for these factors, building codes permit “live-
load reductions” in calculating the design loads for primary members (columns and girders) that support
the roof and floors. The codes use several methods for live-load reduction (CTB&UH 1980):

Percentage Method

—In this method, the live-load reduction increases by a certain percentage

with increasing numbers of floors, with a limit on the maximum value of reduction (typically
50 percent).

Tributary Area Method

—The live load is reduced as the accumulated tributary area that a

member supports is increased. The limiting value depends on the ratio of live load to dead
load. The type of occupancy affects whether a reduction is permitted.

Live Load to Dead Load Ratio

—The permitted reduction depends on the ratio of live load to

dead load, provided that the dead load is greater than the live load.

The 1968 NYC Building Code uses the tributary area method and permits the percentage method as an
alternative for columns, piers, and walls. The 1964 New York State Building Construction Code and the
1967 Municipal Code of Chicago use the tributary area method for beams and girders and the percentage
method for columns and walls. The 1965 BOCA Basic Building Code uses a tributary method that is
similar to the New York State Code.

Figure 4–1 compares the reduced live load for columns, walls, and piers on the basis of the percentage
method for three of the codes. The permitted reductions are similar with the exception of the roof and top
floor, where the 1968 NYC Building Code and the 1967 Municipal Code of Chicago are more
conservative (less reduction permitted) than the 1964 New York State Building Construction Code.

Table 4–3 compares the reduced live loads for beams and girders for the selected codes. For the 1968
NYC Building Code, the reduced value of live load for a given contributory area depends on the live load

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NIST NCSTAR 1-1, WTC Investigation

to dead load ratio, with lower values permitted for lower live load to dead load ratios. For the 1964 New
York State Building Construction Code and the 1965 BOCA Basic Building Code, the values shown in
the table are based on a reduction factor of 0.08 percent/ft

. The lowest reduced value, however, is

limited to 40 percent or

100

−

(4–1)

whichever is larger, where

L/D

is the live load to dead load ratio. As the ratio of live load to dead load

increases, less live-load reduction is permitted. A comparison of the values in Table 4–3 shows that the
1967 Municipal Code of Chicago did not permit as large a reduction in live load for the same contributory
area as the other codes.

4.2.3 Wind

Load

The effect of wind on buildings is accounted for in the building codes by specifying a uniform pressure to
be applied horizontally to a building. These pressures are to be applied in any direction so as to obtain the
most critical loading condition.

The pressure due to wind varies with the square of the wind speed, and wind speed increases with height.
Thus building codes specify minimum design wind pressures that increase with elevation. The variations
of pressure with height, however, are not the same among the building codes compared. Figure 4–2
compares the specified wind pressure versus height relationships for the four selected codes. Several
observations are noted:

•

For buildings up to 600 ft in height, the 1964 New York State Building Construction Code
prescribes the largest wind pressures.

•

The 1967 Municipal Code of Chicago prescribes the lowest wind pressures for buildings up to
900 ft in height.

•

The 1968 NYC Building Code and the 1965 BOCA Basic Building Code provide similar wind
pressures for buildings up to 700 ft in height; for taller buildings the BOCA Code specifies
larger pressures.

For a building height of 1,370 ft (the approximate heights of WTC 1 and WTC 2), the wind pressure
distribution specified by the 1965 BOCA Basic Building Code would result in the largest shear force and
overturning moment at the base of the building.

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NIST NCSTAR 1-1, WTC Investigation

Table 4–3. Reduced live load for beams and girders.

Contributary Area

(ft

)

1968 NYC

Building Code (%)

1967 Chicago

Municipal Code (%)

1956 NY State and

1965 BOCA Codes (%)

100 or less

100

100–149 100

100

150–199 80

95 84

200–299 80

90 76

300–449 60

85 64

450–599 50

85 52

600 and more

40 to 65

85 40

a. Permitted value depends on live load to dead load ratio; less reduction permitted with higher ratio.
b. The lowest value is limited to 40 percent, or 100 percent of (3.33 L/D –1)/(4.33 L/D), whichever is greater.

Assuming wind is blowing in the direction perpendicular to the face of the tower, a comparison using the
specified wind pressures from the aforementioned codes reveals that the largest shear force at the base of
a building the height of the WTC towers is obtained from the BOCA Basic Building Code. Similarly, the
largest overturning moment at the base of a building the height of the WTC towers is also obtained from
the BOCA Basic Building Code. The lowest base shear and moment are obtained from the 1968 and
2001 New York City Codes. The base shear from the New York City Codes is approximately 8 percent
less than that from the BOCA code, while the base moment is approximately 11 percent less (see
Table 4–4).

Table 4–4. Base shears and overturning moments from reviewed codes for a building the

height of WTC towers (1,368 ft).

1968

NYC Building

Code

2001

NYC Building

Code

1964 NY State

Code

1967 Chicago

Municipal

Code

1965

BOCA/BBC

Base

Shear

(kip)

9,250 9,250 9,460 8,610 9,970

Overturning

Moment

(ft kip x 10

footing)

7,621 7,621 7,572 7,446 8,470

The 1968 NYC Building Code permits the designer to use wind pressure values, other than specified
minimums, on the basis of wind tunnel tests and with approval of the building official.

The following

wording is provided in Sec. 6 of Reference Standard RS 9-5, “Minimum Design Wind Pressures.”

In lieu of the design wind pressures established in sections 1 and 2 of this
reference standard, and subject to review and approval of the
commissioner, design wind pressures may be approximated from
suitably conducted model tests. The tests shall be predicated on a basic
wind velocity of 80 mph at the 30 ft level, and shall simulate and include
all factors involved in considerations of wind pressure, including
pressure and suction effects, shape factors, functional effects, gusts, and
internal pressures and suctions.

See Sub-article 904.0, the1968 New York City Building Code.

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The other three contemporaneous codes do not have a similar provision for conducting model tests to
determine the design wind pressure.

Thus the 1968 NYC Building Code presumes a wind with a speed of 80 mph measured 30 ft above the
ground. The 1964 New York State Building Construction Code, on the other hand, states that the
prescribed wind loads “are based on a design wind speed of 75 mph at a height of 30 ft above grade
level.” Both the 1965 BOCA Basic Building Code and the 1967 Municipal Code of Chicago do not
specify the design wind speed.

4.2.4 Earthquake

Load

The 1968 NYC Building Code did not have provisions for earthquake loads. Among the selected
contemporaneous codes, only the 1965 BOCA Basic Building Code had earthquake load provisions.
These are contained in Appendix K-11 of that Code and were adapted from the 1962 edition of the
Uniform Building Code.

The 2001 edition of the NYC Building Code contains seismic design provisions from the 1988 edition of
the Uniform Building Code (UBC 1988), including the 1990 Accumulative Supplement. These provisions
were put into effect in 1996 as a result of Local Law 17 (1995). Significant modifications to the 1988
Uniform Building Code were made, and described in Reference Standard RS 9-6.

For example, the paragraph on “Minimum Seismic Design,” is modified to read:

The following types of construction shall, at a minimum, be designed and
constructed to resist the effects of seismic ground motions as provided in this
section:

new structures on new foundations;

new structures on existing foundations; and

enlargements in and of themselves on new foundations.

Buildings classified in New York City occupancy group J-3 and not
more than three stories in height need not conform to the provisions of
this section. The Commissioner may require that the following types of
construction be designed and constructed to incorporate safety measures
as necessary to provide safety against the effects of seismic ground
motions at least equivalent to that provided in a structure to which the
provisions of the section are applicable:

new buildings classified in occupancy group J-3 and which are three
stories or less in height; and

enlargements in and of themselves where the costs of such enlargement
exceeds sixty percent of the value of the building.

In the subdivision on “Criteria Selection” the following paragraph was added:

Seismic Zone. The seismic zone factor, Z, for buildings, structures and portions
thereof in New York City shall be 0.15. The seismic zone factor is the effective
zero period acceleration for S

type rock.

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NIST NCSTAR 1-1, WTC Investigation

Another significant amendment is the addition of consideration of soil liquefaction that was not found in
the Uniform Building Code.

4.2.5 Other

Loads

Temperature and Shrinkage

The 1968 NYC Building Code included provisions dealing with types of loadings not considered in the
other codes that were compared. Two examples are “thermal forces” and “shrinkage.”
Section C26-905.7 deals with thermal forces and includes the following requirement:

…For exterior exposed frames, arches, or shells regardless of plan
dimensions, the design shall provide for the forces and/or movements
resulting from an assumed expansion and contraction corresponding to
an increase or decrease in temperature of forty degrees F for concrete or
masonry construction and sixty degrees F for metal construction…

Section C26-905.8 on shrinkage includes the following requirement:

The design of reinforced concrete components shall provide for the
forces and/or movements resulting from shrinkage of the concrete in the
amount of 0.0002 times the length between contraction joints for
standard weight concrete, and 0.0003 times the length between
contraction joints for lightweight concrete….

Abnormal loads (Progressive collapse consideration)

The 1968 NYC Building Code did not have provisions for design against progressive collapse of
buildings due to abnormal loads. Abnormal loads would include explosions resulting from ignition of gas
or industrial liquids, vehicle impacts, gross construction errors, and the like. In response to the collapse
of a concrete panel building in Ronant Point, England in 1968, the NYC Building Code by rule

adopted

the progressive collapse provisions in August 2, 1973. However, on August 7, 1973, the Department of
Buildings issued a memorandum to clarify the type of structures to which the new progressive collapse
provisions apply. These include structures with connections that rely on friction due to gravity loads to
transfer tension, compression and shear forces in the structural members. Thus, for cast-in-place concrete
construction having adequate joint reinforcement, the new progressive collapse provisions would not
apply. Similarly, for structural steel construction with bolted, riveted or welded connections to transfer
tension, compression and shear forces, the provisions would not apply. In practical sense, the new
provisions would apply to precast construction wherein joint forces are transferred by friction developed
by gravity loads.

The rules intrepret the code to clarify the intent of the code.

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NIST NCSTAR 1-1, WTC Investigation

4.2.6

Distribution of Loads

Another topic that is addressed only in the 1968 (and 2001) NYC Building Code is the distribution of
loads, which is covered in Article 7 of Sub-chapter 9. Section C26-906.1 deals with vertical loads and
states:

Distribution of vertical loads to supporting members shall be determined
on the basis of a recognized method of elastic analysis or system of
coefficients of approximation. Elastic or inelastic displacements of
supports shall be considered and, for the distribution of dead loads, the
modulus of elasticity of concrete or composition [composite] sections
shall be reduced to consider plastic flow. Secondary effects due to
warping of the floors shall be considered.

Section C26-906.2 deals with distribution of horizontal forces. Because this section provides important
information in the design assumptions to be used in the design of high-rise buildings, several key sections
are repeated here:

The following provisions shall apply to superstructure framing only, and
shall not apply to structures wherein horizontal loads are transmitted to
the foundation by staycables, arches, non-rectangular frames, or by
frames, trusses, or shear walls not oriented in vertical planes.

(a) Distribution of horizontal loads to vertical frames, trusses and
shear walls.

- Horizontal loads on the superstructure shall be assumed to

be distributed to vertical frames, trusses, and shear walls by floor and
roof systems acting as horizontal diaphragms. The proportion of the total
horizontal load to be resisted by any given vertical frame, truss, or shear
wall shall be determined on the basis of relative rigidity, considering the
eccentricity of the applied load with respect to the center of resistance of
the frames, trusses, or shear walls. For vertical trusses, web
deformations shall be considered in evaluating the rigidity.

(b) Distribution of horizontal loads within rigid frames of tier
buildings.

(1) ASSUMPTIONS. - The distribution of horizontal loads within rigid
frames of tier buildings may be determined on the basis of a recognized
method of elastic analysis or, subject to limitations in paragraph two of
this subdivision, may be predicated on one or more of the following
simplifying assumptions:

a. Points of inflection in beams or columns are at their midspan and
midheight, respectively. The story shear is distributed to the columns in
proportion to their stiffnesses.

b. The change in length of columns due to axial effects of the horizontal
loads may be neglected.

c. Vertical column loads due to horizontal forces are taken by the
exterior columns only, or are resisted by the columns in proportion to the
column distances from the neutral axis of the bent.

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(2) LIMITATIONS. -

a. For buildings over 300 ft in height, the change in length of the
columns, due to the effects of the horizontal loads, shall be evaluated or
the framing proportioned to produce regular movements of the
successive joints at each floor so that warping of the floor system may be
neglected.

b. Simplifying assumptions used in design shall be subject to approval
by the commissioner for any of the following conditions or
circumstances:

1. For buildings over 300 ft in height or for buildings with a height-

width ratio greater than five.

2. At two-story entrances or intermediate floors.

3. Where offsets in the building occur.

4. Where transfer columns occur.

5. In any similar circumstances of irregularities or discontinuities in the

framing.

4.3 DESIGN

STANDARDS

Article 10 of the 1968 NYC Building Code is entitled “Structural Work,” and it provides minimum
requirements for materials, design, and construction of all structural elements in buildings. Section 4.3.1
compares design standards in the selected building codes. Section 4.3.2 discusses design load
combinations that were specified in the selected building codes.

4.3.1 Design

Standards

Design standards are those documents that are used to proportion the structural elements and their
connections. The principal structural materials in the WTC buildings were concrete and steel, and the
design standards were those produced by the American Concrete Institute (ACI) and the American
Institute of Steel Construction (AISC). The ACI produced the standard known as ACI 318,

Building

Code Requirements for Reinforced Concrete

and the AISC produced the following:

•

Specification for the Design, Fabrication and Erection of Structural Steel for Buildings
(AISC 1963)

•

Specifications for Structural Steel Buildings–ASD and Plastic Design (AISC 1989)

•

Load and Resistance Factor Design Specifications for Structural Steel Buildings (AISC 1993)

In 1999, the title was changed to

Building Code Requirements for Structural Concrete.

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Table 4–5 summarizes the concrete and steel design standards adopted by the codes that were compared.
The 1964 New York State Building Construction Code was a performance standard and did not adopt
design standards by reference. Thus, at the time the WTC towers were being designed, the other two
codes (Chicago and BOCA) referenced the same concrete and steel design standards as the New York
City code.

Table 4–5. Design standards for concrete and steel.

Material

1968 NYC Code

2001 NYC Code

1967 Chicago

Code

1965 BOCA

Code

Concrete

ACI 318-63

ACI 318-89

ACI 318-63

Steel

AISC 1963

AISC 1989
AISC 1993

AISC 1963

The 1963 edition of ACI 318 permits reinforced concrete members to be designed by either the working
stress (or allowable stress) method or by the ultimate strength method. The 1963 AISC specification, on
the other hand, is based on allowable stress design. The design method affects the loads used in the
design calculations.

4.3.2 Load

Combinations

The loads prescribed by the codes are used in different combinations to assess the governing design
condition. The codes distinguish between sustained loads and loads of short duration or infrequent
occurrence. For allowable stress design, two approaches are used for dealing with these two categories of
loads, as will be discussed. For ultimate strength design, the prescribed loads are multiplied by specified
load factors. In either case, the designer considers all applicable load combinations and determines the
most critical condition, which becomes the design basis for a particular element.

Allowable Stress Design

The 1968 NYC Building Code defines two categories of loads:

•

Basic loads, which include dead load, live load, and reduced live load where applicable; and

•

Loads of infrequent occurrence, which include wind load, thermally induced load, shrinkage
induced load, and unreduced live load where live load reduction is permitted.

Under the 1968 NYC Building Code, stresses in structural elements may not exceed the allowable values
specified in the referenced design standards under the following load combinations

•

The sum of the basic loads multiplied by a factor equal to 1.

•

The factored sum of one or more basic loads and one load of infrequent occurrence, where the
load factor equals 0.75.

See Section C26-1001.4 of the 1968 NYC Building Code.

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•

The factored sum of one or more basic loads plus two or more loads of infrequent occurrence,
where the load factor equals 0.6.

The 2001 NYC Building Code is similar with the exception that it includes earthquake load as another
load of infrequent occurrence.

The other Codes that were compared use a different approach for dealing with loads of infrequent
occurrence. The 1964 New York State Building Construction Code states that stress due to wind load
may be ignored if it is less than one-third of the stress due to dead load plus imposed load excluding wind
load. If the stress due to wind load exceeds this limit, the allowable stress for the material is permitted to
be increased by 1/3.

The 1967 Municipal Code of Chicago uses a similar approach and states: “For combined stresses due to
dead, live, and wind load, the allowable stresses in materials may be increased 1/3, provided the section
thus determined is at least as strong as that required for dead and live load alone. Snow load shall be
considered a live load.”

The 1965 BOCA Basic Building Code is similar except that wind load or earthquake load is considered
along with dead load and live load (including snow load). The same 1/3 increase in allowable stress is
permitted under wind or earthquake load. The BOCA Code also explicitly states that wind load is
permitted to be neglected if it results in stress less than one-third the stress due to dead load plus live load.

Ultimate Strength Design

In the 1960s, ultimate strength design was standardized only for reinforced concrete. As shown in
Table 4-5, the three codes from the 1960s referenced ACI 318-63, which includes the following load
combinations to establish the design loads (U) for structural members:

1. For structures where wind and earthquake loads may be neglected, U = 1.5 D + 1.8 L.

2. For structures where wind load must be included, U = 1.25 (D + L) or U = 0.9 D+ 1.1 W,

whichever produces the most unfavorable condition for the member.

3. For structures where earthquake loading is included, E shall be substituted for W in

condition 2.

4. In structures where effects of shrinkage and temperature are included, the effects of such

items shall be considered on the same basis as the effects of dead load.

The 2001 NYC Building Code refers to ACI 318-99, which includes many more load combinations to be
considered. These are as follows:

1. For all structures, U = 1.4 D + 1.7 L.

2. For structures where wind load must be included, U = 0.75[1.4 D + 1.7 L + 1.7 W)] or

U = 0.9 D + 1.3 W, whichever produces the most unfavorable condition for the member.

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NIST NCSTAR 1-1, WTC Investigation

3. For structures where resistance to earthquakes must be included, the load combinations of

condition 2 are used with 1.1 E substituted for W.

4. For structures where resistance to earth pressure (H) must be included,

U = 1.4 D + 1.7 L + 1.7 H or 0.9 D + 1.7 H, whichever produces the most unfavorable
condition.

5. For structures where resistance to fluid pressure (F) must be included,

U = 1.4 D + 1.7 L + 1.4 F or 0.9 D + 1.7 F, whichever produces the most unfavorable
condition.

6. For structures where resistance shrinkage and temperature (T) must be included,

U = 0.75 (1.4 D + 1.4 T + 1.7 L) > 1.4 (D + T).

7. For structures where resistance to impact must be taken into account, such effects shall be

included with live load L.

4.4

ALTERATION OF EXISTING BUILDINGS

The compared codes have provisions to address code compliance when existing buildings are altered.
The provisions of all codes, other than the 1964 New York State Building Construction Code, are broadly
similar. In general, whether the altered building or only the alternations need to comply with code
requirements depends on the ratio of alterations to the total building expressed either in terms of cost or
dimensions. When the ratio is low, even the alterations may not have to be in compliance with the code,
provided stipulated conditions are met. The 1964 New York State Building Construction Code, however,
requires that any addition or alteration, regardless of building value, shall be made in conformity with that
code. It is silent as to the structure being altered. Table 4–6 summarizes code provisions related to
alterations.

Table 4–6. Compliance requirements for alterations.

Code Provisions

1968 New York City
Building Code

Alterations exceeding 60 percent of building value (in any 12 month period):

The

entire building shall be made to comply with the requirement of the code.

Alterations between 30 percent and 60 percent of building value (in any 12 month
period):

Only those portions of the building altered shall be made to comply with the

requirements of the code.

Alteration under 30 percent of building value (in any 12 month period):

Those

portions altered may, at the option of the owner, be altered in accordance with the
requirement of the code, or altered in compliance with their previously required
condition and with the same or equivalent materials and equipment, provided the
general safety and public welfare are not thereby endangered.

2001 New York City
Building Code

Same as 1968 Code, except that wording for alterations

less than 30 percent

building values was changed to: “those portions of the building altered may, at the
option of the owner, be altered in accordance with the requirements of this code, or
altered in compliance with the applicable laws in existence prior to December sixth,
nineteen hundred sixty-eight, provided the general safety and public welfare are not
thereby endangered.”
In addition, certain alterations are required to conform to the code regardless of

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Code Provisions

magnitude or cost. These include, among others:
Alterations to standpipes, sprinklers, or interior fire alarm and signal systems;
Alterations to equipment for heating or storing water;
Sprinkler, alarm protection, and emergency lighting requirements for places of
assembly.

1964 New York State
Building Construction
Code

Addition or alteration:

Any addition or alteration, regardless of cost, made to a

building shall be made in conformity with applicable regulations of the code.

1967 Municipal Code
of Chicago

More than 50 percent:

Such buildings and structures shall be made to conform to all

requirements of the code that are applicable to new buildings and structures.

25 percent to 50 percent:

All new constructions shall conform to the requirements of

the code for new buildings or structures of like area, height and occupancy.

25 percent or less:

Certain exceptions can be made that allow the use of materials that

conform to the strength and fire resistance for the materials with which the building is
constructed. Otherwise, all new construction shall conform to the requirements of this
code for a new building.

1965 BOCA Basic
Building Code

“In the reconstruction, repair, extension or alteration of existing buildings, the
allowable working stresses used in design shall be as follows:
1.

Building extended:

If altered by an extension in height or area, all existing

structural parts affected by the addition shall be strengthened where necessary and all
new structural parts shall be designed to meet the requirements for buildings hereafter
erected.
2.

Building repaired:

When the uncovered structural parts are found unsound, such

parts shall be made to conform to the requirements for buildings hereafter erected.
3.

Existing live load:

When an existing building heretofore approved is altered or

repaired within the limitation prescribed in Sec. 106.3 (alteration under 50 percent) and
106.4 (alteration under 25 percent), the structure may be designed for the loads and
stresses applicable at the time of erection, provided that public safety is not
endangered.
4.

Posted live load:

May be posted for original approved live loads.”

4.5

MATERIALS AND METHODS OF CONSTRUCTION

The compared codes have requirements for the materials and construction methods. Each code makes
distinctions in materials and methods that depend on the nature of inspection and conformance with
standards.

The 1968 NYC Building Code prescribes testing and inspection requirements for all materials,
assemblies, forms, and methods of construction. A distinction is made between materials and methods
subject to “controlled inspection” and those that are not subject to controlled inspection. Materials and
methods subject to controlled inspections “shall be inspected and/or tested to verify compliance with code
requirements.” In general, activities related to controlled inspections “shall be made and witnessed by or
under the direct supervision of an architect or engineer retained by or on behalf of the owner or lessee,
who shall be, or shall be acceptable to, the architect or engineer who prepared or supervised the
preparation of the plans.” On the other hand, materials and methods not designated for controlled
inspection “shall be inspected and/or tested to verify compliance with code requirements by the person
superintending the use of the material or its incorporation into the work…”

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The 1968 NYC Building Code provides tables to indicate which materials and methods are subject to
controlled inspections and which are not. Table 4–7 includes excerpts from the requirements for
inspection of materials and assemblies. A footnote to the table in the code states that “All structural
materials and assemblies subject to controlled inspection shall be tested and/or inspected at their place of
manufacture and evidence of compliance with the provisions of this subchapter shall be provided as
stipulated in sub-articles 1003.0 through 1011.0.” Table 4–8 is an excerpt of the inspection requirements
for methods of construction. A footnote to the companion table in the code states that “All construction
operations designated for controlled inspection shall be inspected by the architect or engineer designated
for controlled inspection during the performance of such operation.”

Table 4–7. Excerpts of inspection requirements for materials and assemblies in

Article 10 of 1968 NYC Building Code.

Material Elements

Subject

to Controlled Inspection

Elements Not Subject to Controlled

Inspection

Steel

None

All structural elements and connections

Concrete

Materials for all structural elements
proportioned on the basis of calculated
stresses 70 percent or greater, of basic
allowable stresses. See Sec. 1004.0 for
specific requirements relating to “quality
control of materials and batching.”

(1) All materials for all structural elements
proportioned on the basis of calculated stresses
less than 70 percent or greater of basic allowable
values.
(2) Concrete materials for:

(a) Short span floor and roof construction
proportioned as per Sec. 1004.8.
(b) Walls and footings for buildings in
Occupancy Group J-3.

(3) Metal reinforcement.

The 1968 NYC Building Code required that the installation of “sprayed-on fire protection” of structural
members (except those encased in concrete) be subjected to controlled inspection requirements, as
defined above. There were, however, no specific provisions on what testing was required.

The 1964 New York State Building Construction Code and the 1965 BOCA BBC make distinctions
between “controlled” and “ordinary” materials in reference to establishing allowable stresses. For
example BOCA defines “controlled materials” as those that are “certified by an accredited authoritative
agency as meeting accepted engineering standards for quality.” Ordinary materials are those that do not
conform to the requirements for controlled materials.

The 1967 Municipal Code of Chicago specifies that all materials and methods used in the design and
construction of buildings shall be classified as “controlled materials” or “ordinary materials.” According
to the Chicago Code, “controlled materials” means a building, structure, or part thereof, which has been
designed or constructed under the following conditions: (a) All controlled materials must be selected or
tested to meet the special strength, durability and fire resistance requirements upon which the design is
based. (b) The design, preparation of working drawings, including details and connections, the checking
and approval of all shop and field details and the inspection of the work during construction shall be
under the supervision of a registered architect or structural engineer (Sec. 69-3.1).

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Table 4–8. Excerpts of inspection requirements for methods of construction in

Article 10 of 1968 NYC Building Code.

Material

Operations Subject to Controlled

Inspection

Operations Not Subject to Controlled

Inspection

Steel

(1) Welding operations and the tensioning of
high strength bolts in connections where the
calculated stresses in the welds or bolts are 50
percent or more of basic allowable values.
(2) Connection of fittings to wire cables for
suspended structures, except where cables
together with their attached fittings are proof-
loaded to not less than 50 percent of ultimate
capacity.

(1) Welding operations and the tensioning of
high strength bolts in connections where the
calculated stresses in the welds or bolts are less
than 50 percent of basic allowable values.
(2) All other fabrication and erection operations
not designated for controlled inspection.

Concrete

Except for those operations specifically
designated in this table as not subject to
controlled inspection, for all concrete, the
operations described in Sec. 1004.5(a) shall be
subject to controlled inspection.”

(1) All operations relating to the constriction of
members and assemblies (other than prestressed
concrete) which involve the placement of a total
of less than 50 cubic yards of concrete and
wherein said concrete is used at levels of
calculated stress 70 percent or less of basic
allowable values.
(2) placing and curing of concrete for all:

(a) short span floor and roof construction as
per Sec. 1004.8.
(b) Walls and footings for buildings in
Occupancy Group J-3.

(3) Size and location of reinforcement for walls
and footings in Occupancy Group J-3.
(4) All other operations not described in
Secs. C26-1004.5(a).

4.6

STABILITY, BRACING, AND SECONDARY STRESSES

The 1968 and 2001 NYC Building Codes are the only codes of those compared that include provisions for

stability

bracing

, and

secondary

stresses

. The provisions are the same in the two editions of the code.

Stability, in this case, refers to resistance to sliding or overturning of the building on its foundation. The
NYC Building Code requires a factor of safety of 1.5 against failure by sliding or overturning. The
required stability is to be provided solely by the dead load plus any permanent anchorage that is provided.
Bracing refers to lateral support to prevent buckling of compression members (columns and walls). The
NYC Building Code requires that the bracing be proportioned to resist a load of at least 2 percent of the
total design compression load in the braced member plus any transverse shear load on the bracing
member. Secondary stresses refer to stresses associated with transverse deflection of a member. In
trusses, for example, secondary stresses arise because joints are not true pins, and some bending is
introduced, which results in transverse displacements of the individual elements. The NYC Building
Code requires that secondary stresses in trusses be considered in designing the size of the individual
elements.

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4.7 DEFLECTION

LIMITATIONS

All five codes contain limits on vertical deflections of floor and roof assemblies. Except for the NYC
Building Codes (both the 1968 and 2001 versions), the deflection limits relate to crack formation of
plastered building components. The deflection is limited to 1/360 of the span for plastered members and
1/240 of the span for non-plastered members. The NYC Building Codes refer to the reference standards
for deflection limits in addition to the 1/360 of the span limit. For concrete members, ACI 318-63
specifies limits for both short- and long-term deflections of beams and one-way slabs. For steel members,
the 1963 AISC Specification specifies deflection limits to avoid damage to plastered ceilings and to limit
deflections of flat roofs.

4.8 LOAD

TESTS

Building codes generally allow load tests to ascertain the adequacy of load carrying capacity of structural
members. Specifically, building codes allow load tests or tests of in-place materials:

•

To verify adequacy of structural design for a member or an assembly;

•

To verify adequacy of partially completed construction;

•

To prequalify structural members or assemblies before used in service;

•

To verify adequacy of questionable completed structure; and

•

To determine concrete strength by means of core tests.

The NYC Building Codes have provisions to cover all five categories. The New York State Code had
provisions for (1) and (4). The Chicago Municipal Code had provisions for (1), (4) and (5). The
BOCA/Basic Building Code had provisions for (1) and (2).

NIST NCSTAR 1-1, WTC Investigation

Chapter 5

TRUCTURAL

ESIGN OF

WTC

AND

5.1

DESIGN CRITERIA

As stated in Sec. 1.1, the design of World Trade Center (WTC) 1 and WTC 2 was governed by the second
and third drafts of the 1968 New York City (NYC) Building Code. The 1968 Code also governed the
design of WTC 7. However, different design values were allowed by the Building Code if they were
more conservative than minimum design requirements specified in the Building Code.

In a number of cases, the design of the WTC 1 and WTC 2 were based on values that were more
conservative than those specified in the 1968 NYC Building Code, such as live loads for the tenant spaces
outside the central core area and wind loads for the towers. These will be presented in further detail
below. No design calculations are available for review of the actual design criteria used for the design of
WTC 7. The materials presented in this chapter pertain mainly to WTC 1 and WTC 2.

5.1.1 Loads

As presented in Chapter 4, the building codes specify minimum design values for vertical and lateral
loads. In the NYC Building Code, Chapter 26, Article 9 prescribes the minimum loads to be used in the
design of buildings and their parts. Section C26-900.2, Standards, refers to Reference Standard RS-9 for
the minimum dead, live, and wind loads, which are incorporated by reference into Article 9. In no case
does the Code allow for the loads used in design to be less than the minimum values contained in that
article. In this section, actual design loads used for design are presented and compared with the New York
City Code requirements.

Dead Loads

The unit dead loads specified for the various structural members are contained in the Design Criteria for
WTC 1 and WTC 2 (WSHJ 1965a). Different criteria were established for members located inside the
core and outside the core.

Floor Inside of Core

The core area in a representative upper floor of WTC 1 and WTC 2 is illustrated in Fig. 2–13. Unit
design dead loads for the beams, columns, and slabs within the core area of the towers are summarized in
Fig. 5–1.

In all cases, the dead loads in the design criteria were greater than or equal to the

corresponding dead loads prescribed in the Code. Examples of design dead loads in the 1968 NYC
Building Code are listed in Table 4–1. A comprehensive list of the dead loads prescribed in the Code is
given in Annex A1 of the report entitled

Comparison of Building Code Structural Requirements

(NIST

NCSTAR 1-1B). For equivalent uniform loads for partitions (according to C26-901.3(b) of the NYC

In Fig. 5–1, “contact” fireproofing is listed. This is a type of fireproofing that is sprayed on to steel members.

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Building Code), the equivalent uniform partition loads in Reference Standard RS 9-1 may be used in lieu
of actual partition weights when partitions are not shown on the plans. The actual values for design are
given in the design criteria shown in Fig. 5–2. As allowed by the Code, the actual partition loads, which
were less than specified 20 psf in the Code for a partition of 201 plf to 350 plf, were used in the design of
WTC 1 and WTC 2.

Floor Outside of Core

Unit dead loads for areas outside of the core are specified in the design criteria for the following structural
members: one-way long-span floor trusses, one-way short-span floor trusses, two-way floor trusses,
beams on framed floors, bridging, columns, steel deck, and reinforced concrete slabs. The design criteria
vary depending upon the floor level. Figure 5–3 contains sample design criteria for the long-span floor
trusses at typical floor levels. For a further description of dead loads used in design, see NIST
NCSTAR 1-2. The dead loads in the design criteria for all of the structural members were greater than or
equal to the corresponding dead loads prescribed in the Code.

Design Criteria for WTC 7

Design load criteria for WTC 7 are summarized in Fig. 5–4. These criteria appear on Sheet S-24, Typical
Superstructure Sections and Details, in the structural drawings (The Office of Irwin G. Cantor 1983).
Because the actual materials used for the partitions, flooring, and ductwork were not specified, the
reasonableness of these design values cannot be ascertained.

Live Loads

Design Criteria for WTC 1 and WTC 2

Specified live loads are given in the Design Criteria for WTC 1 and WTC 2 (WSHJ 1965a). As in the
case of dead loads, different live-load criteria were established for members located inside the core and
outside the core.

•

Floor inside of core.

Live loads to be used in the design of the beams and columns within the

core area are summarized in Fig. 5–5, Fig. 5–6, and Fig. 5–7. As can be seen from the figures,
except for floor 109 and areas occupied by equipment, the design live load varied from 40 psf
to 100 psf. For all occupancies or use of spaces common to the design criteria and the Code,
the live loads in the design criteria were equal to the corresponding live loads prescribed in the
Code (which are given in Annex A1 of NIST NCSTAR 1-1B/

Comparison of Building Code

Structural Requirements

•

Floor outside of core.

Like the unit dead loads, design live loads outside of the core area

varied with the floor level. At most floor levels, a design live load of 100 psf was specified
for the slabs (see Fig. 5–8 from the Design Criteria). At mechanical floors 7, 41, 75, and 108,
a 75 psf live load was used. Figure 5–9 shows sample design criteria for the columns at the
floor levels noted in the figure. Live loads specified in the design criteria were equal to or
greater than the corresponding live loads prescribed in the Code. It should be noted that the
100 psf live load used is twice the design live load specified in the NYC Building Code.

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Table 5–1 compares live loads used for the design of floors with corresponding values specified in the
1968 NYC Building Code. In most cases, they are the same. Major differences are noted for the design
live loads for corridors within the core, tenant spaces outside of the core, and passenger elevator lobbies
on the tenant floors. Note that the design live load for the tenant spaces are twice the code specified value.

Table 5–1. Live loads used in design of WTC 1 and WTC 2.

Use of Spaces

1968

NYC Code

(psf)

WTC Design

Criteria

(psf)

Cafeteria 100

100

Closets (tenant floors)

100

Concourse 100

100

Corridors within core (mechanical equipment
floor)

75 100

Corridors within core (skylobby floor)

100

Corridors within core (typical office floor)

Duct offset space

Electric closet

Electric substation & transformer room

Expansion tank room

Janitor’s closets

100

Kitchen 100

100

Local passenger elevator lobbies (skylobby floors)

100

Main shuttle elevator lobbies (skylobby floors)

100

Mechanical equipment rooms

Men’s toilets

Observation lobby

100

Tenant space outside core

100

Passenger elevator lobbies (tenant floors)

100

Powder rooms

Restaurant 100

100

Roof 30

Secondary motor rooms

Service room (mechanical equipment floor)

100

Service room (tenant floor)

100

Sprinkler tank room

Stairs 75

100

Telephone closets

Tenant spaces within core

Woman’s toilets

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NIST NCSTAR 1-1, WTC Investigation

Design Criteria for WTC 7

As noted previously, design criteria for WTC 7 are summarized in Fig. 5–4. These criteria appear on
Sheet S-24, Typical Superstructure Sections and Details, in the structural drawings (The Office of Irwin
G. Cantor 1983). For the floor levels where the type of occupancy was noted on Sheet S-24, the live
loads in the design criteria were equal to those given in the Code.

5.1.2

Live Load Reduction

Code Requirements

Provisions for live-load reduction in the 1968 NYC Building Code are contained in Sub-article 903.0,
Live Load Reduction. According to C26-903.1, live load reduction is not permitted on roofs. The
allowable reduced live load for floor members is determined by multiplying the basic live load value from
Reference Standard RS 9-2 (see above) by the percentages given in Table 9-1 of the Code, which is
reproduced in Table 5–2. These percentages are a function of the contributory floor area, which is defined
in C26-903.3, and the ratio of live load to dead load.

Table 5–2. Percentage of live load per the 1968 NYC Building Code.

Ratio of Live Load to Dead Load

Contributory

Area (ft

)

0.625 or less

2 or more

149 or less

100

150–299 80

300–449 60

450–599 50

600 or more

a. For intermediate values of live load/dead load, the applicable percentages of live load

may be interpolated.

Contributory floor areas are computed as follows (C26-903.3):

•

For one-way and two-way slabs: product of the shorter span length and a width equal to one-
half the shorter span length. Ribbed slabs shall be considered as though the slabs were solid.

•

For flat plate or flat slab construction: one-half the area of the panel.

•

For columns, girders, or trusses framing into columns: the loaded area directly supported by
the column, girder, or truss. For columns supporting more than one floor, the loaded area
shall be the cumulative total area of all the floors that are supported.

•

For joists and similar multiple members framing into girders or trusses, or minor framing
around openings: twice the loaded area directly supported but not more than the area of the
panel in which the framing occurs.

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NIST NCSTAR 1-1, WTC Investigation

No live load reduction is permitted (C26-903.2(b)) for members and connections (other than columns,
piers, and walls) supporting:

•

Floor areas used for storage (including warehouses, library stacks, and record storage);

•

Areas for parking of vehicles;

•

Areas for places of assembly, for manufacturing; and

•

Areas for retail or wholesale sales.

The maximum live load reduction is 20 percent for columns, piers, and walls supporting such areas.

Live-load reduction is also not permitted for calculating shear stresses at the heads of columns in flat slab
or flat plate construction (C26-903.2)).

As an alternative procedure, live load reduction for columns, piers, and walls may be taken as 15 percent
on the top floor, increased successively at the rate of 5 percent on each successive lower floor, with a
maximum reduction of 50 percent. For girders supporting 200 ft

or more of floor area, the live-load

reduction is 15 percent.

Design Criteria for WTC 1 and WTC 2

Live-load reduction criteria from the Design Criteria for WTC 1 and WTC 2 are given in Fig. 5–9
(WSHJ 1965a). The figure shows the percentage of design live load from the Design Criteria that was
used in the design of structural members. For floor members, these percentages were the same as those
from the 1968 Code, except in the case where the live load to dead load ratio was 2 or more and the
loaded area tributary to the floor member was between 150 ft

and 299 ft

; in this case, the code-

prescribed percentage is 85 percent, while the value in the Design Criteria was 90 percent, which is more
stringent than the code requirement (see Fig. 5–10).

Figure 5–11 shows the design live loads from the Design Criteria for the tenant areas inside of the core.
The solid line represents the reduced live load that was used in the design of the beams; these values were
computed in accordance with the live-load reduction provisions in the Design Criteria (see Fig. 5–10).
The unreduced live load specified in the Design Criteria for tenant spaces inside the core was 100 psf,
which matches the design live load shown in Fig. 5–12 for tributary areas up to 200 ft

. Also included in

this figure are two other sets of data points: one set represents the reduced live load computed in
accordance with the 1968 Code provisions with a live load to dead load ratio equal to one and the other
set is the Code equivalent uniform load for partitions, which is a constant 6 psf for partition weights up to
100 plf. The Code requires a 50 psf live load in tenant areas (office areas without storage) per Reference
Standard RS 9-2. The 50 psf live load plus the 6 psf partition load is shown in the figure for tributary
areas up to 150 ft

. Figure 5–12 clearly shows that the design live loads specified in the Design Criteria,

including live load reduction, were greater than those required by the Code for office areas without
storage.

Figure 5–13 contains the design criteria for live load reduction for the floor areas outside of the core for
the floor levels that are noted in the figure. These criteria are the same as those for the tenant space inside

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of the core (see Fig. 5–12). Similar criteria were also provided in the Design Criteria for other floor
levels.

5.1.3 Wind

Load

In lieu of using its prescribed pressures, the 1968 NYC Building Code allows “suitably conducted model
tests” to establish design wind pressures, subject to review and approval of the Building Commissioner
(Item 6 in Reference Standard RS 9-5). The tests are to be based on a basic (fastest-mile) wind velocity
of 80 mph at 30 ft above ground and are to simulate and include all factors involved in consideration of
wind pressure, including pressure and suction effects, shape factors, functional effects, gusts, and internal
pressures and suctions.

Design Criteria for WTC 1 and WTC 2

Design wind forces on the towers were determined based on a series of wind tunnel tests that were
conducted at the Colorado State University (CSU) and the National Physical Laboratory (NPL) in the
United Kingdom. Specific details on these tests can be found in NIST NCSTAR 1-1A.

The design wind loadings of the exterior walls of WTC 1 and WTC 2 consisted of shear forces and
overturning moments that were computed at each floor level in the two principal directions of the towers
due to the equivalent design wind velocity of 98 mph from 24 wind directions equally spaced at
15 degrees intervals around the tower. The equivalent design wind velocity was defined as the mean wind
velocity averaged over a 20 min period at a height of 1,500 ft above the ground and was based on a
50 year return period.

The shear forces

and overturning moments

at each floor level were comprised of static and dynamic

components:

′

(5–1)

where the first and second terms indicate, respectively, the mean or steady-state components and the
dynamic components. The static components of the shear and moments were calculated from the
following equations.

)

(

)

(

)

(

)

(

DHC

(5–2)

where:

design air density = 0.0023 slugs per cubic foot

mean design wind velocity = 98 mph averaged over 20 min at a height of 1,500 ft above

ground

Draft for Public Comment

Structural Design of WTC 1, 2, and 7

NIST NCSTAR 1-1, WTC Investigation

shear force coefficients from wind tunnel tests

overturning moment coefficients from wind tunnel tests

plan dimension of building

height

building

The dynamic components of the shear forces and overturning moments at any height

were calculated

from the following equations.

)

(

)

(

)

(

)

(

)

(

∫

′

(5–3)

In the first of these equations,

is the natural frequency of oscillation of the building, and

is the

amplitude of oscillation at the top of the tower corresponding to a mean design wind velocity. The
quantity )

(

is the mass per unit height of the building, and

)

(

is the mode amplitude at height

for

unit amplitude at the top of the building. Using sets of shear and overturning moment coefficients
obtained from the wind tunnel tests (WSHJ 1966a), the shear forces and overturning moments at each
floor were computed.

A comparison of the base shear and moment obtained from using the wind pressures from the 1968 NYC
Building Code and the wind tunnel test results are shown in Table 5–3. The code-based values of base
shear and overturning moment occur simultaneously on the same face of the tower, whereas the base
shear and the overturning moment obtained from the wind tunnel tests represent the largest values related
to most unfavorable wind direction, thus they may not occur simultaneously on the same face of the
tower. For the description used to compute the values based on the wind tunnel tests, see NIST
NCSTAR 1-2. The wind load used to design the towers are greater than that based on the code specified
wind pressure values.

Table 5–3. Base shears and overturning

moments based on the 1968 NYC Building Code

and wind tunnel tests.

1968

NYC Building Code

Wind

Tunnel Tests

Base Shear

(kip)

9,250 13,100

Overturning

Moment

(10

ft kip)

7,621 12,600

For external cladding and glazing, design wind pressures were specified in the WTC Design Criteria.
Outward (negative) pressure acting normal to the surface varied from 65 psf below the 7th floor to
125 psf at the 109th floor. Inward (positive) pressures varied from 45 psf below the 7th floor to 55 psf at

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NIST NCSTAR 1-1, WTC Investigation

the 108th floor. These pressures are based on the results of a series of wind tunnel tests that were
performed specifically for this purpose (WSHJ 1967a).

Design criteria were also established for the antenna mast located on top of WTC 1 (WSHJ 1973). The
antenna and its components were to be designed for the following conditions:

•

A mean wind speed of 140 mph in any direction and no ice coating;

•

A mean wind speed of 110 mph in any direction with an ice coating of ½ in. over all exposed
unheated metallic surfaces with a minimum air temperature of 20

•

A mean wind speed of 110 mph in any direction and no ice coating under a range of air
temperatures from 10

F to 90

•

A mean wind speed of 40 mph in any direction and no ice coating under a range of air
temperatures from –15

F to 105

F; and

•

Dynamic effects of wind associated with the mean wind speeds specified above (dynamic
effects of wind gusts were obtained by multiplying the mean wind forces by a factor of 5).

The requirement of a ½ in. thick coating of ice is consistent with the requirement in C26-905.6 of the
1968 NYC Building Code for the design of open-framed or guyed towers. Also, the NYC Code requires
that exterior exposed frames, arches, or shells be designed for the forces and/or movements resulting from
an increase or decrease in temperatures of 60

F for metal construction (C26-905.7). These requirements

are less stringent than those contained in the design criteria. The design criteria contain a section on how
the wind forces were computed based on these velocities.

Design Criteria for WTC 7

No design criteria or calculations were available for WTC 7 with respect to wind loads. However, a wind
tunnel study of WTC 7 was carried out in 1983 by the University of Western Ontario at the request of the
structural engineer of record, Irwin G. Cantor, Consulting Engineers (Isyumov 1983). No document is
available to show whether the wind tunnel test results were used in design of WTC 7.

5.1.4 Aircraft

Impact

No building code in the United States has specific design requirements for impact of an aircraft, and thus,
buildings are not specifically designed to withstand the impact of fuel-laden commercial aircraft.
However, since the collision of a B-25 bomber into the Empire State Building in 1945, designers of high-
rise buildings have become aware of the potential of the crash of aircrafts into buildings. A three-page
document from the Port Authority of New York and New Jersey (PANYNJ or Port Authority) indicates
that the impact of a Boeing 707 aircraft flying at 600 mph was analyzed during the design stage of the
WTC towers in February/March 1964.

Letter with an attachment dated November 13, 2003 from John R. Dragonette (Retired Project Administrator, Physical

Facilities Division, World Trade Department) to Saroj Bhol (Design and Engineering Department, PANYNJ).

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NIST NCSTAR 1-1, WTC Investigation

No documents on the aircraft impact analysis are available to review the criteria and method used in the
impact analysis of a Boeing 707 aircraft on the WTC tower and to verify the assertion in the three-page
document that “…such collision would result in only local damage which could not cause collapse or
substantial damage to the building and would not endanger the lives and safety of occupants not in the
immediate area of impact.” Without the original calculations of the aircraft impact analysis, any
comment on the document would be a speculation. In March 1964, a calculation was made by the Port
Authority to determine the period of vibration of the tower due to an aircraft impact at the 80th floor.

Although no conclusion was stated on the calculation sheet, it clearly indicates that the Port Authority
recognized during the design stage the possibility of an aircraft impact on the tower.

5.2

STRUCTURAL DESIGN REQUIREMENTS

According to sub-article 1002.0 of the NYC Building Code (Adequacy of the Structural Design), the
design of structural members was to conform to the applicable material standards mentioned in sub-
articles 1003.0 through 1011.0 (C26-1002.1). If such computations as prescribed in these standards
cannot be executed due to “practical difficulties,” the structural design can be deemed adequate if the
member or assembly performs satisfactorily when subjected to load tests in accordance with 1002.4(a).
Provisions to determine the adequacy of completed or partially completed structures are also provided.
Prequalifying load tests (C26-1002.4(a)) can be used to establish the strength of a member or assembly
prior to having such members or assemblies incorporated into a structure. The test specimens are to be a
true representation of the actual members or assemblies in all aspects, including the type and grade of
material used. Support conditions for the members or assemblies being tested are to simulate the
conditions of support in the building, except that conditions of partial fixity might be approximated by
conditions of full or zero restraint, whichever produces a more severe stress condition in the member
being tested. In regard to strength requirements, the member or assembly must be capable of supporting
the following (note: no specific reference to a particular type of building material is given in this section
of the Code):

Without visible damage (other than hairline cracks) its own weight plus a test load equal to
150 percent of the design live load plus 150 percent of any dead load that will be added at the
site, and

Without collapse its own weight plus a test load equal to 50 percent of its own weight plus
250 percent of the design live load plus 250 percent of any dead load that will be added at the
site.

The latter loading is to remain in place for a minimum period of one week, and all loading conditions in
Article 9 of the Code are to be considered. Exceptions to the above load conditions are also given in this
section.

The member or assembly is also subject to the following deflection requirements: the recovery of the
deflection caused by the superimposed loads listed in item 1 above must be at least 75 percent. Also, the
deflection under the design live load is limited to the values prescribed in C26-1001.5.

A three-page calculation dated March 2, 1964 by E. Liu (Structural Engineer, the Port of New York Authority)

(WTCI-408-LERA)

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The Code also gives requirements for tests on models less than full size. The similitude, scaling, and
validity of the analysis are to be attested to by an officer or principal of the firm or corporation making the
analysis. The firm or corporation is to be approved by the Building Commissioner.

5.2.1 Concrete

Requirements

According to sub-article 1004.0, design of reinforced concrete structural members was to conform to the
requirements in that section and Reference Standard RS 10-3, which is the 1963 edition of

Building Code

Requirements for Reinforced Concrete

(ACI 1963) with modifications, which was applicable to the

design of WTC 1 and WTC 2. These modifications include the replacement of the requirements of
ACI 318 Secs. 902 (Design loads) and 903 (Resistance to wind, earthquake, and other forces) with the
following: “Building code requirements for loads and infrequent stress conditions shall apply.”
“Infrequent stress conditions” refer to such conditions as wind and earthquake. In other words, all loads
are to be determined in accordance with the 1968 Code. In case of concrete structures designed by the
ultimate strength design method, design (factored) loads are to be determined in accordance with
Sec. 1506 of ACI 318-63.

According to the specifications for WTC 7 (WTC 7 Project Specifications 1984), the 1983 edition of
ACI 318 was applicable (ACI 1983).

5.2.2 Steel

Requirements

Design of steel structural members was to conform to the requirements in sub-article 1005.0 and
Reference Standard RS 10-5, which is the 1963 edition of

Specification for the Design, Fabrication, and

Erection of Structural Steel for Buildings

(AISC 1963b) with modifications, which was applicable to the

design of WTC 1 and WTC 2. Similar to the design of reinforced concrete members, the NYC Building
Code replaced the provisions of Sec. 1.3 (Loads and Forces) of the AISC Specification with a statement:
“The provisions of the building code for loads shall apply.” Other notable modifications to the AISC
Specification are:

•

The following paragraph is added to the definition of composite construction in Sec. 1.11.1:
“Concrete materials shall meet the applicable requirements of the building code. Where
concrete having a unit weight less than 130 pcf is used, the capacity of the shear connectors to
resist applied load under the proposed conditions of use shall be investigated…”

•

Sec. 1.25.5 on field connections during erection is deleted and replaced with the following:
“…No holes, copes or cuts of any type shall be made to facilitate erection unless specifically
shown on the shop drawings or authorized in writing by the party or parties designated for
inspection of such work.”

The 1968 NYC Building Code requires that Reference Standards RS 10-6 and 10-7 be used for light
gauge cold formed steel and open web steel joists, respectively (see

Comparison of Building Regulatory

and Code Requirements for WTC 1, 2, and 7

[NIST NCSTAR 1-1B]).

According to the specifications for WTC 7 (WTC 7 Project Specifications 1984), the 1978 edition of the
AISC Specification was applicable (AISC 1978).

NIST NCSTAR 1-1, WTC Investigation

Chapter 6

NNOVATIVE

EATURES

NCORPORATED IN

TRUCTURAL

ESIGN

6.1 INNOVATIVE

FEATURES

A number of innovative features, which were applied to the design of a super high-rise steel building for
the first time, were incorporated in the structural design of World Trade Center (WTC) 1 and WTC 2.
They were incorporated in both the lateral-load-resisting system and the gravity-load-carrying system.

These features include the following:

•

Application of the framed-tube system to resist lateral loads.

•

Uniform exterior column geometry (14 in. by 14 in. cross-section) was maintained over most
of the height of the 110-story buildings by using 12 different grades of steel.

•

Use of deep spandrel plates as beam elements connecting perimeter columns.

•

Use of long-span composite steel trusses for the floor system to develop diaphragm action in
super tall buildings and to develop composite action by extending truss diagonals into the
concrete slab.

•

Application of sprayed fire resistive materials on open-web steel trusses for fire protection.

•

Application of viscoelastic dampers connecting the floor trusses to the perimeter framed tube
system to control dynamic response.

•

Use of wind tunnel test data to establish the wind loads used in the design of the towers.

Several prominent features are described below in detail.

6.2 LATERAL-LOAD-RESISTING

SYSTEM

The structural design of high-rise buildings (over 40 stories) is usually controlled by lateral loads. It is
well known that for high-rise buildings, the most efficient way to resist lateral loads is by mobilizing the
exterior framing system. As described in Sec. 2.2.2, the lateral-load-resisting system of WTC 1 and
WTC 2 used the framed-tube concept wherein the lateral loads are resisted by the exterior frames. A
framed-tube system does not depend on shear walls or other bracing systems to resist lateral loads.
Typically, the exterior wall is comprised of moment resisting frames with closely spaced columns and
deep spandrel beams to form a Vierendeel-truss-type structural form.

In the United States, the first application of a framed-tube system was the 43-story DeWitt-Chestnut
apartment building (later renamed The Plaza on DeWitt) in Chicago, which was completed in 1965. This
building used reinforced concrete for the structural framing system. Since then, many variations of this

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NIST NCSTAR 1-1, WTC Investigation

structural system have been used in a number of buildings. WTC 1 and WTC 2 were the first super high-
rise steel buildings that were designed using the framed-tube concept.

The framed-tube system of WTC 1 and WTC 2 was comprised of closely spaced steel columns that were
connected by deep spandrel plates. To assess the stiffness characteristics of the wall panel, a series of
model tests using one-quarter scale models made of thermoplastics were carried out prior to final design
of the frame-tube system (Gardner 1966). The model tests allowed the evaluation of changes in the
overall stiffness of the wall panels as the sizes of the members that made up the wall panels varied, which
included columns, spandrels, and stiffeners. The results of the model tests guided the design of the wall
panels. Detailed descriptions of the tests are given in NIST NCSTAR 1-1A.

The columns and spandrels were shop-assembled and welded into 36 ft high by 10 ft wide panels, which
consisted of three columns and three spandrels as shown in Fig. 2–9. These panels were erected on site
by bolting the base plate of an upper column to a cap plate of a lower column (see Fig. 2–10). Such
splices were staggered so that only one-third of the panels were spliced at each story level, except at the
base of the building and at the mechanical floors where all of the panels were spliced at the same level. In
such cases, supplemental welds were employed to improve connection capacity. Spandrels were
connected at midspan with high-strength bolted shear connections.

6.3

COMPOSITE FLOOR SYSTEM

As described in Sec. 2.2.2, outside of the central core area, floor construction of WTC 1 and WTC 2
typically consisted of 4 in. of lightweight concrete on fluted metal deck supported by a series of
composite floor trusses that spanned between the core and the exterior walls. The floor trusses consisted
of double angles that were used for the top and bottom chords and round bars that were used for the
diagonals. What made the floor system in WTC 1 and WTC 2 innovative was that (1) use of the
lightweight composite floor system, comprised of lightweight concrete slab on long-span open-web steel
trusses, to provide lateral stability of columns and diaphragm action in super tall buildings,
(2) development of composite action by extending truss diagonals into the concrete slab (see Fig. 2–14),
and (3) application of sprayed fire resistive materials on open-web steel trusses for fire protection (For
detailed description, see NIST NCSTAR 1-6B).

The first recorded tests on composite open-web steel joists were conducted under a project jointly
sponsored by Granco Steel Products and Laclede Steel Company (who manufactured the trusses for
WTC 1 and WTC 2) in September 1964.

In this study, the overall performance of non-composite joists

was compared with composite joists. The joists were manufactured with their webs projecting above the
top chord. The tests revealed that the composite joists had greater moment capacities and smaller
deflections than the non-composite joists.

Since composite action was achieved by the “knuckle” functioning as a shear connector, a test program
was carried out by Laclede Steel Company to determine the failure loads of the shear knuckles. The shear
knuckle tests are described in detail in NIST NCSTAR 1-1A. The test results indicated that shear
strengths of the knuckles were found to be well over the allowable values used in the design of the
composite trusses.

See Sec. 1.1 of Sen and Galambos (1968).

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NIST NCSTAR 1-1, WTC Investigation

Additional tests on open-web joists were performed at Washington University (Tide and Galambos 1968).
The findings, which were reported in February of 1968, were similar to those reported from the previous
tests. In particular, the specimens with extended web diagonals into the concrete slab serving as shear
connectors were shown to be strong and stiff, and failure was due to crushing of the concrete near the
connectors. Further tests conducted at Washington University are reported in Sen and Galambos (1968).
In summary, the findings from this study confirmed those obtained from earlier research programs.

The composite floor trusses used in the WTC towers were similar to those that were tested only in the
sense that the webs were used as shear connectors. Other than that, they were different in all other
aspects, including member sizes and overall lengths. It may have been the first time that this type of floor
construction was used in a high-rise building, especially of this size.

6.4

VISCOELASTIC DAMPING UNITS

Viscoelastic damping units were used in the structural system of WTC 1 and WTC 2 to supplement the
tubular steel frame in limiting wind-induced building oscillations. According to Mahmoodi (1987), “The
selection, quantity, shape, and location of the dampers was based on the dynamic analysis of the towers
(computer modeling, wind tunnel, etc.) and of the damping required to achieve performance standards.”
This may have been the first application of damping units for this purpose in tall building structures, and
would certainly qualify it as an innovative system at that time.

The damping units were uniformly distributed throughout both of the buildings. One hundred four (104)
dampers were used on each floor from the 7th to the 107th floor. The planned locations of damping units
on the various floors of the buildings are contained in structural drawings D-AB1-2 through D-AB1-14.2
(WSHJ 1967). As the buildings oscillated from the wind, part of the energy of oscillation was dissipated
by shear deformations in the viscoelastic part of the damping units.

Two testing programs were carried out to test the effectiveness and efficiency of the damping units in
controlling building motion due to wind. The Minnesota Mining and Manufacturing Company (3M)
conducted the first set of tests in May of 1967.

The Massachusetts Institute of Technology (MIT)

conducted the second test program during 1968 and 1969.

These tests included variations in

(1) amplitude and frequency of the applied cyclic axial deformation, (2) ambient temperature, and (3) a
static preload superimposed on the simple harmonic loading. In general, it was found that “…the energy
absorbing capabilities of the elements are generally adequate to provide the expected damping under
design conditions and that the elements do perform satisfactorily under limited variations of loading
conditions, speed of oscillation, duration of oscillation, and ambient temperature.” Detailed descriptions
of these tests are given in NIST NCSTAR 1-1A.

Two different types of damping units were used in WTC 1 and WTC 2. Type A damping units were used
on floors with trusses spanning between the core and the outside wall, and were located between the
bottom chords of the floor trusses and the columns of the outside wall (Fig. 2–16). Type B damping units
were used on floors that had wide-flange beams spanning between the core and the outside walls (i.e.,

Letter dated June 22, 1967 and enclosure from Don Caldwell of 3M to Peter Chen of SHCR (WTCI-501-L;see Appendix B of

NIST NCSTAR 1-1A without appendixes that are contained in WTCI-501-L).

“Test Program for World Trade Center Viscoelastic Damping Units,” by Stephen H. Crandall of MIT, May 20, 1968 (WTCI-

501-L, see Appendix B of NIST NCSTAR 1-1A).

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NIST NCSTAR 1-1, WTC Investigation

floors 7, 9, 41, 43, 75, 77, and 107). This type of damping unit was located between the bottom flanges
of the floor beams and the outside wall, as shown in Fig. 6–1. The use of dampers increased significantly
the critical damping ratio of the towers. The reduction of oscillation during strong winds was estimated to
be about 12 percent of the amplitudes without dampers. Expected time period during which building
oscillation might be perceived by the occupants was estimated to be reduced by about 34 percent
(SHCR 1967).

Type B damping units were slightly longer than Type A damping units. Also, the connections between
Type A damping units and the floor trusses were different than those between Type B damping units and
the wide-flange beams. Sheet DA-3 in the structural drawings shows specific details for each type of
damping unit (WSHJ 1967).

Worthington, Skilling, Helle & Jackson (WSHJ) initially inquired about different types of viscoelastic
damping materials in a letter to 3M in 1964.

A follow-up letter from them to 3M contained the physical

and mechanical properties required for the viscoelastic material, based on calculations they had
performed.

Additional correspondence on various aspects of the damping units, including the results of

tests that were run at 3M that measured the properties of the damper material and the strength of an
assembled damping unit prototype, was exchanged subsequent to these letters. In particular, it was noted
that testing of an assembled truss damping unit by 3M was completed and that the results agreed with the
theoretical predictions.

6.5

WIND TUNNEL TESTS

Wind tunnel tests were part of an overall wind program that was developed by WSHJ for the design of the
WTC (WSHJ 1964). Details of the wind program are given in NIST NCSTAR 1-1A. Briefly, the
program consisted of four parts:

•

Meteorological Program

was to determine the mean wind speeds, the return periods, the

magnitude of wind shear and gradient, the directional characteristics of the wind, and the
energy spectra of wind gusts that were expected at the site of the WTC.

•

Wind Tunnel Program

was to (a) develop a physical model of lower Manhattan and subject

the model to wind velocities obtained from the meteorological program, (b) obtain static and
dynamic responses of the WTC towers, (c) study construction problems, and (d) study the
effect of the structural parameters on the integrity of the towers.

•

Structure Damping Program

was to determine the critical damping ratio of the structural

system and to determine ways of increasing this ratio.

Letter dated July 16, 1964 from Alan G. Davenport of WSHJ to Carl A. Dahlquist of 3M (WTCI-450-L; see NIST

NCSTAR 1-1A, Appendix D).

Letter dated November 23, 1964 from Richard D. Steyert of WSHJ to Carl A. Dahlquist of 3M (WTCI-450-L; see NIST

NCSTAR 1-1A, Appendix D).

Internal correspondence dated February 1966 by Richard D. Steyert of WSHJ (WTCI-450-L; see NIST NCSTAR 1-1A,

Appendix D).

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NIST NCSTAR 1-1, WTC Investigation

•

Physiological Program

was to determine acceptable levels of response to wind-induced

excitations as measured by perception levels of a cross-section of the population.

Wind tunnel tests were conducted at Colorado State University (CSU) and the National Physical
Laboratory (NPL), located in Teddington, Middlesex, United Kingdom (WSHJ 1964). Tests were
conducted on single-tower and twin-tower configurations subject to uniform and turbulent flow
conditions.

6.5.1

Tests Conducted at CSU

Over 2,000 tests were conducted at the CSU Microclimatological Wind Tunnel to study the behavior of
static and aeroelastic models (WSHJ 1964). One of the most important requirements in the modeling
process was to achieve correct simulation of the wind velocity profile (considering both surface roughness
and its influence on wind velocity with respect to height) as it approached the model of lower Manhattan.
From the southeast direction, wind traveled across Brooklyn to the site of the WTC, which was a
relatively rough urban area. From the southwest, wind traveled mainly across open water.

Aside from wind velocity, the principal variables in the wind tunnel tests were the following:

•

Spacing of towers

•

Number of towers

•

Damping

•

Wind direction

•

Boundary layer characteristics

•

Relative stiffnesses of the models

It was found that the models oscillated in the wind due to vortex shedding, gust buffeting, and wake
buffeting under certain combinations of the variables listed above.

Two hundred tests were run at CSU to study the effect of tower spacing on the response of the buildings.
It was concluded that the “as planned” spacing was satisfactory.

Aeroelastic tests and measurements of steady pressure for single-tower and twin-tower configurations in
uniform flow provided a comparison between the performance of the models at CSU and at the NPL. The
CSU report concluded that the aeroelastic tests at the two locations were in good qualitative and
quantitative agreement (WSHJ 1965c). Models used for the pressure tests at the CSU were constructed of
clear acrylic plastic at a scale of 1/500, the same scale used in the aeroelastic tests

The aeroelastic tests were designed to determine the predominant sway motion (i.e., deflections or
amplitudes) of the towers and to provide a check of the steady-state component of the overturning
moment at the base. To determine the pressure distribution on the towers, tests were conducted using
models with pressure points along a regular grid. From these tests, shear forces and overturning moments
were obtained along the height of the towers.

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NIST NCSTAR 1-1, WTC Investigation

The tests also indicated that large lateral deflections at the top of the building occurred for wind velocities
in the range of 125 mph to 130 mph for angles of incidence within approximately 10 degrees of normal
(see Fig. 6–2). The results are plotted in Figs. 19 and 20 in WSHJ (1965c). The deflections showed a
consistent dependence on the degree of damping and were shown to be inversely proportional to the
damping ratio.

Tests were also conducted at CSU using the southeast and southwest models of lower Manhattan
subjected to turbulent flow conditions (WSHJ 1966c).

Both single-tower and twin-tower configurations

were considered. Definition of the grid system and tower configurations used in the tests is illustrated in
Fig. 6–3. Also shown in the figure are the experimentally determined fundamental frequencies of the
towers in the two principal directions in cycles per second (cps). Included in these tests were
measurements of the maximum deflections at the tops of the towers (aeroelastic tests; wood models) and
pressures along the height of the towers (thermoplastic models).

Similar to the other tests described above, test results for the single-tower model indicated that the most
severe oscillations were transverse to the wind and occurred with the wind blowing within a small range
of angles on either side of the normal to a face. The results also showed that an increase in turbulence,
which was characteristic of the southeast model of lower Manhattan, appeared to suppress vortex
shedding but gave rise to turbulence excitation with increased wind speed. Finally, it was observed that
greater levels of damping reduced the dynamic response of the single tower in all cases, more so in
uniform flow conditions than in turbulent conditions.

Based on the results obtained from the twin-tower wind tunnel tests, it was concluded that the response of
the WTC towers was governed by three aerodynamic factors: (1) Magnitude of the effective turbulence
forces induced by the wind flow, (2) Magnitude of the effective forces induced by vortex shedding and
turbulence in the structure’s own wake, and (3) Effective aerodynamic damping and coupling forces
generated by the motion of the tower through the airflow. It was also noted that the effective mass, the
effective stiffness, the mode of vibration, and the mechanical damping of the towers influenced these
factors (WSHJ 1966).

A theoretical method was derived and was used to predict the dynamic behavior of the towers
(WSHJ 1966c). Results from the theoretical models were compared to the results from the wind tunnel
tests. A comprehensive discussion on this comparison can be found in WSHJ (1966c).

The results from the wind tunnel tests were used in the design of the exterior columns and spandrels,
which is discussed in Sec. 2.3.2 of this report.

The extensive wind tunnel testing that was performed to establish the lateral wind loads used in the design
of WTC 1 and WTC 2 was state-of-the-art at that time.

The meteorological program found that winds were stronger from westerly and northerly quadrants. Wind from the southeast

direction was chosen in the wind tunnel program not because the velocity from this direction was the greatest, but because
winds from this direction were the most turbulent (wind in this direction traveled over Brooklyn, which is a relatively rough
urban terrain). Turbulence plays an important part in the dynamic excitation of structures, especially tall, slender structures. A
fundamental discussion on turbulence and resulting aeroelastic phenomena can be found in Simiu and Scanlon (1996).

NIST NCSTAR 1-1, WTC Investigation

Chapter 7

ABRICATION AND

ONSTRUCTION

NSPECTIONS AND

ARIANCES

7.1 INTRODUCTION

The contract documents for World Trade Center (WTC) 1 and WTC 2 between Port Authority of New
York and New Jersey (PANYNJ or Port Authority) and the steel fabricators and erector, and the
construction contract specifications for WTC 7, indicate that inspection programs were instituted at the
steel fabrication sites. For WTC 1 and WTC 2, the documents reviewed revealed that the inspection
requirements were part of the contract. For WTC 7, the project specifications list inspection
requirements. The records of inspections for both the WTC 1 and WTC 2 and the WTC 7 projects were
not available to the investigation. According to PANYNJ, the records for WTC 1 and WTC 2, which
were kept in WTC 1, were destroyed, and the records for WTC 7 were discarded by the general contractor
after retaining them for 7 years. In this section, the inspection requirements for WTC 1 and WTC 2 and
for WTC 7 are described briefly. NIST NCSTAR 1-1A provides more detailed description of the
inspection requirements.

7.2

FABRICATION INSPECTION REQUIREMENTS FOR WTC 1 AND WTC 2

As described in Sec. 1.3, the Port of New York Authority (PONYA) instructed the consultants to comply
with the second and third drafts of the 1968 New York City (NYC) Building Code for their designs of
WTC 1 and WTC 2. The Code contains provisions that govern the fabrication and inspection of materials
used in buildings. Thus, in general, the requirements in the specifications of the contracts with various
steel fabricators were equivalent to those in the Code at a minimum. However, in a number of cases, the
contract specifications were more comprehensive and stringent than the corresponding provisions in the
Code. Section C26-1000.7, Material and Methods for Construction, of the Code refers to the
requirements in Reference Standard RS 10-5, which is the 1963 American Institute of Steel Construction
(AISC)

Specification for the Design, Fabrication, and Erection of Structural Steel for Buildings

(AISC 1963). The AISC Specification, Sec. 1.23 contained minimum fabrication requirements for the
following:

•

Straightening of materials

•

Gas cutting

•

Planing of edges

•

Riveted and bolted construction – holes

•

Riveted and high strength bolted construction – assembling

•

Welded construction

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NIST NCSTAR 1-1, WTC Investigation

•

Finishing

•

Tolerances

Specific inspection requirements during fabrication of various structural members were covered in the
contract documents between PONYA and individual fabricators. The individual contract documents
which contain the inspection requirements during fabrication are found in appendixes of NIST
NCSTAR 1-1A.

Some salient features of the fabrication inspection requirements for different structural framing systems
are presented below.

7.2.1 Floor

Trusses

The contract between the PONYA and the Laclede Steel Company, the manufacture of the floor trusses,
contained the specifications for fabrication, including welding of structural steel, and also a quality
control and inspection program (see Appendix E of NIST NCSTAR 1-1A). In addition to the quality
control requirements for steel fabrication in the AISC Specification (AISC 1963), Chapter three of the
contract included a list of specific requirements for inspection during fabrication, including: continual
visual inspection and surveillance of the fabrication process of steel trusses by qualified contractor’s
supervisory personnel; physical and nondestructive testing welding of truss panel points; and full-scale
testing of completely fabricated steel truss components.

7.2.2

Box Core Columns and Built-Up Beams

The contract between the PONYA and the Stanray Pacific Corporation contained the specifications for
the box core columns and built-up beams from the 9th story to the roof. The requirements for fabrication,
including welding of structural steel, inspection, and quality control, were in the contract specification.
Appendix E of NIST NCSTAR 1-1A addresses the applicable sections of the contract specifications and
other quality control requirements in detail.

In addition to the inspection requirements in the contract, special requirements were added for inspection,
testing, coordination, and supervision by an independent testing agency at the fabrication plant before
structural components left the fabrication yard. These additional requirements were necessary because a
major portion of the steel used for the core structural members was to be produced in Japan and
England.

The description of a comprehensive program for “supervision, coordination, inspection, and

testing based on the use of the personnel and facilities of a local independent testing agency supervised by
a resident engineer (a professional engineer employed full time by the structural engineer Skilling, Helle,
Christiansen, & Robertson [SHCR])” was attached to the letter sent from Leslie Robertson of SHCR to
Malcolm P. Levy of PONYA (see footnote 31). The scope of this program was two-fold:

•

To provide the Port Authority assurance through adequate documentation that fabricated steel
conformed to the contract documents and to ensure on-time delivery of fabricated steel.

Letter dated June 5, 1967, from Leslie E. Robertson of SHCR to Malcolm P. Levy of PONYA (WTCI-491-L; see NIST

NCSTAR 1-1A, Appendix E).

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•

To provide detailed inspection by checklist and by non-destructive testing prior to final
acceptance of the members.

The responsibilities of the resident engineer included the following items:

•

Prior to fabrication, performing a complete study of the fabricator’s quality control
procedures, proposed fabrication procedures, provisions for storage of incoming material, and
provisions for loading and shipping of completed building components.

•

Ensuring proper interpretation of the contract drawings and specifications.

•

Directing the work performed by the independent testing agency and its inspectors.

•

Performing surveillance of the quality of work on a continuous basis.

The structural engineer (SHCR) also recommended that an independent testing agency be hired for mill
inspection of Japanese steel.

The main responsibility of the testing agency was to verify the accuracy of

the certified mill testing reports by witnessing tests at the manufacturing mill. Procedures were
established for witnessing the tests at both Stanray Pacific and Pacific Car & Foundry in the United
States. The Port Authority subsequently contracted with Superintendence Inc., an international inspection
agency, who provided the mill inspections in both countries.

The Port Authority set forth requirements for the independent testing portion of the mill inspection
program.

The requirements, which were part of PONYA’s overall quality control program on fabricated

steel for the WTC, depended on whether the steel was from a domestic source or from a foreign source.
For steel obtained from domestic sources, the independent testing portion of the mill inspection program
consisted of the following:

•

For steel with yield points less than 50,000 psi, one tensile test and one check analysis on
samples selected at random from 1 out of 10 heats.

•

For steel with yield points of 50,000 psi and higher, one tensile test, one bend test, and a check
analysis on samples selected at random from 1 out of 10 heats.

For steel obtained from foreign sources:

•

For steel with yield points less than 50,000 psi, one tensile test and one check analysis on
samples selected at random from 1 out of 10 heats to be performed abroad. In addition, one
sample suitable for a tensile test from 1 out of 4 heats was to be shipped by the inspection
agency to a laboratory in the United States for tensile testing and check analysis.

Letter dated April 5, 1967 from Leslie E. Robertson of SHCR to Malcolm P. Levy of PONYA (WTCI-489-L; see NIST

NCSTAR 1-1A, Appendix E).

Letter dated September 21, 1967 from R. M. Monti of PONYA to R. E. Morris of the Stanray Pacific Corporation

(WTCI-490-L; see NIST NCSTAR 1-1A, Appendix E).

Letter dated November 13, 1967 from R. M. Monti of PONYA to R. E. Morris of Stanray Pacific Corp. (WTCI-498-L; see

NIST NCSTAR 1-1A, Appendix E).

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NIST NCSTAR 1-1, WTC Investigation

•

For steel with yield points of 50,000 psi and higher, one tensile test, one bend test, and a check
analysis on samples selected at random from 1 out of 10 heats to be performed abroad. In
addition, one set of samples suitable for machining into a tensile specimen and a bending
specimen was to be selected at random from 1 out of 4 heats and shipped by the inspection
agency to a laboratory in the United States for testing.

The Port Authority paid special attention to the quality control of structural steel members fabricated
using steels produced in Japan and England.

7.2.3

Exterior Columns from Elevation 363 ft to the 9th Floor Splice

The Pittsburgh-Des Moines Steel Company (PDM) fabricated the column trees, as depicted in Fig 2–8,
from elevation 363 ft to the 9th floor splice. Per the contract specifications, PDM developed the
procedures for quality control and welding (see Appendix E of NIST NCSTAR 1-1A). The final draft of
the quality control program was submitted to the PONYA on September 28, 1967, and was subsequently
approved by SHCR.

Different specifications were written by PDM for the different types of welds that were to be used in the
manufacture of the column trees. These specifications were reviewed and approved by SHCR, and
subsequently approved by the PONYA.

The PONYA hired the Pittsburg Testing Laboratory, an independent inspection company, in 1967, for
mill inspection at PDM’s suppliers’ plants and for fabrication inspection at PDM’s shop.

7.2.4

Exterior Columns Above the 9th floor Splice

The contract between the PONYA and the Pacific Car & Foundry Co. (PCF) contains the specifications
for the exterior walls (box columns and spandrel plates as shown in Figs. 2–9 and 2–11) from the 9th
story splice to the roof. Requirements for fabrication and welding of structural steel are in the
specification, and inspection and quality control requirements are in Sec. 105 of the contract. These
requirements can be found in Appendix E of NIST NCSTAR 1-1A.

The quality control and welding procedures were prepared by PCF, and subsequently reviewed by SHCR
and approved the PONYA, subject to the following conditions:

•

The first three full penetration spandrel butt welds performed by each new welding machine
operator or welder was to be subjected to ultrasonic testing.

•

Where a spandrel weld was rejected, all welds made by the same welder or welding machine
were to be tested by the ultrasonic testing technique for the spandrel in question, as well as for
the spandrels produced immediately before and after the subject spandrel.

•

Approval of the Pacific Car & Foundry Co. quality control and testing program did not
include approval of any welding process or procedure subject to American Welding Society
qualification tests.

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101

•

Visual inspection was to be carried out by certified Pacific Car & Foundry Co. inspection
personnel on 100 percent of all types of welds included in the work.

Weekly inspection reports were submitted by the SHCR resident engineer at the Pacific Car & Foundry
plant in Seattle, WA, to the SHCR home office in New York.

These reports reference a test jig that was

built by Pacific Car & Foundry. Fabricated wall panels were checked for compliance with required
tolerances on the jig before they were approved for shipment.

7.2.5

Rolled Columns and Beams

The contract between the PONYA and the Montague-Betts Company, Inc. contains the specifications for
the rolled core columns, interior columns, louver wall struts, and rolled beams that were used in both
towers. Requirements for fabrication and welding of structural steel are in the specifications, and
inspection and quality control requirements are in the contract. These requirements can be found in
Appendix E of NIST NCSTAR 1-1A.

The quality control and testing program was part of the contract. In particular, the following specific
points were to be included in the quality control program:

•

Material received should be checked against the certified mill test reports for size, grade, heat
number, and color code. One copy of each certified mill report should be submitted to
PONYA and SHCR.

•

Overhangs, gross laminations, excessive slag inclusions, and similar defects should be defined
and repair procedures for these defects should be outlined.

•

Certification papers for each welder and welding machine operator should be submitted to
PONYA and SHCR. Welding procedures must be prepared and the fabricator must perform
qualification tests where applicable. All welds should receive 100 percent visual inspection.
Non-destructive testing of welds needs to be described.

•

The amount of periodic inspection of work in progress and the persons performing this
inspection should be described. The inspection of finished work should be documented in
reports submitted to PONYA and SHCR.

7.2.6 Other

Requirements

Where problems arose in the fabrication yards, particularly when it came to fabrication tolerances,
specific requirements that addressed the specific problems were adopted. The typical method used to
remedy a problem was for the fabricator to submit a procedure for correction to the PONYA. The
procedure was subsequently accepted or rejected by SHCR, and final approval by the PONYA was
contingent upon the fabricator satisfying the requirements set forth by SHCR. These variances from the
original specifications are presented in Chapter 8 of this report.

Weekly inspection reports contained in WTCI-749-L.

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NIST NCSTAR 1-1, WTC Investigation

7.3

FABRICATION INSPECTION REQUIREMENTS FOR WTC 7

No contract documents were available to review for the inspection requirements during fabrication of
structural members for WTC 7. However, WTC 7 project specifications for structural steel referred to the
following codes and standards for fabrication:

•

New York City (NYC) Building Code (1968)

•

Specification for the Design, Fabrication, and Erection of Structural Steel for Buildings, AISC

•

Specifications for Structural Joints using ASTM International (ASTM) High Strength Bolts,
ASTM A 141 Rivets, and ASTM A 307 Unfinished Bolts, Research Council on Riveted and
Bolted Structural Joints

•

Specifications for Structural Joints using ASTM A 325 or A 490 Bolts, AISC

•

Code of Standard Practice

, AISC (except that the first sentence of Sec. 4, paragraph d shall

not apply)

•

Code of Arc and Gas Welding in Building Construction, AWS Standard Code D1.1, American
Welding Society

•

Steel Structures Painting Manual

, Vols. 1 and 2, Steel Structures Painting Council (SSPC)

•

Handbook of Bolts, Nut and Rivet Standards, Industrial Fasteners Institute

Structural steel was to be fabricated and assembled in the shop to the “greatest extent possible” according
to these codes and standards.

The project specification called for a separate contract for testing and inspection of fabrication including
welding. This contract was not available to the NIST investigation, and implementation of this contract
could not be ascertained.

7.4

INSPECTION DURING CONSTRUCTION

Construction of WTC 1 and WTC 2 was overseen and managed by the Tishman Realty & Construction
Company (TRCC), acting as the construction manager. In that role, TRCC as the general contractor
coordinated the scheduling of the various activities required on the project, including the day-to-day
construction activities at the site. The Port Authority required that all correspondence pertaining to
administration of a prime contractor’s contract, including contract changes, matters pertaining to field
problems, job progress, and schedule be submitted to TRCC.

Karl Koch Erecting Co. (KKE) performed

structural steel erection (see NIST NCSTAR 1-1A). As pointed out in Sec. 7.1, the record of construction
and inspection were not available to the investigation. However, construction inspection identified a
number of problems during the erection of WTC 1 and WTC 2, such as material defects, damaged

General instructions from Malcolm P. Levy of PONYA to prime contractors for WTC contracts [WTCI-239-P; see Appendix F

of NIST NCSTAR 1-1A].

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103

structural members, fabrication errors, and fabricated and field welding defects. In a number of cases, the
Port Authority granted construction variance and repair requests by the fabricators and the erector, and
they are presented in Sec. 7.5.

Although WTC 7 project specifications have general erection requirements for fasteners, anchor bolts,
column bases, installation, and bracing, no inspection requirements during construction are given in the
specifications. Further, the records of construction and inspection documents were not available to the
investigation.

7.4.1

Erection Marks and Marking System WTC 1 and WTC 2

To facilitate steel erection, a marking system for structural steel in WTC 1 and WTC 2 was developed by
the Port of New York Authority and Nassau Bridge Detailers. The marking system was devised to
identify following structural members:

•

Exterior wall columns – below the first story splice

•

Exterior wall columns – above the first story splice

•

Core columns

•

Louver wall struts

•

Vertical bracings at exterior wall columns

•

Vertical bracings at core columns

•

Interior pipe posts and hangers

•

Floor beams

•

Horizontal bracings at exterior walls

•

Prefabricated floor units

•

Loose deck and loose power/telephone cells for beam-framed areas

•

Anchor bars and anchor plates

•

Shear studs

•

Viscoelastic damping units

•

Grillages, column base plates and anchor bolts

This system was used by the fabricators to properly identify the different steel members/pieces that were
used in the tower construction. For detailed description of the marking system, see Appendix F of NIST
NCSTAR 1-1A.

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7.4.2

Quality Control and Inspection Program for WTC 1 and WTC 2

A quality control and inspection program was developed by Karl Koch Erecting Co., who performed
structural steel erection work, submitted to the Port Authority for approval. The quality control and
inspection program included information on the following:

•

Survey control

•

Control of construction and erection loads

•

Field welding

•

Bolting of structural steel

•

Control of stud welding operations

•

Erection procedures

•

Control of workmanship

•

Control of erection tolerances

•

As-built drawings

•

Safety programs

For detailed description of the inspection program, see Appendix F of NIST NCSTAR 1-1A. Section 7.5
cites a number of construction errors identified during construction inspection.

7.5 VARIANCES

GRANTED

The WTC towers were very complex steel frame buildings. The structural frames of the towers
incorporated many beams, columns and trusses that were formed by welding steel plates. During
fabrication and erection of structural members, errors were noted by the steel fabricators and the erector.
Such errors included mainly dimensional deviations of structural members from the design and
fabrication drawings. The PONYA was requested by the fabricators and the erector to approve variances
to contract drawings and specifications.

For variance requests, the following procedure was established by the PONYA. All variances resulting
from difficulties encountered in complying with the contractual requirements for fabrication or erection
were submitted by the fabricators or erector to the Office of the Construction Manager of PONYA.
Variances were also requested when, in the opinion of a fabricator or erector, an alternative detail or
procedure was warranted. For expediency, such requests were usually submitted at the same time to the
SHCR.

Typically, the Office of the Construction Manager approved a variance after SHCR reviewed the details
of the variance and recommended its approval. In many cases, SHCR submitted alternative methods,
which were incorporated into the variance.

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The variances that were granted may be categorized into the following groups:

•

Fabrication/erection tolerances

•

Defective (cracked, laminated, misfit) components

•

Fabricator/erector-preferred procedure

•

Material substitutions

•

Frequency/rate of weld inspections

Listed below are types of variances granted by PONYA for each of the above categories. NIST
NCSTAR 1-1A gives a detailed listing of fabrication and erection related variance requests by fabricators
and the erector.

7.5.1

Variances Relating to Fabrication and Erection Tolerances

The following is a list of approved variance requests for fabrication and erection of box beams, box
columns, and floor trusses.

•

Flange offset of 3/16 in. instead of 1/8 in. from the web for box-column sections fabricated by
Stanray Pacific Corporation.

•

Out-of-square tolerances of box beams and a maximum twist of box columns by Mosher Steel
Company.

•

Greater depth of the floor truss end bearing of 20 trusses (4.5 in. vs. specified 4 in.) by
Laclede Steel Company.

•

Field modification procedures for vertical struts of floor trusses to meet erection tolerances by
Karl Koch Erecting Company.

•

Change of fabrication tolerances of floor trusses by Laclede Steel Company.

•

Fabrication modifications of floor trusses to avoid erection difficulties to Laclede Steel
Company by the PONYA.

7.5.2

Variances Relating to Defective Components

The following is a list of specific requests relating to variances for defective components of column trees
and floor trusses.

•

Fabrication error for truss connectors that were 1/4 in. narrower than the required width by
Laclede Steel Company.

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NIST NCSTAR 1-1, WTC Investigation

•

Fabrication errors of placing filler plate at incorrect locations at the bearing end of floor
trusses by Laclede Steel Company. These errors were approved by the inspection company
Pittsburgh Testing Laboratory subject to approval by SHCR.

•

Repair procedures to correct fabrication errors by Laclede Steel Company.

•

Repair procedure to correct laminations in column trees by Pittsburgh-Des Moines Steel
Company.

•

Repair procedure to correct cracks that developed in a number of column trees by Pittsburgh-
Des Moines Steel Company.

•

Repair procedure to correct fabrication errors by adding back up plates by Pittsburgh-Des
Moines Steel Company.

•

Repair procedure for butt welds in column trees by Pittsburgh-Des Moines Steel Company.

7.5.3

Variances Relating to Alternate Fabrication and Erection Procedures

The following is a list of specific requests relating to variances for alternate fabrication and erection
procedures of core columns, floor trusses, exterior wall columns, and beam seats.

•

Deviation of weld splice location of core columns from the contract drawings by Stanray
Pacific Corporation.

•

Use of Hobart automatic arc welding equipment to expedite welding process by Laclede Steel
Company.

•

Elimination of clipped corners of stiffener plates in the exterior wall columns by Pacific Car &
Foundry.

•

Substitution of different beam seat angles of (8 by 6 by 7/8) in. with (8 by 6 by 1) in. angles
by Pacific Car & Foundry.

7.5.4

Variances Relating to Product Substitutions

The following is a list of specific requests relating to variances for product substitutions in the exterior
wall.

•

Substitution of different steel plates with yield strengths ranging from 42 ksi to 100 ksi for
specific plates that were originally specified for use in the exterior wall by Pacific Car &
Foundry.

•

Substitution of 3/4 in. thick plates for 5/8 in. and ½ in. thick plates by Pittsburgh-Des Moines
Steel Company.

•

Substitution of ASTM A36 steel with ASTM A 441 modified steel by Pittsburgh-Des Moines
Steel Company.

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Chapter 8

TRUCTURAL

AINTENANCE AND

ODIFICATIONS DURING

CCUPANCY

8.1 INTRODUCTION

Both architectural and structural modifications were made to meet the occupancy needs of individual
tenants throughout the history of occupancy of the World Trade Center (WTC) 1, 2, and 7.

Examples of

modifications include openings cut in existing floors to construct new stairways linking two or more
floors, and reinforcement of floor framing members to accommodate heavy loads imposed by tenants.
All modifications were reviewed by the Port of New York Authority (PONYA), later called the Port
Authority of New York and New Jersey (PANYNJ), to maintain structural integrity of the buildings and
to ensure that modifications were compatible with existing building conditions. In order to guide tenants
in their modification process, the PONYA issued

Tenant Alteration Review Manual

(PONYA 1971).

This manual was first issued in 1971 soon after the first tenant occupied WTC 1 in December 1970, and
subsequently updated periodically through 1997.

In anticipation of structural degradation, in 1986, the PANYNJ issued the

Standard for Structural

Integrity Inspection of the World Trade Center Towers A & B

(PANYNJ 1986) to guide periodical

inspection of structural members. Deteriorated and damaged members were identified for repair. The
standard was used by consultants who were retained by PANYNJ for systematic examination of WTC 1
and WTC 2.

In 1998, the PANYNJ issued the

Standards for Architectural and Structural Design

(PANYNJ 1998) for

modification works. The standards included not only the design guide, but also included specifications
and standard details to be used in modification works. Tenants proposing any modifications were
required to follow the requirement specified in the standards.

In this chapter, the documents described above are presented, and significant modification and repairs to
the structural systems of WTC 1, 2, and 7 are summarized. NIST NCSTAR 1-1C provides more detailed
description of structural maintenance and modification during occupancy of WTC 1, 2, and 7.

8.2

TENANT CONSTRUCTION REVIEW MANUALS

PONYA issued the first edition of the Tenant Construction Review Manual in 1971, shortly after the first
tenants occupied WTC 1 in December 1970 and prior to initial occupancy of WTC 2 in January 1972.
Subsequent editions were issued in 1979, 1984, 1990, and 1997.

The manual and standards mentioned below and the records on structural modifications, inspection and maintenance presented

in this chapter were made available to the NIST investigation by PANYNJ. The

Tenanat Alteration Review Manual,

the

Standard for Structural Integrity Inspection of the World Trade Center Towers A & B, and

the

Standard for Structural

Integrity Inspection of the World Trade Center Towers A & B

are given in Appendices A through F of NIST NCSTAR 1-1C.

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NIST NCSTAR 1-1, WTC Investigation

The purpose of these manuals was to present the technical criteria, standards, and requirements that were
to be followed by tenants that were planning construction work in any PONYA facility. The manuals
included the criteria that were used by the Engineering Department of the Port Authority when reviewing
proposed construction or alterations. Requirements were given for alterations and modifications to
architectural, structural, geotechnical, civil, mechanical, plumbing, and fire protection systems.

The General Requirements section of the manual required that all tenants submit an application form to
the Port Authority outlining the scope of work, the design criteria, and the plans prior to construction. The
design was to be performed by a registered architect or licensed professional engineer. Contractors were
required to comply with all applicable provisions of federal, state, municipal, local, and departmental
laws, ordinances, rules, regulations, and orders, except where stricter requirements were contained in the
project specifications.

8.2.1 1971

Edition

The 1971 edition of the Tenant Construction Review Manual (see footnote 37) requires that all structural
modifications conform to the provisions of the 1968 New York City (NYC) Building Code. Registered
design professional were to submit structural calculations for review by PONYA. The PONYA structural
reviewer was responsible for the structural integrity of all walls and partitions. Building frames were
checked for stability and sidesway, including the effects of these on the columns.

A comprehensive inspection program was implemented for all construction. Inspection was required
during various phases of construction and was mainly to be performed in accordance with the 1968 NYC
Building Code Sec. C26-106.3 (Materials, Assemblies, Forms and Methods of Construction; Inspection
Requirements) and Sec. C26-107.3 (Service Equipment; Inspection Requirements). The architect,
engineer, or other person who supervised the work was required by PONYA to be present at final
inspection.

8.2.2 1979

Edition

In the 1979 edition of the Tenant Construction Review Manual (see footnote 37), structural requirements
were modified and expanded. Significant differences between the 1971 and 1979 editions were based on
updates of the Structural Chapter of the 1968 NYC Building Code. These include:

•

Rules and regulations relating to resistance to progressive collapse under extreme local loads.

•

Rules and regulations for the design of composite construction with metal decks or
lightweight concrete.

•

Rules related to structural design based on electronic computer computations.

•

Rules for application and protection of sprayed-on fireproofing (BSA Cal. #118-68-GR).

Denotes number of the New York City Board of Standards and Appeals (BSA) document.

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•

Rules for arc and gas welding and oxygen cutting of steel covering the specifications for
design, fabrication, and inspection of arc and gas welded steel structures and the qualification
of welders and supervisors (BSA Cal. #1-38-SR).

Significant updates to controlled inspection of materials, operations, and equipment include:

•

Inspection requirements for proper use of admixtures for concrete.

•

Deletion of the requirement for checking welders’ licenses or qualifications as this item was
covered in the new rules and regulations relating to structural items.

•

Acceptance of mill, manufacturers’, and suppliers’ inspection and test reports as evidence of
compliance with the provisions of the Code for all structural materials and assemblies.

•

Inspection of spray-on fireproofing since such inspection was added for the first time in
C26-502.2(f) of the 1968 NYC Building Code in 1976 (Local Law 55). This new section of
the code required that the installation of all sprayed-on fire protection of structural members,
except those encased in concrete, be subject to the controlled inspection requirements of
C26-106.3, which requires all materials designated for controlled inspection were to be
inspected and/or tested to verify compliance with code requirements. All required inspections
and tests were to be made and witnessed by or under the direct supervision of an architect or
engineer who the owner retained and who was acceptable to the architect or engineer who
prepared the plans. The architect or engineer was to file with the NYC Building Department
signed copies of all inspection and test reports, together with a signed statement that the
material and its use or incorporation into the building complied with code requirements.

8.2.3

1984 Edition, Revised 1990

Except for some editorial changes, the requirements of the 1984 edition of the Tenant Construction
Review Manual remained virtually the same as those of the 1979 edition. In the revised March 1990
edition, requirements were added concerning the role of consultants working on the project who were not
the architect or engineer of record.

The scope of structural review of the alterations and/or modifications consisted of compliance with the
applicable codes, standards, and design criteria given in the Structural Review section of the manual. In
particular, the provisions of the then applicable New York City (NYC) Building Code were to be satisfied
for work performed in New York City.

The revised March 1990 edition (see footnote 37) of the manual included a requirement that all structures
were to be designed for earthquake zone 2 forces in accordance with the BOCA (Building Officials Code
Administrators, Inc.) code. Local laws that contained seismic provisions more stringent than those in the
BOCA code were to take precedence. Also, reference was made to ASTM International (ASTM) E 580,
Standard Practice for Application of Ceiling Suspension Systems for Acoustical Tile and Lay-in Panels in
Areas Requiring Moderate Seismic Restraint for lightweight ceilings to resist seismic forces.

The Controlled Inspection section of the manual contained a comprehensive inspection program that was
to be implemented for all construction. Controlled inspection requirements were abstracted from the

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NYC Building Code Secs. C26-106.3 and 107.3. All materials, equipment, and construction designated
by the Code for controlled inspection were required to be inspected and/or tested to verify compliance
with the Code. Controlled inspection was required to be made and witnessed by or under the direct
supervision of a registered architect or professional engineer retained by the tenant and acceptable to the
architect or engineer responsible for the plans.

The inspection requirements were significantly reorganized and modified in the revised March 1990
edition of the manual. Requirements for approval/acceptance of materials and controlled inspections were
abstracted from the applicable sections of the NYC Building Code.

8.2.4 1997

Edition

The requirements of the 1997 edition of the Tenant Construction Review Manual are essentially the same
as those in previous editions of the manual. The most notable change was related to earthquake design.
The manual added horizontal force factors for overhead signs, anchorage for suspended ceilings weighing
more than 4 psf, elevator and counterweight guardrails and supports, sprinkler piping, gas and high hazard
piping, other piping, and heating, ventilation, and air conditioning (HVAC) ducts, along with new notes
pertaining to sprinkler piping, other piping, and HVAC ducts. These requirements were added to the
manual to ensure that potential overhead hazards would not fall on building occupants during a seismic
event.

No significant changes were made to the inspection requirements from the 1990 edition of the manual.

8.3

STANDARDS FOR STRUCTURAL INTEGRITY INSPECTION OF THE WTC
TOWERS

To guide the PANYNJ in the evaluation of the structural integrity of the WTC towers, the Engineering
Department of PANYNJ issued

Standards for Structural Integrity Inspection of the World Trade Center

Towers A & B

(PANYNJ 1986)

in 1986. These standards were used to identify structural degradation,

and to repair damaged structural members.

Three methods were used to evaluate the structural integrity of the towers: (1) statistical inspections,
(2) review of maintenance and tenant complaint reports, and (3) building movement and deformation
measurements.

In the first method, periodic visual inspection of selected structural components in “higher-potential
trouble areas” was to be made initially by qualified outside consultants. The periodic inspections were to
be supplemented by occasional visual inspections when the structure was exposed during tenant
remodeling or general maintenance work.

In the second method, various reports were to be examined by the Engineering Department of PANYNJ,
which could possibly shed light on underlying structural problems. Maintenance reports of non-structural
repairs, water leakage, and tenant complaints about unusual building movements, vibration, or noise are
examples of such reports.

See Appendix E of NIST NCSTAR 1-1C for the complete document.

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In the third method, the performance of systems within the buildings was to be evaluated by the
Engineering Department through measurement of movement or deformation using appropriate tests and
instruments. Measurements were to be performed on individual components in the towers as well as on
the entire towers themselves.

8.3.1 Visual

Inspections

Since the visual inspection of the entire structure, or even a major portion of WTC 1 or WTC 2, was not
practical, a statistical inspection program was implemented. This approach involved sampling those
components and systems that were important to structural integrity at locations with “a relatively higher
potential for occurrence of defects or problems.”

Visual inspection was to be supplemented by the use of simple hand tools, measurements, and recording
techniques, as required. Loose, cracked, or rust-stained spray-on fireproofing and concrete or masonry
encasement covering structural steel members and connections was to be removed prior to examining the
steel. After inspection, any removed fireproofing had to be properly replaced. Also, where it was
necessary to drill a hole through a structural steel element to provide access for a borescope or any other
device for inspection, the access hole was to be sealed with weld metal, body putty, or caulking, as
appropriate.

Periodic inspection of WTC 1 and WTC 2 was to be carried out on the following components at various
time intervals:

TV antenna mast on the top of WTC 1.

This program consisted of four parts:

(a) inspection of the structural steel elements in the antenna, (b) inspection of the high tensile
bolts and studs, (c) inspection of the weatherproof enclosure, and (4) inspection of the
radomes. Inspection of these components was to be performed on a “continuing basis,” as
weather and operational restrictions permitted. A complete inspection of the mast structure
within the weatherproof enclosure was to be performed at least once a year; the other
components were to be inspected at least once every 3 years.

Exterior roof and wall elements.

Every year, the exterior roof and wall elements were to be

inspected for signs of water intrusion. Roof leakage was to be ascertained from an
examination of the spaces immediately below the roof areas. Wall leakage was to be
determined from signs of water staining on interior finishes.

Room occupancies.

An inspection of room occupancies and uses throughout both towers

was to be performed on an annual basis to verify that design live load was not exceeded.

Accessible column envelopes, including fireproofing.

Every 2nd year, accessible columns

were to be inspected for bowing or deviation from plumb. Also, fireproofing was to be
examined for signs of rust or cracking. Inspection for lateral displacement or rotation of
columns in elevator shafts, where the columns were braced on only one axis by connecting
beams or concrete slabs, was required.

Fireproofing and masonry partitions on diagonal bracing and transfer trusses.

Fireproofing and masonry partitions enclosing the diagonal bracing on exterior column lines

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in both towers below the Service Level Floor and the transfer trusses below floor 1 in WTC 2
under exterior and core columns were to be inspected every 2nd year for cracking, stains, and
other possible signs of structural distress.

Hat truss members.

Every 2nd year, the hat truss members between floor 107 and the roof

in the core area were to be inspected.

Exterior box columns and spandrel plates.

Exterior box columns and spandrel plates

under column trees below floor 7 were to be inspected every 4th year. Exterior aluminum
covers and spray-on fireproofing were to be removed to gain access to the exterior surfaces of
the box columns and spandrel plates. Both the columns and plates were to be visually
inspected for bowing or distortion, cracking, and corrosion. Visual inspection was also
required for accessible welds. Ultrasonic testing of full or partial penetration welds and
adjacent base metal was to be performed where base metal thickness exceeded 1.5 in.

The interior of the box columns was to be examined by a borescope for the presence of water
and the existence of rust on the interior plate surface. This was to be accomplished by
drilling an access hole in the column or the spandrel plate.

The “tree” junction where the three superstructure columns merged was also to be inspected.
The top surface of the horizontal diaphragm plate that capped the tapered box column just
below the point where the three separate columns merged was to be examined, as was the
exterior column plate between this location and the column splice at elevation +372 ft 4 in.

Steel floor framing over mechanical spaces.

Every 4th year, the steel floor framing over

mechanical spaces and other areas without suspended ceilings was to be inspected.

Concrete slabs, partitions, and finishes.

Concrete slabs, partitions, and finishes were to be

inspected every 4th year for signs of distress, which could indicate excessive structural
deformation.

Occasional inspections were also to be made of the structural steel framing, connections, and concrete
slabs when general repairs or remodeling was done that involved removing ceilings, partitions, finishes,
or other coverings. In particular, the top of the concrete slab was to be examined for cracking, spalling,
and exposed or corroded top reinforcement. Where reinforcing bars were corroded and where concrete
had spalled, repairs were to be made as tenant relocation permitted.

8.3.2 Review

Reports

General maintenance reports and complaints from tenants were to be used to search for possible problems
related to underlying structural defects. Water damage caused by leaks at the roof level or at the exterior
walls, broken plumbing, and cracks in partitions or the concrete floor slab were to be reviewed to
determine whether such events were caused by structural deformations. Records were to be kept of tenant
complaints of building sway, floor vibration, sagging ceilings, unusual noise, and other items. Visual
inspection of the appropriate area of the building was to be performed where a reasonable assessment of
the data in the reports or logs was tied to a specific structural element or system. Reports and log data
were to be correlated with testing and measurements.

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8.3.3 Periodic

Measurements

Periodic measurements of various types of deformation and vibration were to be made by the Engineering
Department of PANYNJ for the purposes of monitoring changes in certain important characteristics of the
buildings. Adverse changes in such measurements were assumed to reflect possible structural
deterioration.

Measurements of the following items were to be performed on a periodic basis:

Natural frequency of the towers.

Accelerometers were to be used to measure natural

frequencies of the towers on a monthly basis. Wind speed and direction were also to be
recorded at that time.

Accelerometers were installed only in WTC 1.

Natural frequency of the TV mast on WTC 1.

Accelerometers and amplifiers were to be

installed within the heated enclosure of the TV mast on the top of WTC 1 at a level of about
2/3 of the height of the mast above its base. One accelerometer was to be oriented to measure
N-S displacements, and one was to be oriented to measure E-W displacements. Displacement
measurements, as well as wind speed and direction, were to be recorded once a month.

There is no evidence that accelerometers were installed on the TV mast.

Natural frequency of the floor construction.

The natural frequency of the floor

construction was to be measured when floor space had been emptied due to tenant change or
remodeling. The natural frequency and damping values of the floor structure were to be
measured by performing a “heel drop” test. In such tests, vibrations induced in the floor
structure by a vertical impact were recorded using an accelerometer attached to the floor.
Vibration measurements were also taken for an impact load of 100 lb dropped from
approximately 6 in. above the floor slab on to a 1 in. thick neoprene pad.

Viscoelastic dampers.

This program consisted of continuously measuring and recording the

movements of WTC 1. Wind speed and direction were also to be measured. It was anticipated
that such measurements would continue until the end of 1985 or longer, depending on
available funds.

Twelve viscoelastic damping units (four units from each of three floors) were to be removed
from WTC 1 annually and were to be tested by the Minnesota Mining and Manufacturing
Company (3M), who were the manufacturers of the damping units. Temperature effects and
shear strength were to be tested.

Plumbness and level.

Building plumbness and floor level checks were to be performed

semiannually for each tower, preferably in the early morning hours in August when wind
velocity was low and outside air temperatures were moderate.

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Building plumbness was to be determined by measuring the offsets from a vertical laser
beam, which was to be projected up from the bottom of freight elevator shaft 50, to the shaft
walls. Offset measurements were to be taken at 20-story intervals.

Floor levelness was to be determined by measuring the relative elevation of 16 benchmarks
on the floor slab at floor 70 of each tower.

8.3.4 Recordkeeping

Standards for Structural Integrity Inspection

lists the defects and signs of distress that were to be noted

and recorded during inspection of the structural steel and the reinforced concrete. Detailed descriptions of
the defects and signs of distress are given in NIST NCSTAR 1-1C.

In general, a description was to be made of the defect or indication of distress. Measurements, sketches,
and photographs were to be provided in those cases where a written description was not adequate. The
use of a tape recorder was also permitted.

If defects that appeared to require more than routine attention were uncovered, a separate report of such
findings was to be submitted to the Engineering Department, PANYNJ. For conditions of a serious
nature, immediate notification was to be made to the Engineering Department in person.

Three categories of urgency were established for repairs. Repairs falling into the “immediate” category
included possible closure of the area and/or structure affected until interim remedial action (such as
shoring or removal of a potentially unsafe element or structure) could be implemented. Such action was
to be undertaken immediately after discovery, and a description of the action taken and recommendations
for permanent repair were to be included in the inspection report.

The “priority” category was for those conditions where no immediate action was required, or for which
immediate action had been completed, but for which further investigation, design, and implementation of
interim or long-term repairs should be undertaken on a priority basis (i.e., taking precedence over all other
scheduled work).

Repairs falling into the “routine” or “non-priority” category could be undertaken as part of a scheduled
major work program or other scheduled project, or when routine facility maintenance was to be
performed, depending on the type of repair that was required.

Standards for Structural Integrity Inspection

outline the various measurements and test data that were to

be recorded during the inspection process. Also given are the criteria that determine whether a possible
problem may exist, based on the recorded measurements.

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8.4

STANDARDS FOR ARCHITECTURAL AND STRUCTURAL DESIGN

Standards for Architectural and Structural Design

was issued on February 27, 1998, by PANYNJ and

contained architectural and structural design requirements, specifications and standard details for tenant
alterations that were to be made specifically at WTC 1 and WTC 2.

Prior to any design work, the tenant’s consultants were required to perform a field inspection of all areas
that would be affected by the alterations so that the latest information was available for all structural
elements, including, but not limited to truss reinforcement, stair openings in slabs, and core-hole
locations.

Tenants are required to submit calculations and construction drawings to PANYNJ for review and
approval. All construction documents were required to be signed and sealed by a professional engineer or
registered architect licensed to practice in the state of New York.

Minimum loads to be used in the calculations were also specified. Calculations to compare the proposed
loading with the allowable loads were required to conform to the latest edition of the NYC Building
Code. Both allowable stress design and load-and-resistance-factor design were acceptable design
methods.

All work was required to conform to the latest edition of the NYC Building Code, including any
revisions. Provisions in the latest editions of the following codes took precedence over those in the NYC
Building Code whenever they were more stringent:

•

American Institute of Steel Construction, Specification for the Design, Fabrication and
Erection of Structural Steel for Buildings. Supplement 1 is specifically excluded.

•

American Concrete Institute, Standard Building Code Requirements for Reinforced Concrete,
ACI 318.

•

American Welding Society, Structural Welding Code – Structural Steel (AWS D1.1) and
Reinforcing Steel (AWS D1.4).

Any steel plates that were added to reinforce existing framing or for other reasons were required to
conform to ASTM A36, and any reinforcing bars that were added were required to conform to
ASTM A 615 Grade 60.

Welding materials for structural steel and reinforcing steel were required to be E7018 conforming to
American Welding Society (AWS) A5.1

Specifications for Covered Carbon Steel Arc Welding

Electrodes

. Specifications for non-shrink grout were also specified.

8.5

STRUCTURAL INSPECTION PROGRAMS

Beginning in 1990, the Port Authority of New York and New Jersey implemented a systematic facility
condition survey program of WTC 1 and WTC 2 using

Standards for Structural Integrity Inspection

The complete document is in Appendix F of NIST NCSTAR 1-1C.

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which was developed in 1986. WTC 7, which was not owned by PANYNJ, was also inspected based on
the criteria in

Standards for Structural Integrity Inspection

. Prior to 1990, both WTC 1 and WTC 2 were

inspected occasionally by the Engineering Department of PANYNJ.

The survey program included:

Condition survey of WTC 2 in 1990 by the Engineering Quality Assurance Division of
PANYNJ.

Condition survey of WTC 1 in 1991 by the Office of Irwin G. Cantor, Consulting Engineers,
for the Engineering Quality Assurance Division of PANYNJ.

Condition survey of WTC 7 in 1997 by Ammann & Whitney for the Engineering Quality
Assurance Division of PANYNJ.

Due diligence physical condition survey of WTC 1 and WTC 2 in 2000 by Merrit and Harris
for PANYNJ prior to entering into a long-term leasing contract for the WTC buildings with
Silverstein Properties.

In addition to these four separate condition surveys, Leslie E. Robertson Associates (LERA) and other
engineering firms conducted periodic inspections of the towers under the WTC Structural Integrity
Inspection (SII) Program, which was based on the proposal originally submitted to PANYNJ by LERA
in 1990.

This section summarizes the findings of these condition surveys. Detailed descriptions of the condition
survey programs and findings are given in NIST NCSTAR 1-1C.

8.5.1

Facility Condition Survey of WTC 2

The scope of work, which was designed to minimize impact on tenant and facility operations, included
inspection of (1) the exterior wall system (columns, spandrel plates, and splices), (2) core columns
(including column splices and lateral bracing below the 7th floor), (3) the space frame (hat trusses),
(4) floor systems (floor slabs and decks, trusses, rolled beams, bridging, and connections), and (5) the
damping system. Thirty floors throughout WTC 2 were selected for inspection, including all four of the
two-story mechanical equipment rooms.

Inspection Procedures and Methodology

To assess the condition of the structural system in the tower, both visual inspection and nondestructive
testing methods were performed. The thickness of steel members was checked using an ultrasonic
thickness gauge. Fillet welds were tested for cracks and discontinuities using magnetic particle or dye
penetration test methods, and groove welds were tested using the ultrasonic method.

Exterior Walls (Columns and Spandrels)

Exterior columns and spandrels were inspected at (1) column field splice connections, (2) spandrel field
splice connections, and (3) the inside of the spandrel plate face at the column/floor truss seat connections.

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A total of 59 column splices were inspected and all were found to be in good condition. On two of the
floor levels, the columns had only three bolts at the splice location, although the design called for four.
According to the report, this had no effect on structural integrity.

Spandrel plates, splice plates, and spandrel bolted connections were also found to be in good condition.
Scattered rust stains were observed on the spandrel fireproofing as well as on the inside of some of the
steel box columns.

No priority recommendations were made in the report. The report recommended that a long-term
maintenance program be developed and implemented to clean and paint the inside surfaces of the exterior
box columns to prevent further corrosion of the structural steel.

Core Columns

Core columns were inspected from elevator shafts and from office area floors. Twenty-five elevator shafts
were randomly selected for inspection, and the elevator core framing was primarily inspected with
fireproofing materials in place. In general, some defects were found in the fireproofing. In most of the
shafts, several small regions and a few large areas of fireproofing were found to missing from core
framing members. In the worst case, 100 percent of the fireproofing was found to be missing from the
south face of column 908 between floors 27 and 29 in elevator shaft number 1. Exposed steel members
exhibited only isolated locations of light surface corrosion.

Gypsum wallboards surrounding the elevator shafts were also found to be in good condition, although
isolated holes were detected at various locations.

Inspection of column splices and eccentric-braced column connections with fireproofing removed showed
that all bolts, welds, and structural steel were in good condition.

No priority recommendations for repair were made in the 1990 report. The report recommended that the
fireproofing that was missing from the framing members in the elevator shaft be replaced, including those
regions where the fireproofing was removed for inspection. It also recommended that the holes in the
gypsum wallboards surrounding the elevators be repaired.

Space Frame (Hat Trusses)

From floor 107 to floor 110, a steel space frame system, also known as “hat trusses” was interconnected
with columns in the core and the exterior walls. The hat trusses were constructed of rolled wide-flange
and welded-box sections, and were designed to support a future antenna. Thirty three locations were
inspected visually from the floor below. Visual inspections of truss connections were made closely at six
locations after removing the fire resistant material (“fireproofing”). The space frame system was found to
be in good condition. The exposed areas exhibited light surface corrosion. Both bolted and welded
connections were found to be in good condition with no significant deterioration.

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Floor Framing

In the main lobby, beams and connections that were inspected within the core area were found to be in
good condition. Fireproofing was missing at various locations, exposing light surface corrosion on these
elements.

On floors 9 through 41, the floor framing that was inspected was also found to be in good condition.
Light corrosion was observed on all core beams and beam connections, and on floor truss connections.
The most significant deficiencies were found at the damping units, where a number of such units were
missing from 1 to 4 fasteners in the connections to the framing members.

Floor framing on floors 43 through 75 were found to be in good condition. The most significant
deficiencies were found on floors 64 and 75. A deformed bottom chord was found on the main truss
along column 343 on floor 64; no signs of distress were observed. On the 75th floor, untightened bolts
were found at truss seat connections at several locations, which, according to the report, had no
significant affect on the structural integrity of the framing, since they served for erection purposes.

The floor framing on floors 77 through 107 was found to be in good condition with light surface
corrosion observed on all core framing beams and connections. The most significant deficiencies were
concrete slabs that had separated from the metal deck at floors 93 and 108. According to the report,
structural integrity was not comprised, since the metal deck served as only formwork for the concrete.

The floor framing and slabs were found to be in good condition except where hairline cracks were found
in concrete beam encasement at various locations on all four mechanical equipment room levels.

No priority recommendations for repair were made in the report. All of the deficiencies noted above were
considered to have no significant effect on structural integrity.

Damping System

Visual inspection of damping units noted missing non-structural fasteners. Of 30 floors examined, at
least 1 damping unit on each of 4 floors (18, 29, 37 and 38) had missing fasteners, ranging from 1 to 4.
No priority recommendations for repair were made in the report.

Mechanical Equipment Rooms (Floors 7-8, 41-42, 75-76, 108-109)

The floor framing and slab inspected on the Mechanical Equipment Room (MER) floors were found to be
in good condition. On all MER floors, most of the structural framing was inaccessible due to HVAC
ducts, fans, electrical equipment, or plumbing.

Hairline cracks were found in concrete beam encasement at various locations on all four MER floors.
Exposed steel exhibited light surface corrosion, and no deterioration was found at the underside of floor
slabs.

The report recommended no priority repair. As mentioned above, all the deficiencies found were
considered to have no significant effect on structural integrity. The report recommended that utility
supports found to be bowed or vibrating be replaced as part of the facility’s regular maintenance program.

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Roof

Roof framing, which consisted of rolled steel wide flange beams supporting a structural concrete slab,
was found to be in good condition. Hairline cracks in the fireproofing and in the underside of the
concrete slab were found at various locations.

No priority recommendations for repair were made in the report. The report recommended repairs that
were none structural in nature.

8.5.2

Facility Condition Survey of WTC 1

The scope of the survey was based on experience gained from the survey of WTC 2 in 1990. As in the
case of the WTC 2 survey, the scope of work was designed to minimize impact on tenant and facility
operations. The exterior wall system, core columns, floor framing, damping system, space frame (hat
truss), mechanical equipment rooms, and roof were inspected.

Exterior Walls (Columns and Spandrels)

A total of 28 exterior column splices were inspected throughout 14 office floors on floors 9 through 106.
Nondestructive testing was performed on the plate splice welds, and ultrasonic testing was performed to
verify plate thickness at 26 of these locations. All inspected columns splices were found to be in good
condition.

The inside faces of the steel box column plates exhibited scattered areas of light to moderate corrosion
and peeling paint. Ultrasonic thickness testing on the outer column plates above and below the splice
location indicated no cross-section loss.

Spandrel plates, splice plates, and bolted connections were also found to be in good condition. Scattered
rust stains were observed on the spandrel fireproofing.

On the floors above 106, only the joints at floor 108 were inspected. No structurally significant
deterioration was found.

No priority recommendations for repair were made in the report. The report noted that missing
fireproofing should be replaced on the spandrel plates and splices.

Core Columns

Core columns were inspected in 13 elevator shafts with fireproofing left in place. Corner core column
splices were inspected on two office area floors. Core floor beam to column connections were also
inspected at 25 of 56 locations on 14 floors.

The exterior wall column splices were found to be in good condition. Results from nondestructive testing
of the splice plate welds were acceptable, and results from ultrasonic thickness testing showed no
significant loss in member thickness.

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Several small areas and a few large areas of fireproofing were missing from some of the steel beams and
columns in the express elevator shafts. According to the report, the probable cause of missing
fireproofing on the columns was due to the high speed of the elevators moving up and down the shafts.
All exposed steel was found to be in good condition with light to medium surface rust.

Gypsum wallboards were found to be in good condition, except for two isolated holes in two elevator
shafts at the 58th and 69th floors.

Similar to the case of the express elevator shafts, fireproofing was found to be missing on some of the
steel columns and beams, and some isolated holes were found in some of the gypsum wallboards in the
local elevator shafts.

Inspection of core corner column splices and floor beam to column connections showed all of the
elements to be in good condition.

No priority recommendations were made in the report. The report recommended that missing
fireproofing from the framing members in the elevator shafts be replaced, including those regions where
the fireproofing was removed for inspection during the condition survey. It also recommended that the
holes in the gypsum wallboards be repaired.

Floor Framing

Fourteen office floors (11, 13, 22, 30, 35, 52, 54, 61, 65, 78, 84, 86, 90, and 93) were selected for
inspection. Inspection of the structural elements at these levels followed the following sequence:

Six long-span trusses and two short-span trusses were selected from the plans for even,
random distribution of inspection locations throughout the floor area.

Floor framing, damping unit, utility supports, steel decking, inside faces of steel spandrel
plates, spandrel splices, and core concrete or rolled steel members were visually inspected.
Structural steel members were examined for signs of deformation or corrosion with
fireproofing still in place.

Fireproofing was removed to inspect the condition of steel framing members at the following
locations: (a) six truss locations, (b) one core floor beam, (c) two spandrel plate splices, and
(d) two exterior columns (plaster removal). Visual inspections were made using lights,
scrapers, wire brushes, and mirrors for signs of cracking, deformation, or corrosion.

Nondestructive testing was performed on column splice welds and welded floor framing
connections. Testing was performed by the Port Authority’s Materials and Research
Division.

On 2 of the 14 floors inspected, column splices on 7 core columns were inspected after removal of the
gypsum board firewalls. Top sides of exposed concrete floor slabs were also inspected where carpeting or
floor tiles had previously been removed.

Two typical conditions were observed during inspection of the floor trusses: (1) small areas of
fireproofing were missing at scattered locations throughout the floor framing, and (2) the underside of the
floor trusses exhibited light rust. Welds were tested at various connections and were found to be in good

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condition. In some cases, the connection of the truss to the exterior spandrel plate had one bolt and a
weld instead of the typical two-bolt connection. These field welds were also tested and were found to be
in good condition.

The metal deck and concrete slabs that were inspected were also found to be in good condition, except for
the slab in the southeast corner of the 60th floor where cracks were found on the top surface.

The report made no priority recommendations for repair for any of the floor framing members. Routine
recommendations were made as follows: (1) patch elastomeric sealer at the construction joint south of
columns 504 and 505 under the 13th floor, (2) even though the modifications made to the bridging trusses
at the 10th and 61st floors did not meet the original design, no further modifications were needed, (3)
patch spalls that were created in concrete slabs when partition rails were removed, and (4) patch cracks on
the 60th floor with elastomeric sealer

Damping System

At all of the locations that were inspected, the damping units did not have fireproofing covering them.
Light rust was observed on the surfaces of the units. A non-structural bolt was missing on one of the
damping units under the 30th floor.

Space Frames (Hat Trusses)

A total of 199 members were inspected in the space frame (hat truss). Light rust was found on diagonal
braces, beams, and connections where fireproofing was missing. No priority recommendations were
made in the report. Routine recommendations were made to replace missing fireproofing.

Mechanical Equipment Rooms and Space Frame

All four mechanical equipment room floor levels (floors 7-8, 41-42, 75-76, and 108-109) were inspected.
Floor slabs at these levels were found to be in good condition with scattered cracks found on the slab
surfaces. Scattered patches of fireproofing were found missing from the underside of the metal decks
outside the core area.

A concrete encased beam on the 110th floor was subjected to steam from a leaking steam valve.
Moderate rusting was found on the member, but no significant section loss was found.

Hangers supporting ducts and piping were visually inspected, and some were found to be subject to
excessive vibration. Loose hanger rods and fatigue of pipe supports were also found at various locations.
Beams that supported the duct hangers had fireproofing missing where the hangers were mounted.

The report included a priority recommendation to replace the leaking valve under the 110th floor that
caused the floor beam to corrode. Routine recommendations were made to repair cracks in the concrete
slabs and to repair hangers that were found to be vibrating, bowed, sagged, and/or deformed.

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Roof

No significant structural deficiencies were found at the roof level. Cracking and spalling of the concrete
slab was found in localized areas of the roof.

No priority recommendations were made in the report. Routine recommendations included removing and
replacing existing patches in the roof slab and patching spalled areas in the concrete slabs.

8.5.3

Facility Condition Survey of WTC 7

The scope of work was designed to minimize impact on tenant and facility operations and was limited to
unoccupied floors and floors that had vacant space. The column splices, wind bracing system, interior
beam connections, floor slabs, and the Consolidated Edison (Con Edison) Substation were all inspected.

According to the report, no problems or deterioration were found on the column splices, wind bracing, or
the interior beam connections at any of the locations that were inspected. Rust buildup was found
between the flanges of members that rested on top of one another at the main roof level where the steel
framing was exposed. The report recommended that the steel be cleaned and painted to prevent further
deterioration, even though this was not considered to be a structural problem.

Fireproofing was found to be missing from the steel framing at various locations where utility supports
were installed on all of the floors that were inspected. It was most prominent on the fifth floor framing
above the main lobby and the second floor framing above the loading dock area. It was recommended in
the report that the fireproofing be replaced.

Loose concrete was found on the north face of column 51 on the 46th floor of the cooling tower area.
Silverstein Properties personnel immediately removed the loose concrete.

Floor slabs were found to be in good condition. No deficiencies were found, except for some shrinkage
cracks on the top of some of the exposed slabs and some damage to the metal deck.

The Con Edison station was found to be in very good condition, and no action was required at that time.

8.5.4

Due Diligence Condition Survey of WTC 1 and WTC 2

This section discusses the findings of the condition survey of WTC 1 and WTC 2, which was performed
by Merritt & Harris, Inc. in 2000 for PANYNJ (Merritt & Harris 2000). On-site evaluations were
performed to assess the general physical condition of the property, as it existed at that time. In particular,
WTC 1, WTC 2, WTC 4, WTC 5, the retail mall and plaza, central services, and the subgrade were
inspected. The following discussion focuses on the findings for WTC 1 and WTC 2.

Inspection Procedures and Methodology

Observations were limited to those portions of the project that were visible during walk-through. In many
areas, building finishes concealed structural components from view.

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Findings of Inspection

Merritt & Harris reported the following findings and recommendations for both WTC 1 and WTC 2.

According to the report, the building structure appeared to be in good overall condition, based on
observations of portions of the structure that were not concealed by building finishes. No apparent
movement or settlement of foundations was observed, and interior slabs were reported to be in good
condition.

The report noted that LERA and other engineering firms had performed, on a regular basis, SII of various
structural systems and that those studies had indicated the following deficiencies: (1) rusting of steel
columns in the elevator shafts, (2) missing fireproofing, and (3) floor coring damage.

The due diligence

condition survey report went on to note that the most recent SII recommended repairs were underway at
the time the report was written.

Damping units had been tested every 5 years, most recently in 1996. The report noted that approximately
two-dozen damping units were kept in stock for replacement. The report also stated that LERA strongly
recommended that the analysis of wind acceleration measurements be continued.

The report noted that an ongoing program of re-fireproofing structural steel members was in place at the
time of the inspection. Re-fireproofing the structural steel was supposed to provide a 2 h fire rating for
those members. Such work was performed on an entire floor when the space was being built-out for new
occupancy. At the time of inspection by Merritt & Harris, Inc., approximately 30 floors had been
completed in the two towers.

8.5.5

Structural Integrity Inspection Program

In 1986, PANYNJ implemented an inspection program to detect, record, and correct any signs of distress,
deterioration, or deformation that could signal structural problems. This structural integrity inspection
program, which was based on an inspection and testing plan prepared by LERA, contained detailed
guidelines on inspection, record-keeping, and follow-up procedures.

Inspection findings under this program were to be categorized as “Immediate,” “Priority,” or “Routine.”
Repairs falling into the “immediate” category included possible closure of the area and/or structure
affected until interim remedial action (such as shoring or removal of a potentially unsafe element or
structure) could be implemented. Such action was to be undertaken immediately after discovery, and a
description of the action taken and recommendations for permanent repair were to be included in the
inspection report. The “priority” category was for those conditions where no immediate action was
required, or for which immediate action had been completed, but for which further investigation, design,
and implementation of interim or long-term repairs should be undertaken on a priority basis (i.e., taking
precedence over all other scheduled work). Repairs falling into the “routine” or “non-priority” category
could be undertaken as part of a scheduled major work program or other scheduled project, or when
routine facility maintenance was to be performed, depending on the type of repair that was required. An
important requirement in the inspection program was that where inspection procedures involved the
removal of fireproofing, such fireproofing was to be properly replaced on completion of inspection.

Detailed findings of the Structural Integrity Inspections are given in NIST NCSTAR 1-1C.

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A summary of the findings from the SII can be found in NIST NCSTAR 1-1C.

LERA’s proposed plan to monitor the structural integrity of the World Trade Center Complex included
WTC 1, WTC 2, WTC 4, WTC 5, WTC 6, the Vista Hotel, and the subgrade.

The plan called for

inspection/monitoring of the following items in WTC 1 and WTC 2:

•

TV mast (WTC 1 only)

•

Roof water tightness and curtain wall

•

Space usage

•

Accessible columns, including exterior box columns at locations of spandrel intersections and
“tree’ junctions below floor 7 and above floor 1 (Plaza Level)

•

Bracing at exterior column line below elevation 294 ft 0 in., and in WTC 2 only, the transfer
trusses below floor 1 under exterior columns

•

Hat truss between floor 107 and the roof

•

Floor framing for mechanical spaces

•

Floor framing for tenant areas

•

Concrete slabs, partitions, and finishes

•

Steel framing, slabs, and the like where exposed for general repairs or tenant remodeling

•

Measurement of natural frequency of tower and TV mast

•

Floor natural frequency

•

Damping units

•

Plaster ceilings in main lobby

•

Marble wall panel supports

•

Review of maintenance reports

•

Fire stairs

Inspection and monitoring of these items were to occur at regular intervals. A summary of the structural
integrity inspections conducted and their corresponding dates is given in Table 8–1.

Letter dated January 12, 1990, from Saw-Teen See of Leslie E. Robertson Associates to Suren Batra of the Port Authority of

New York and New Jersey (WTCI-123-P).

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Table 8–1. Summary of Structural Integrity Inspections Completed for

WTC 1 and WTC 2.

Inspection Program

Date(s) of Inspection Reports

Space Usage Survey

1995, 1996, 1997, 1998, 1999

Accessible Columns

1993, 1995, 1996, 1997, 1998

Plaza Level Box Columns

1998

Bracing Below Elevation 294 ft 0 in.

1991, 1995

Hat Trusses

1992, 1995

Floor Framing Over Mechanical Areas

1992, 1996, 1999

Floor Framing Over Tenant Areas

1992, 1995, 1997, 1999

Natural Frequency Measurements

1993, 1995, 2000

Natural Frequencies of Floors

1995

Viscoelastic Damping Units

1996

Space Usage Surveys

The purpose of the space usage surveys was to identify possible structural overloading of the slabs and
floor framing due to changes in occupancies and uses and/or due to additions of heavy equipment or
furniture. Surveys were conducted annually over a 5-year period starting in 1995, with two surveys
conducted in 1996. The only priority recommendation was made in the 1995 report, which advised
PANYNJ to distribute the load of the granite slabs on floor 106 of WTC 1 over a larger area.

Accessible Columns

Surveys of the accessible columns (columns in the core area that are not enclosed by an architectural
finish, which can be visually inspected) in the elevator shafts of WTC 1 and WTC 2 were performed to:

•

Ascertain the condition of the accessible columns with respect to rusting, cracking, bowing,
and deviation from plumb;

•

Identify specific locations of structural distress or damage;

•

Identify locations of damage to the fireproofing;

•

Identify lateral displacement or rotation of the column about a vertical axis where the column
is directly braced on only one axis by connecting beams or concrete slabs; and

•

Identify deformations of the slabs-on-ground surrounding each column at the sublevel.

Accessible column surveys were performed approximately every two years, starting in 1993 and ending
in 1998. Priority recommendations were made in the 1996 report and also in the 1998 report. The later
report recommended that missing fireproofing be repaired as part of a scheduled work program on
columns at various locations in WTC 1 and WTC 2.

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Plaza Level Box Columns

The purpose of the inspection of the Plaza Level box columns was to assess their overall structural
integrity, including the condition of the fireproofing. The proposed inspection interval was four years.
One such inspection was conducted in April of 1998. The east face of WTC 1 and the north face of
WTC 2 were visually inspected between the Concourse Level ceiling and the underside of the Plaza Level
slab. The columns between the Concourse floor level and ceiling level were inaccessible due to their
enclosures.

The report recommended no immediate and priority repairs. However, it noted that fireproofing was
missing from approximately 2 percent to 3 percent of the Plaza Level box columns and seated beam
connections in WTC 1 and about 1 percent to 2 percent in WTC 2. The report recommended that

PANYNJ to clean all exposed steel on Plaza Level columns 236, 242, and 248 in WTC 1 and
repair damaged fireproofing on columns and seated beam connections in both towers.

Bracing Below Elevation 294 ft 0 in.

Below Elevation 294 ft 0 in. (Sublevel 1) in both WTC 1 and WTC 2, diagonal bracing was used in place
of deep spandrels between the exterior columns to resist lateral loads from the tower above.

The purpose of the inspection of the bracing system below elevation 294 ft 0 in. in the perimeter walls of
the towers was to:

•

Assess the overall performance and structural integrity of the bracing (and, in 1991 only, the
transfer trusses below elevation 310 ft 0 in. in WTC 2);

•

Identify specific locations of structural distress or damage;

•

Identify locations of damage to the structural fireproofing systems; and

•

Provide recommendations for remedial work for both structural and fireproofing damage.

Bracing surveys were performed in 1991 and 1995. No immediate and priority recommendations were
made in these reports. Routine recommendations were made to repair damaged fireproofing at several
locations. These included PANYNJ to clean, repair, and reinstall fireproofing on structural members at
Level B6 in the Mechanical Equipment Rooms of both towers. PANYNJ to repair all spray fireproofing
on the braces in WTC 1 and 2 and the transfer truss in WTC 2 between columns 242 and 248.

Hat Trusses

The purpose of the inspection of the hat trusses between floor 107 and the roof was to:

•

Assess the overall performance and structural integrity of the hat trusses;

•

Identify specific locations of structural distress or damage;

•

Identify locations of damage to the structural fireproofing systems; and

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•

Provide recommendations for remedial work for both structural and fireproofing damage.

Hat truss surveys were performed in 1992 and 1995. Although no immediate and priority
recommendations were made in the SII reports, routine recommendations were made to repair
fireproofing and gypsum wallboard at various locations in WTC 1 and WTC 2. No follow-up actions
were stated in the report. Fireproofing that was removed during the Facility Condition Survey inspections
performed by the PANYNJ in 1990 and 1991 for WTC1 and WTC 2, respectively, was found not to be
repaired.

Floor Framing for Mechanical Areas

The inspection program for the floor framing supporting the mechanical equipment rooms (MER)
consisted of the following:

•

Assess the overall performance and structural integrity of the steel and concrete framing

•

Identify locations of defects and signs of distress in slabs, partitions, column enclosures, and
concrete supports for mechanical equipment;

•

Identify locations of damaged fireproofing;

•

Compare the findings with those of previous inspections; and

•

Provide recommendations and procedures for remedial work for both structural and
fireproofing damages and/or inadequacies.

Surveys were conducted in 1992, 1996, and 1999. No immediate repair recommendation was reported in
the 1992, 1996 and 1999 reports. Priority recommendations were made in the 1996 and 1999 reports.
The 1996 priority recommendation called for reapplication of fireproofing at various locations in WTC 1
and WTC 2. The 1999 priority recommendation called for repair of a water leak in an overhead pipe on
floor 75 of WTC 1. The report included routine recommendations for repair of cracked and spalled
concrete slabs in the 1996 and 1999 reports.

The reports indicated that damaged concrete masonry unit (CMU) walls in Level B6 of WTC 1 and
WTC 2 still existed in 1996. These damages were found initially during the structural integrity inspection
of the diagonal bracing in 1991 and again in 1995. Similarly, damaged fireproofing on the perimeter
diagonal bracing members at this level in WTC 1 and WTC 2, which was initially found in 1991 and
1995, still existed in 1999.

Floor Framing for Tenant Areas

The inspection program for the floor framing supporting the tenant areas consisted of the following:

•

Assess the overall performance and integrity of the steel and concrete framing;

•

Identify locations and signs of distress in slabs, partitions, column enclosures, and steel
framing;

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•

Identify locations of inadequate fireproofing; and

•

Provide recommendations and procedures for remedial work for both structural and
fireproofing damage and/or inadequacies.

Surveys for floor framing supporting tenant areas were conducted in 1992, 1995, 1997, and 1999. No
immediate recommendation was made in the 1992, 1995, 1997 and 1999 reports. Priority
recommendations were made in the 1999 report concerning restoration of fireproofing on a truss on
floor 89 in WTC 2, repair of spalled concrete on floor 89 of WTC 2 and floors 33 and 91 in WTC 1, and
repair of damaged reinforcement on floor 91 of WTC 1.

In the 1992, 1995 and 1997 reports,

PANYNJ was directed to replace or repair damaged fireproofing on steel members.

Natural Frequency Measurements

The purpose of this inspection program was to determine the natural frequencies of oscillation of WTC 1
due to wind excitation. Only WTC 1 was instrumented with accelerometers at six locations on floor 108,
which measured the accelerations in both principal directions of the building with respect to time due to
wind. These natural frequencies were to be compared with corresponding values that had been
determined in the past. A significant change in the tower’s dynamic behavior was considered to be a
possible indication of diminishing structural integrity. According to the reports, characteristics that may
have been observed or inferred by review of the recorded acceleration data were:

•

Integrity of the lateral-load-resisting system;

•

Condition of the viscoelastic damping system;

•

Condition of other sources of inherent structural damping; and

•

Other changes that affect fundamental characteristics of the lateral-load-resisting system.

Reports were prepared by LERA in 1993, 1995, and 2000.

The 1993 and 1995 reports compared the

available measured first mode natural frequencies of WTC 1 to those determined by the structural
engineer in 1966, which were 0.084 Hz in the north-south direction and 0.096 in the east-west direction
(WSHJ 1966). A summary of the measured first mode natural frequencies from the 1995 report, which
contained the most current data, is shown in Table 8–2. No recordings were reported for the period
between 1981 and 1991.

These reports were prepared by Leslie E. Robertson Associates [WTCI 4073/66-L, 4056/66-L, 4094/66-L].

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Table 8–2. Measured first mode natural frequencies for WTC 1.

Measured Frequency (Hz)

Date

Wind Speed (mph) and

Direction

N-S E-W

10/11/78 11.5,

E/SE 0.098

0.105

01/24/79 33.0,

E/SE 0.089

0.093

03/21/80 41.0,

E/SE 0.085

0.092

12/11/92 49.0

0.087

0.092

02/02/93 20.0,

NW 0.085

0.095

03/13/93 32.0,

NW 0.085

0.094

03/10/94 14.0,

W 0.094

0.094

12/25/94 37.0,

W 0.081

0.091

a. No direction was given in the report.

Both the 1993 and 1995 reports concluded that the measured and computed first mode frequencies
compared well, especially for the greater wind speeds. The 1995 report also concluded that the February
1993 bombing had no permanent measurable effect on the dynamic response of WTC 1. Both reports
recommended that WTC 2 be instrumented similarly to WTC 1.

The 2000 SII report pointed out that PANYNJ had not been able to analyze the data acquired from the
instrumentation of WTC 1 since 1998 because the PANYNJ laboratory that contained playback and
analytical equipment necessary to assess the recorded data was dismantled in the fall of 1998. The report
recommended that the capability to assess and analyze the accelerometer data be re-established as soon as
possible. The report further recommended that WTC 1 be additionally instrumented at a mid-level floor,
and that WTC 2 be instrumented at its top floor and at a mid-level floor.

Natural Frequencies of Floors

The purpose of this inspection program was to determine the natural frequencies of the floor systems in
WTC 1 and WTC 2 and to compare them with corresponding values that had been determined in the past.
A significant change in the vibration characteristics of the floor system was considered to be a possible
indication of diminishing structural integrity.

For purposes of determining the natural frequencies of the floor construction, a typical tower floor was
divided into three zones, which corresponded to the type of floor truss that was used in that zone: short-
span zone, long-span zone, and two-way zone.

Vibration characteristics of the floor systems were studied both analytically and experimentally. In 1971,
Teledyne Geotronics of Long Beach, CA made field measurements of vertical vibration on floors 13, 27,
and 32 of WTC 1 using seismometers. The field measurements were obtained under the direction of
Skilling, Helle, Christiansen, Robertson (SHCR). SHCR also made analytical estimates of the natural
frequencies of the floor systems at that time (SHCR 1971). They determined that the natural frequencies
of the long-span and short-span trusses, considering viscoelastic damping, were 4.6 Hz and 7.9 Hz,
respectively. A summary of the natural frequency test results for WTC 1 is contained in Table 8–3.

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Table 8–3. Summary of natural frequency

test results for floors of WTC 1,

March 1971.

Floor Zone

Frequency Range (Hz)

Long-span

4.6 to 5.1

Two-way

4.6 to 5.7
7.0 to 7.9

Short-span 7.9

In March of 1995, Cerami and Associates, of New York, NY, made field measurements on floors 17, 22,
26, 38, and 88 of WTC 1 and floors 23, 24, an 58 of WTC 2 using the following equipment: piezo-electric
accelerometer, vibration meter, peak band pass filter, and strip chart recorder (Cerami 1996). The floors
were subjected to a standard heel-drop test or by jumping in place. All field work was performed under
the direction of LERA. A summary of the test results for WTC 1 and WTC 2 are given in Table 8–4.

The SII report produced by LERA in April of 1995 summarized the analytical and experimental results to
date (LERA 1995). Based on the available data, the report concluded that there had been no significant
measurable change in the performance of the typical floor systems in WTC 1 and WTC 2.

Table 8–4. Summary of natural frequency

test results for floors of WTC 1 and WTC 2,

March 1995.

Floor Zone

Frequency Range (Hz)

WTC 1

Long-span

4.5 to 5.3

Two-way

4.6 to 4.9
6.6 to 7.6

Short-span

7.8 to 8.8

WTC 2

Long-span

4.8 to 5.6

Two-way

4.9 to 5.4
7.5 to 7.8

Short-span

7.9 to 8.0

Viscoelastic Damping Units

A summary of the integrity of the viscoelastic damping units in WTC 1 and WTC 2 was given in a report
by LERA in 1996 (LERA 1996). Also given in the report is a historical review related to the performance
of the damping units.

The report concluded that based on the then available studies, the integrity of the damping units was
good, and that no action was required at that time beyond the routine testing of the damping units.

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Inspections Related to Explosion of February 16, 1993

Six different inspections were performed before and after repairs were made to WTC 1 in the aftermath of
the terrorist attack in February 1993. Summaries of these inspections were reported in a series of
inspection reports prepared for the PANYNJ.

No anomalies were detected in the welds used to repair

structural members.

8.5.6

Summary of Structural Integrity Inspection Programs

In general, the structural integrity inspections found that the structural systems of WTC 1, 2, and 7 were
in good condition. The inspection consultants made numerous routine and some priority
recommendations for repairs to the PANYNJ. According to the PANYNJ, all of the construction records
on repairs following the inspections were lost on September 11, 2001. Thus, it cannot be determined
whether all of the recommended repairs were performed. However, in 1999, the PANYNJ issued
guidelines requiring that fireproofing be upgraded for steel floor trusses when full floors were undergoing
alterations.

8.5.7

Modifications and Repairs to Structural Framing Systems of WTC 1, 2, and 7

Most of the modifications to the structural systems of WTC 1, 2, and 7 were done to accommodate tenant
requirements (see NIST NCSTAR 1-1C). These generally involved cutting holes in existing floor slabs to
construct new stairways linking two or more floors or reconstructing the floor system over previously cut
openings. In other cases floor or column members were reinforced to accommodate new floor loadings
imposed by tenant requirements.

Modifications to the structural systems were to follow the Tenant Construction Review Manuals of
PANYNJ, which are summarized in Sec. 8.2.

Modifications and Repairs Made to WTC 1

Openings Made in Floor Slabs

Slab openings were made in the floor slabs on the following floors during the following years:

Floors 93 to 95, 1978 (openings were made in floors 94 and 95 between columns 901, 902,
1001, and 1002 in core)

Floors 99 to 101, 1979 (openings were made in floors 100 and 101 between columns 707,
708, 806, and 807 in core, and in floor 99 between columns 701, 702, 801, and 802)

Floors 89 and 90, 1985 (opening was made in floor 90 between columns 901, 902, 1001, and
1002)

Floor 107, 1995 (opening was made in floor 107; location could not be determined)

These reports can be found in WTCI-67-L.

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Floors 105 to roof, 1997 (opening was made in floor 105 near columns 704 and 804A in the
core)

Floors 93 to 100, 1999 (openings were made on all floors; location could not be determined)

Most openings were made to accommodate new stairs and elevators.

Closing of Previously Opened Floor Slabs

Openings that had been cut primarily for stairways were subsequently closed on the following floors
during the following years:

Floor 95, 1972 and 1985 (new beams and floor deck were added near lines 124 and 239)

Floors 91 and 92, 1987 (new beams and floor deck were added between columns 901, 902,
1001, and 1002 in the core)

Floors 96 and 100, 1998 (new beams and floor deck were added between columns 119 and
123 on floor 96 outside of the core and near columns 707, 708, 806, and 807 on floor 100)

Structural Members that were Reinforced

Various floor members were reinforced to accommodate floor loads that were greater than the original
design loads. Members were reinforced on the following floors during the following years:

Floors 97 to 100, 1979 (cover plates were added on existing beams on floor 98 between
columns 601 and 602 and between columns 701 and 702; on floor 98, diagonals were added
to existing floor trusses on the east side of the core between columns 218 and 221; on floor
99, floor trusses along lines 309, 311, 313, 315, 317, 319, 321, 323, 325, 327, and 329 were
reinforced; and, on floor 99, core perimeter columns were reinforced)

Floor 86, 1996 (floor trusses were reinforced in the northwest corner of the building)

Floor 85, 1998 (cover plates were added to existing beams and existing floor trusses were
reinforced)

Floors 47 and 48, 2001 (floor trusses were reinforced)

Repair Work Following the February 26, 1993, Explosion

Damage from the Explosion

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Briefly, the explosion tore out the diagonal brace between column 324 at Level B2 and column 327 at
Level B1 and severely bent the diagonal brace between column 324 at Level B2 and column 321 at
Level B1. Spandrel beams at Level B1 from column 321 to 324 and from column 324 to 327 were also
damaged by the blast. Spandrels were bowed and cracked and some had missing bolts.

The explosion caused a crack along the field splice in column 324. Ultrasonic testing determined that the
crack extended across the full width of the weld on the south face of the column and at each end of the
weld on the north face. A magnetic-particle testing procedure determined that the crack extended across
the east face of the column and the majority of the weld on the west face as well.

The explosion also damaged floor beams framing into the tower side of column 324 at Levels B1 and B2.
Concrete spandrel beams at Level B3 between columns 318 and 330 also sustained damage. Masonry
walls in WTC 1 were breached over distances of approximately 50 ft to the east and 120 ft to the west of
the blast origin.

Repair Work

The diagonal bracing members between Levels B1 and B2 that were damaged by the explosion were
removed and replaced with new members.

New plates were added to the damaged spandrel beam at Level B1 between columns 324 and 327 and
between columns 321 and 324. Also, the cracked weld on the south face of the spandrel beam at
Level B1 near column 324 was removed and replaced.

An eight-step procedure was prescribed for repair of the crack in column 324 immediately adjacent to the
field weld at the column splice above Level B2. No documentation was found to confirm that this crack
was repaired according to that procedure.

Repairs were made to the floor beams framing into columns 321, 324, and 327. Repairs were also made to
connections between floor beams and columns on Levels B3 and B4. Along the south face of WTC 1, the
damaged concrete spandrel beams were demolished and replaced.

Modifications and Repairs Made to WTC 2

Openings Made in Floor Slabs

Slab openings were made in the floor slabs on the following floors during the following years:

Floor 77, 1979 (openings were made at nine locations in the northeast quadrant of the
building)

Floor 96, 1987 (opening was made near columns 901 and 902 in the southeast quadrant of the
building)

Floors 94 and 95, 1993 (opening was made between columns 507, 508, 607 and 608)

Floors 99 to 101, 1997 (openings were made; locations could not be determined)

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Floor 99, 1998 (opening as made between columns 601, 602, 701, and 702 in the core on
Floor 99)

Floors 25 and 26, 1999 (opening was made near column 901 in the core)

Floors 88 and 89, 1999 (openings were made; locations could not be determined)

Most openings were made to accommodate new stairs.

Closings of Previously Opened Floor Slabs

Openings that had been cut were subsequently closed on the following floors during the following years:

Floors 37 and 38, 1997 (new framing and floor deck was added near column 608)

Floors 95 and 96, 2000 (new beams and floor deck were added between columns 901, 902,
1001, and 1002)

Structural Members that were Reinforced

Members were reinforced on the following floors during the following years:

Floor 96, 1993 (a number of floor trusses and their connections were reinforced in the
northeast quadrant of the building)

Floor 81, 1991 (two-way floor trusses were reinforced in area occupied by United Parcel
Service)

Other Modifications

In 1994, the slab in the elevator lobby on floor 90 (bounded by columns 702, 703, 902, and 903) was
repaired for Fiduciary Trust; NIST has not found evidence of the reason for this modification. The
existing slab was demolished and was replaced with a 5 in. thick lightweight aggregate concrete slab.

Modifications and Repairs Made to WTC 7

Modifications Made due to New Loading Requirements

Members were reinforced primarily to accommodate floor loads that were greater than the loads for
which these members were originally designed. Members were reinforced on the following floors during
the following years:

Floor 38, 1988 (cover plates were added to existing beams along lines 30, 35, 37 and 40)

Floor 24, 1989 (cover plates were added to existing beam on line 45 and to two adjacent
beams)

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Floor 47, 1989 (cover plates were added to existing beams on line 2-5 and to the existing
girder on line 56)

Floors 11 and 12, 1990 (cover plates were added to eight existing beams and girders in the
northwest corner of the building on floor 11, and to three existing beams between lines 48
and 49 and to the girder between columns 70 and 73 on floor 12)

Floor 19, 1991 (cover plates were added to existing beams; location could not be determined)

Floor 12, 1992 (cover plates were added to 11 existing beams in the northwest corner of the
building, and a new beam was added between existing beams)

Floors 18 and 19, 1992 (cover plates were added to existing beams on lines 31, 32, and 33)

Floor 28, 1993 (additional shear studs were added to existing beams located in the
mechanical/electrical room)

Floors 7 and 8, 1993 (a new beam was added between lines 7 and 8)

10.

Floors 7-29, 1994 (cover plates were added to 22 existing beams between lines 5 and 25 on
the south side of the building and on each floor between levels 7 and 29, and to eight existing
beams on the east side of the building between lines 31 and 37)

11.

Floor 20, 1995 (cover plates were added to existing beams along lines 23 and 25, and WT
sections and cover plates were added to existing beams east of line 19)

12.

Floor 37, 1999 (a new beam was added between two existing beams along lines 76 and 77)

13.

Floor 13, 1999 (additional shear studs were added to an existing beam; location could not be
determined)

14.

Floor 40, 1999 (four new beams were added near column 76 and WT sections were welded to
the bottom of two existing beams)

15.

Floor 31, 2000 (cover plates were added to an existing beam between columns 77 and 80)

16.

Floor 38, 2000 (cover plates were added to existing beams between columns 76 and 77 and
between columns 77 and 78, and to existing girders between columns 76 and 79, 77 and 80,
and 78 and 81)

17.

Floor 39, 2000 (new beams were added between columns 76 and 77)

Openings Made in Floor Slabs

The floor slabs on floors 41 and 43 were completely removed on the east side of the building to
accommodate the trading floors for Salomon Brothers Inc. Columns 76, 78, 79, 80, and 81 were
reinforced with plates that ran from the top of the 39th floor to the underside of the 49th floor due to the
removal of the floor slab at the 39th floor. Similarly, column 74 was reinforced with plates that ran from

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the top of the 43rd floor to the underside of the 44th floor due to the removal of the floor slab at the 43rd
floor.

Other slab openings were made in the floor slabs on the following floors during the following years:

Floors 3 and 4, 1989 (openings were made on the 3rd floor on the west, north, and east sides
of the building; on the 4th floor, openings were made on the north side of the building)

Floor 3, 1989 (openings were made near columns 24 and 25)

Floor 11, 1990 (opening was made between columns 77, 78, 80, and 81)

Floor 43, 1994 (opening was made near column 71 in the core area)

Modifications Made to Beam Webs and Flanges

Modifications were made to beam webs and flanges on the following floors during the following years:

Floor 28, 1993 (openings were cut in the web of an existing beam; location could not be
determined)

Floors 4 to 7, 16, 21, 29, 38, and 45, 1993 (notches were cut in the bottom flanges of various
beams and plates were welded to the upper side of the bottom flanges)

Floor 1, 1998 (notch was cut into the top flange of an existing beam, and two plates were
welded under the top flange; location could not be determined)

Floors 36 to 44, 1999 (openings were cut in the web of an existing beam framing into column
75 on all floor levels; the beams was reinforced with web plates and a WT section welded to
its bottom flange)

Floors 42 and 44, 1999 (openings were cut in the webs of numerous beams along the north
and east sides of the building)

Other Modifications

A list of structural modifications that were made to WTC 7 prior to April of 1997 is given in Chapter IV,
Sec. A(5) of the Facility Condition Survey Report for WTC 7 (PANYNJ 1997). The following is a
summary of the modifications that are noted in that report:

In the Convention Area on the 3rd floor between column numbers 45 and 48A, steel plates
were installed around the perimeter of the room between the slab and the floor surface
(behind the wall coverings and above the suspended ceiling). According to the PANYNJ
report, these plates were installed to protect attendees of the Convention Center from the
magnetic field generated from the Con Ed Substation beneath the conference rooms. No
documentation was located that provides any additional details on this modification.

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Chapter 9

OMPARISON OF

IRE

AFETY

ODES AND

RACTICES

9.1

COMPARISON OF FIRE PROVISIONS IN BUILDING CODES

9.1.1 Introduction

The 1968 New York City (NYC) Building Code (NYCBC 1968) was compared with four other building
codes to determine the extent to which the codes and mandatory referenced standards were utilized in the
design and construction of the towers. The other codes are: the 1964 New York State Building
Construction Code (NYSBC 1964); the 1965 Building Officials and Code Administrators/Basic Building
Code (BOCA/BBC 1965); the 1967 Municipal Code of Chicago Relating to Buildings (MCC 1967); and
the 2001 edition of the NYC Building Code (NYCBC 2001). In addition, comparisons of fire safety
requirements were made to the 1966 edition of the National Fire Protection Association (NFPA 101),
Code for Safety to Life in Buildings and Structures. While not a building code, National Fire Protection
Association (NFPA) 101 is widely adopted for its requirements on life safety in fires.

The codes selected for comparison are laws or nationally or regionally recognized model regulations that
reflect the standards of practice of the time. The 1964 New York State Building Construction Code was
the governing building code outside the New York City limits. The 1965 BOCA Basic Building Code
was typically adopted by local jurisdictions in the northeastern region of the United States. The 1968
NYC Building Code is compared with the 1967 Municipal Code of Chicago (MCC) to see whether there
are any substantial differences in the fire safety requirements of the two codes. In the late 1960s and early
1970s, several tall buildings were built in Chicago including the Sears Tower (110 stories) and the John
Hancock Tower (100 stories), both of which were classified as business use and incorporated innovative
design features. In addition, the 2001 edition of the NYC Building Code was compared with the 1968
version to examine the extent to which Local Laws modified the code provisions, and in most cases, is
only addressed in areas where changes occurred between the two versions.

A provision by provision comparison was made between the 1968 NYC Building Code and these codes.
The code provisions that were compared are limited to the requirements related to structural stability,
active and passive fire safety, and emergency egress and are presented in the reports NIST NCSTAR 1-E
and 1-F.

The NYC Building Code was regularly modified by local laws, two of which, Local Law 5 (1973) and
Local Law 16 (1984), had a significant influence on the fire and life safety features of WTC 1 and
WTC 2, even though the buildings were completed and occupied at the time of adoption. Normal practice
is not to apply building code changes to existing buildings, although Local Laws 5 (1973) and 16 (1984)
did contain some retroactive provisions. The Port Authority of New York and New Jersey (PANYNJ or
Port Authority) chose to follow the revised provisions and to retrofit the buildings as required under the
new provisions. The resulting changes to WTC 1 and WTC 2 are discussed primarily in the sections on
modifications to the building systems and in reports NIST NCSTAR 1-G and 1-H. Local Laws 5 (1973)
and 16 (1984) were in place at the time WTC 7 was designed and constructed, and the requirements of

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these local laws were reflected in that building. There were no significant revisions to the NYC Building
Code that affected the fire and life safety features of WTC 7.

9.1.2

Interrelation of Codes, Standards, and Practices

The provisions contained in building codes generally specify what is required under specific conditions.
The building codes contain references to standards that provide further details on how the provisions are
to be implemented to meet the intent of the code. New York City makes use of nationally recognized
technical standards but adopts them with modifications to meet local needs and accepted practices. These
modified standards are known as Reference Standards (RS) and are available from the city. Reference
standards take on the force of law when they are included in the building regulation as mandatory
references and enforced by the regulatory official. For the WTC towers, the Port Authority utilized the
New York City reference standards and the source standards from NFPA and others in design guidelines,
manuals, and procurement contracts associated with system upgrades.

In some cases, trade associations and professional societies develop practices that may guide how
building design and construction work is done. While not strictly enforceable unless referenced in the
code, such practices represent a consensus of what is reasonable or prudent. A few, relevant practices are
discussed in this section.

9.1.3

Comparison of New York City and Contemporary Building Codes

While New York City developed its own building code, its code development committees were
influenced by the same forces that bore on the national model codes. Thus, there were relatively few
differences between the NYC Building Code and the others.

Construction Classification

The model building codes classify building constructions into different “Types.” Although there are some
variations in categories, they are reasonably consistent.

The main categories are Type 1 (fire resistive),

Type 2 (non-combustible), Type 3 (combustible), Type 4 (heavy timber), and Type 5 (ordinary).

Types 1 and 2 are constructed with non-combustible exterior and interior bearing walls and columns. Fire
resistance ratings (see Fire Ratings) are greatest for Type 1, and Type 2 is any (non-combustible)
construction not meeting Type I requirements. For Type 3, exterior bearing walls are non-combustible
and interior bearing walls and some columns may employ approved combustible materials. Type 4 is
known as

heavy timber,

which utilizes large, solid cross section wooden members such as in post and

beam construction. Type 5 is traditional wood frame construction. Common non-combustible structural
elements are made of steel or reinforced concrete. Combustible structural elements are usually made of
solid- or engineered-wood and laminates.

Combustibility of the materials in a structural element is determined in an ASTM International
(ASTM) E 136 test in which the material is placed in a furnace at 750

C (1,380 °F), which is a “typical”

Construction type definitions varied among the model codes until an effort in the 1970s by the Board for the Coordination of

the Model Codes to eliminate unnecessary differences.

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fire temperature. Some minor surface burning (e.g., from paint or coatings) is allowed in the first 30 s,
but there cannot be any significant energy release as indicated by more than a 30

C (54

F) increase in

the furnace temperature, and the test specimen cannot lose more than half its initial mass. Materials that
pass are designated non-combustible, and the rest are combustible.

Within each construction type, there are several sub-categories determined by the fire resistance ratings of
the columns, beams, and floor supports. In some codes, these sub-categories are identified by letters
following the type (e.g., 1B or 3A) or by a set of three numbers that represent the fire resistance required
(in hours) of the columns, beams, and floors, respectively (e.g., Type 1 [3,3,2]).

For unsprinklered office buildings, the following construction classes are permitted in the five building
codes reviewed.

•

Type 1A and 1B—NYCBC 1968, NYSBC 1964, BOCA/BBC 1965 (Unlimited height)

•

Type 1A, 1B, 1C, 1D—NYCBC 2001 (Height limited to 75 ft)

•

Type 1A only—MCC 1967 (Unlimited height)

The 1938 NYC Building Code did not include Type 1B construction for office occupancies. The reasons
for the inclusion of Type 1B construction for office occupancies in the 1968 NYC Building Code are not
recorded (recordkeeping in the codes and standards development process was very poor prior to the
Hydrolevel vs. ASME Supreme Court decision in 1982). The codes then and now tend to follow each
other, as champions of changes to one code usually try to change all of the codes.

The 1950 edition of the Basic Building Code (BOCA), the regional model code used in the Northeastern
United States, included a Type 1B construction class with unlimited height and area for business and low
hazard storage occupancies without sprinklers. Among other model codes, the Standard Building Code
(1946-47 edition, SBCCI) had a Type 2 construction similar to Type 1B for business occupancies and
buildings more than 80 ft in height; the National Building Code (1934 edition, NBFU) had a semi-
fireproof construction similar to Type 1B for buildings above 75 ft; and the Uniform Building Code (1927
edition, ICBO) had a Type 2 construction similar to Type 1B for buildings above 75 ft. The 1968 NYC
Building Code is consistent with the 1950 BOCA in its inclusion of the Type 1B construction.

Mandatory sprinkler requirements for new high-rise buildings was first introduced in the NYC Building
Code in 1984 (by Local Law 16), in BOCA in 1984, and in the Chicago Building Code (which allows a
compartmentation alternative) in 1975. Before Local Law 16 was adopted, the 1968 NYC Building Code
permitted Type 1A, 1B, 1C, and 1D construction for sprinklered office buildings of unlimited height. In
the 2001 NYC Building Code, the minimum permitted construction classification for sprinklered office
buildings of unlimited height is Type 1C.

Selection of Construction Type for WTC Towers

The 1938 NYC Building Code recognized one construction type for buildings of unlimited height and
area, namely Class 1—Fireproof Structures, which required a 4 h fire rating for columns and a 3 h rating
for floors. In the 1968 NYC Building Code, Group I (Noncombustible) construction was subdivided into
“Class 1A—4-hr protected” and “Class 1B—3-hr protected” construction. Class 1A specified similar

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protection as the previous Class 1, and Class 1B specified a 3 h rating for columns and girders supporting
more than one floor and a 2 h rating for floors including beams. Both Class 1A and Class 1B construction
permitted unlimited height and area for unsprinklered business occupancy.

If a building qualifies for more than one construction classification, such as Class 1A or Class 1B, all of
the building codes do not say which classification should be used. In such situations, the classification
selected for construction is at the discretion of the owner/architect.

Fire Ratings

The structural elements of a building are protected against failure in fire for a specified period, as
determined in the ASTM E 119 test. The intent of the fire rating requirements is for the structure as a
minimum to withstand design loads (including fire) without local structural collapse until occupants can
escape and the fire service can complete search and rescue operations.

Fire resistance requirements in the building codes are greatest for structural members that are essential to
the stability of the building as a whole. These include columns and other major gravity load carrying
members that connect directly to columns such as girders and trusses.

For various construction classes, the building codes specify different fire resistance ratings. The building
codes reviewed all specify fire resistance ratings for high-rise office occupancies as follows:

•

Type 1A

−

Columns: 4 h (supporting more than one floor)

−

Beams: 3 h (floor construction)

•

Type 1B

−

Columns: 3 h (supporting more than one floor)

−

Beams: 2 h (floor construction).

•

Type 1C (for sprinklered buildings only)

−

Columns: 2 h (supporting more than one floor)

−

Beams: 1½ h (floor construction).

The choice among permitted construction classes for a particular building is made by the architect and/or
the owner. Thus, an unsprinklered high-rise office building that was designed according to the
1968 version of the NYC Building Code could follow either Type 1A or 1B, and if designed subsequent
to the passage of Local Law 16/1984, a high-rise office building would have to be sprinklered but it could
follow Type 1C as a minimum classification. Similar reductions in the minimum required fire resistance
ratings for sprinklered buildings are found in all national model building codes over this period, as
requirements for fire sprinklers, especially in high-rise buildings, have become common.

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Type 1B, and eventually Type 1C, construction was permitted for high-rise office occupancies because
this occupancy is considered low risk. Most other use groups in high-rise buildings were restricted to
Type 1A, which is the construction type with the maximum structural fire protection defined in these
codes.

Practice Related to Fire Resistance Ratings

Building codes specify fire resistance ratings for the structural members of buildings as a function of

•

Building height,

•

Construction type, and

•

Use group (Occupancy),

with modifications for the presence of fire sprinklers. In the International Building Code (IBC 2003),
Section 703.2 states,

The fire-resistance ratings of building elements shall be determined in
accordance with the test procedures set forth in ASTM E 119 or in
Section 703.3. Where materials, systems or devices that have not been
tested as part of a fire-resistance-rated assembly are incorporated into the
assembly, sufficient data shall be made available to the building official
to show that the required fire-resistance rating is not reduced.

This section appears to have been based on the 1981 BOCA (Sec. 1403.1.1) which required that, “The fire
resistance ratings of building assemblies and structural elements shall be determined in accordance with
the test procedures set forth in ASTM E 119…”

IBC Section 703.3 allows the fire resistance of a building element to be established by:

•

Fire resistance designs documented in approved sources

•

Prescriptive designs of fire-resistance-rated building elements prescribed in Section 720, or

•

Calculations in accordance with Section 721.

These alternative methods were included in the other model building codes on which the IBC is based.
For example, the 1997 Uniform Building Code contains tables of prescribed ratings for specific materials
and assemblies which may be depended on as an alternate to ASTM E 119 testing. The 1994 Standard
Building Code permits calculated fire resistance using specified methods or testing by ASTM E 119.

In the 1968 New York City (NYC) Building Code, Section C26-501.1 requires that,

Samples of all materials or assemblies required by this code to have a
fire-resistance rating, … shall be tested under the applicable test
procedures specified herein … . The fire-resistance rating of materials
and assemblies listed in reference standard RS 5-1 (which references

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ASTM E 119) may be used to determine conformance with the fire
resistance requirements of this code.

In traditional practice, the architect (sometimes different from the design architect, called the code
architect) specifies the fire resistance ratings needed to comply with the building code. The required
ratings are normally not shown on the architectural drawings (although the construction type may be);
rather they are shown in the supporting material submitted to the building department for plans review.

Building codes require that an ASTM E 119 test be performed to determine the details of the assembly
that would meet the requirement. In some cases the architect may choose to use an assembly that has
already been tested and rated. Such assemblies are listed by testing laboratories in directories, databases
accessed on test laboratory web sites, or in test reports available from manufacturers of materials used in
the assemblies such as the producers of fireproofing products. These sources are very detailed and
indicate the thickness of the specific product tested that is required to achieve a specific hourly rating.
Enforcement officials are expected to verify that even the smallest details are followed or that the
variations do not affect performance. Alternatively, some codes include descriptions of generic
assemblies that can be assumed to achieve specified ratings, or provide calculation methods to determine
the thickness of spray applied material needed to achieve a specific rating. These methods are often based
on W/D (width to depth) ratios, which must be applied differently as a function of member geometry.

An additional variable that affects the needed thickness of fireproofing is whether the assembly is
restrained. It is traditionally assumed that an assembly that is thermally restrained requires less
fireproofing. Note that the NIST tests of the floor assemblies used in the towers showed the opposite.
However, the definition of restrained is not trivial and needs to be specified by the structural engineer.

In some cases, it is not clear who actually determines the required thickness of fireproofing material. If
the bid specifications for the fireproofing contract simply require the assemblies to be sprayed to achieve
a specific hourly rating (which may be the case where a specific product is not identified to be used), then
the thickness determination may be left to the fireproofing contractor.

Standards of practice for spray applied fireproofing are contained in the Underwriters Laboratories (UL)
guide card (UL 2001) (although technically this only applies when a UL Listed assembly is used) and in
manuals published by the Association of Wall and Ceiling Industries (AWCI 1997). There is also the
American Institute of Architects Masterspec on Spray Applied Fireproofing (AIA 2000) that is similar to
the Association of Wall and Ceiling Industries (AWCI) manual. Additional guidance may be provided by
the manufacturers of fireproofing materials that are specific to the characteristics of those products.

The UL guide information (BXUV) includes a number of limitations on the application of listed
assemblies, including:

•

Limits on the size (flange width and web depth, pipe outer diameter) without the use of a
mechanical break such as metal lath or fasteners,

•

Use of bonding agents or conduct of a bond strength test in accordance with ASTM E 736
whenever the steel is painted (other than a paint specified in the listing)

•

Conduct of thickness testing in accordance with ASTM E 605.

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The AWCI Technical Manual contains similar limitations and instructions, including the production of a
test report on thickness and density, bond strength, correction of deficiencies, and patching procedures.
Further, the AWCI manual describes a quality assurance program and requires that the fireproofing
contractor retain the services of the licensed engineer or architect qualified to make the determination of
restrained and unrestrained members. The AWCI manual is not cited as a mandatory reference in any
model code; rather it is voluntarily followed by contractors that are members of the association. The
American Institute of Architects (AIA) Masterspec is similar to the AWCI manual, except that it is
written to be incorporated into the fireproofing contract, which would make it enforceable against the
contractor.

Some building codes require that spray applied fireproofing on steel structural members be subjected to
inspection at the time of installation. Local Law 55 (1976) amended the 1968 NYC Building Code to
require that all required, spray-applied fireproofing of structural members except those encased in
concrete be subjected to a controlled inspection, meaning that it must be conducted under the supervision
of a building inspector or a licensed design professional who assumes responsibility for compliance. This
provision applied to all installations after the date of enactment (November 1, 1976) and was not
retroactive. The inspection was to include verification of the thickness of the material, its density, and its
adhesion, each utilizing a specific ASTM test method. There are no code requirements nor general
practice by which spray applied fireproofing is inspected over the life of the building. Most building
codes contain a requirement that spray applied fireproofing that is installed in areas where it is subject to
mechanical damage shall be protected and maintained in a serviceable condition. For a detailed
discussion of the fireproofing system found in the towers, see NIST NCSTAR 1-6A.

9.1.4 Occupancy

Group

All building codes define categories of occupancy (which may have more than one sub-class). The group
designations vary in different codes. The ones presented here are those used in the 1968 NYC Building
Code. These are:

•

High Hazard (Group A)

•

Storage (Groups B-1 and B-2)

•

Mercantile (Group C)

•

Industrial (Group D-1 and D-2)

•

Business (Group E)

•

General Assembly (Group F-1 through F-4)

•

Educational (Group G)

•

Institutional (Groups H-1 and H-2)

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•

Residential (Groups J-1 through J-3)

•

Miscellaneous (Group K)

Building codes use occupancy as a surrogate for risk factors that determine the level of performance
needed. For example, occupancy is determined by a combination of factors such as types and quantity of
combustible contents, common ignition sources, and typical occupant characteristics. Business
occupancies (which includes office buildings) are considered among the lowest risk because they
typically contain grades of furniture that constitute relatively low combustible loads, few ignition sources,
and a population that is predominately adult, and not sleeping. The most risky occupancies are High
Hazard, in which are found highly flammable, toxic, or explosive materials, and Institutional
(e.g., hospitals and prisons) in which occupants are likely to be incapable of unassisted egress.

In some codes, including the 1968 NYC Building Code, occupancy groups are subclassified with a “fire
index” rating in hours. For example, “high hazard” occupancy is assigned a fire index of 4 h, while
“business” occupancy is assigned a fire index of 2 h. These fire indexes are used to specify the
performance of separations between spaces of different use in a mixed-use building. For example, spaces
of different use with the same fire index are separated by a partition of lower rating than for uses with a
different fire index. Many buildings are mixed use because they contain spaces used for different
purposes as defined in the building codes.

Business (Group E)

The business use group includes all office buildings, but this can range from a construction office in a
trailer temporarily located on a construction site to a high-rise office building like the World Trade
Center. Business occupancies are characterized by an average occupant load, occupants who are
generally physically fit and do not sleep in the space. Combustibles are average in quantity and include
higher quality furniture and paper.

Assembly (Group F)

The assembly use group includes any place used for the gathering of more than 50 people for civic, social,
or religious functions, recreation, food and drink, or awaiting transportation. Assembly use is
characterized by the highest occupant loads, which may include families with small children and older
adults. Combustibles are light in quantity and vary in character depending on specific use.

9.1.5 Egress

Systems

The 1968 NYC Building Code contains requirements for the number and capacity of stairs and for the
assumed occupant load that are similar to requirements in the other contemporaneous codes
(see Appendix A). Codes of the time required that multiple stairs be located “as remote from each other
as practicable.” New York City permits scissor stairs,

and the code requires the exit doors to be at least

15 ft apart. Local Law 16 (1984) first imposed a remoteness requirement of 30 ft or one-third the

Scissor stairs refers to two separate interior stairways contained within the same enclosure and separated by a fire rated

partition.

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maximum travel distance of the floor (whichever is greater), which was not retroactive, so it did not apply
to WTC 1 and WTC 2 but did apply to WTC 7.

The 1968 NYC Building Code also contains a requirement that, “ …vertical exits should extend in a
continuous enclosure to discharge directly to an exterior space or at a yard, court, exit passageway or
street floor lobby …” (C26-602.4). Similar requirements are found in the 1965 BOCA Basic Building
Code and in 1966 NFPA 101, but not in the 1964 New York State Building Construction Code or the
1966 Municipal Code of Chicago. Current code language (2003 IBC, section 1003.6) defines continuous
as: not “ … interrupted by any building element other than a means of egress component.”

The requirement for exit stairs to discharge to a public way was the subject of ongoing discussion with
respect to the A and C stairs in WTC 1 and WTC 2 terminating at the mezzanine level, which was not at
street level but rather at the Plaza level. The Port Authority’s position was that the Plaza was a street and
the Concourse was an underground street, and that the arrangement met the intent of the Code. NIST
found PANYNJ documents indicating that the NYC Department of Buildings agreed with this
interpretation (e.g., Solomon 1975), but did not find any documents from the NYC Department of
Buildings confirming this. Thus, the issue continued to come up as a variance with the Code as late as
1996 (see Section 11.4).

9.2

SUMMARY OF DIFFERENCES BETWEEN CODES

In Construction Classifications, NYC Building Code 1968, NYS Building Code 1964 and
BOCA/BBC 1965 all recognized Class 1A or Class 1B (with the same fire resistance ratings for building
elements) for most unsprinklered buildings of unlimited height while MCC 1967 recognized only
Class 1A. New York City imposed a 75 ft height limit on unsprinklered buildings with the adoption of
LL 16 (1984).

At the time of construction, sprinklers were primarily for property protection and were rare even in high-
rise buildings (except for underground spaces). Fire alarm systems were mostly manually initiated, but
there was a concern about smoke being recirculated through the heating, ventilating, and air conditioning
(HVAC) systems, so smoke detectors controlled dampers at return shafts to prevent this. This is the
arrangement of the fire alarms system originally installed in the towers. Voice communication systems
were a response to phased evacuation with the recognition that it was necessary to provide instructions to
occupants who were relocated or held within the building at least until they were told to leave.
Requirements for voice systems first appeared in national standards in the mid-1980s (e.g., the 1985
edition of NFPA 72F), at the same time as New York City adopted LL 16-1984.

All building codes rely on referenced technical standards to provide the details of design, installation,
operation, and maintenance of required systems. Most building codes reference national (consensus)
standards as published, but New York City cites its own Reference Standards that are based on the
national standards but are often highly modified. For example, fire alarm systems and fire sprinkler
systems are addressed in RS 17, with Class E fire alarm systems (required in office occupancies) covered
in RS 17-3A and general fire alarm system requirements in RS 17-5. The former is written entirely by a
NYC code committee, and the latter is based on NFPA 72 (National Fire Alarm Code), but highly
modified by the deletion of many sections and modification of many others. One major modification is
the fact that RS 17 does not incorporate the NFPA 72 “Survivability” section for high-rise voice

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Chapter 10

NFLUENCE OF

ODES AND

TANDARDS ON THE

ESIGN AND

ONSTRUCTION OF

WTC

AND

WTC

10.1

EGRESS SYSTEM DESIGN

One of the largest impacts to the design of World Trade Center (WTC) 1 and WTC 2 resulting from the
decision to follow the 1968 New York City (NYC) Building Code rather than the 1938 Code was the
impact on the emergency egress system. The other large impact was the use of the 1-B Construction
Class introduced in the 1968 Code rather than the 1-A Class that would have been required under the
1938 Code, in Sec. 9.1.3, see “Selection of Construction Type.” In 1963, the Port Authority of New York
and New Jersey (PANYNJ or Port Authority) instructed the designers of the WTC to follow the then
current 1938 NYC Building Code. During this time, the code was in the process of being revised, and in
1965, the Port Authority directed its designers to adopt the draft version of the new code for their final
designs. Some of the advantages of the new draft code were noted to be the following (Levy 1965):

•

Fire towers

could be eliminated;

•

Provisions for exit stairs were more “lenient;” and

•

Criteria for partition weights were more “realistic.”

It was not certain whether all the changes being proposed to the 1938 code would be incorporated into the
final version of the new code. Thus in 1966, the Chief Engineer of the Port Authority suggested that the
“architect/engineers prepare a listing of the elements of the design which do not conform to old code
requirements, but are acceptable under the new. With this list in hand, we could initiate discussions, at
top level in the Building Department, to see if we can secure agreement to go along with our design
(Kyle 1966).”

A one-page document,

dated “2/15/67”, with the initials “CKP” listed the following items:

1. Fire tower corridors [sic] eliminated.

2. Number of stairs reduced from 6 to 3. (Old plans had 5 stairs at 3’-8”

and 1 stair at 4’-8” for a total population of 390. New plans have 2
stairs at 3’-8” and 1 stair at 4’-8” allowing a population of 390.)

3. The size of doors leading to the stairs are [sic] changed from

3’-8” to 3’-0”.

A “fire tower” is a stair tower enclosed within a 4 h fire rated shaft that is entered through a naturally ventilated vestibule. The

1938 Code stipulated that one of the required exits in most buildings over 75 ft in height be a fire tower.

“Changes to Building to Conform to New York City Building Code,” dated 2/15/67.

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4. All stairs exit through a lobby. Old plans had fire tower stair exiting

through a fire enclosed corridor.

5. Shaft walls are changed from a 3-hour rating to a 2-hour rating.

6. Corridors are limited to a 100’ dead end and with a 2-hour rating.

7. Additional [word(s) missing] changed from 20 pounds per square

foot to 6 pounds per square foot (based on partition weight of
50 pounds to 100 pounds per linear foot).

Apparently, this list represents elements of the WTC design that would not have satisfied the 1938 code,
but did satisfy the then-current draft version of the new code.

Thus, the provision of three egress stairs located within the core exactly provided the 6½ units of required
capacity for the occupant load in the office spaces. By locating the stairs at the edges of the core it, could
be argued that they were as far apart as practical, but on some floors the provision of transfer corridors to
go around equipment and to recover tenant space from the termination of local elevator shafts brought the
stairs quite close with far less than the one third the maximum travel distance of the floor requirement of
the 1968 NYC Building Code. The proximity of the stairs on some floors also resulted in standpipe
spacings that exceeded the maximum 140 ft distance from any point on a floor in the 1968 NYC Building
Code, since the standpipes were located in the stairways.

10.1.1

Egress Provisions from Windows on the World

The 106th and 107th floors of WTC 1 contained the

Windows on the World

complex, consisting of the

Windows on the World

restaurant, the

Greatest Bar on Earth

, numerous banquet and function rooms,

kitchens and support areas, and offices from which the operation was run. While the configuration of the
space may have changed over the life of the building these functions were all present from the time the
building was first occupied.

Restaurants, bars, and function rooms are classified in building codes as assembly use, which carries a
significant increase in occupant load and consequent provisions for egress. The design occupant load for
assembly space is 15 ft

per occupant as opposed to the 100 ft

per occupant for the office use on most of

the floors. Thus, while the design number of occupants on an office floor was 390, the design number of
occupants for these floors was over 1,000 each (the exact number depends on the area of kitchens,
dishwashing, and office space on the floor, all of which is at 100 ft

per occupant).

Locating assembly space high in a building poses particular challenges to egress design since the capacity
of an egress component is not permitted to be decreased in the direction of travel. Thus where more or
wider stairs are provided to meet capacity requirements these must be continued all the way down through
the building with the associated impacts on space utilization.

Since Windows on the World first opened in April 1976 (Bhol 2005), it is unclear what conditions existed
from that date to the time the agreed solution was implemented in 1995. The dates suggest that the need
to provide for egress by the large occupant load of these floors was identified as a result of the
Memorandum of Understandings between the Port Authority and the NYC Department of Buildings and
FDNY executed in 1993 following the bombing. The Windows on the World facilities were closed

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following the bombing and reopened in 1996 after a complete refurbishment that included the egress
system changes (Bhol 2005).

A letter dated January 27, 1995, from the PANYNJ to the Deputy Commissioner of the NYC Department
of Buildings documents the confirmation of a meeting on December 6, 1994, at which they reached
agreement on a plan to address egress requirements from the 106th and 107th floors (Fasullo 1995). The
details of the agreed solution are summarized below. The Deputy Commissioner of the NYC Department
of Buildings signed the letter to show concurrence with the agreed solution, as verification of meeting
code requirements.

The basis for the agreed solution was to divide each floor into three areas of refuge in accordance with
Section 27-372 of the (then current) NYC Building Code to provide additional capacity to the existing
stairs in accordance with Section 27-367 of the (then current) NYC Building Code. Identical provisions
were included in the version of the 1968 NYC Building Code in effect when the buildings were built as
sections C26-604.5 and C26-603.3 respectively (the NYC Building Code was renumbered as the result of
changes in New York State Laws, effective September 1, 1986).

The code provisions cited above allow for a doubling of allowed stair capacity when one area of refuge is
provided on a floor and tripling the stair capacity for two or more areas of refuge on a floor. These areas
of refuge must be separated by 2 h construction, be large enough for the expected occupant load at 3 ft

per occupant, each contain at least one stair, and have access to at least one elevator (above the 11th
floor). Since three distinct areas of refuge were provided on each floor, the tripling of the capacity of the
three stairs resulted in a maximum permitted occupant load of 1,170 people per floor.

Attached to (and referenced in) the letter were two plans entitled “106th Floor Egress Plan” and
“107th Floor Egress Plan” that detailed the arrangement. The 2 h separation walls snaked across the
floors and were not aligned on the two floors. Some areas that needed to remain open to free passage
were protected with so-called Won-doors (accordion doors that are fire rated and are closed automatically
on activation of the fire alarm system). Details of the egress system design calculations and
corresponding NYC Building Code requirements are included on the plans to demonstrate that they met
code requirements. Figures 10–1 and 10–2 are the actual attachments to the letter which included both
diagrams of the arrangement of the rated partitions and the calculation of occupant loads for the 106th and
107th floors, respectively. Important details of the calculations have been enlarged by NIST.

By comparison to the (current) model building codes, the International Building Code and NFPA 5000,
permit a doubling (but not tripling except in IBC Type I-2 and I-3 institutional uses) of the stair capacity
for the provision of a horizontal exit on a floor. The horizontal exit must consist of a 2 h fire rated
separation, contain at least one stair on each side, and have sufficient space for the expected occupant
load at 3 ft

per person. A horizontal exit must be continuous down through the building to grade

(NFPA 11.2.4.3.1 and IBC 1021.2), unless the floor assemblies are at least 2 h with no unprotected
openings.

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The concept of using horizontal exits to create areas of refuge was to provide a protected space in which
occupants could wait to get into stairs that do not have adequate capacity for the numbers of people. In
the world of post-September 11, 2001, it is unclear whether people will be comfortable waiting in a large
queue to enter an egress stair, and what the impact would be of such a large group of people moving
down the stairs on the orderly evacuation of lower floors. The decedent analysis in NIST NCSTAR 1-7
estimates the number of people in Windows on the World at 188. The early hour of the attacks saw much
fewer patrons (such as the early arrivals for a breakfast meeting) that would have been expected later in
the day. The occupant load would have permitted more than 2,000 people on these two floors, from
which there were no survivors.

10.1.2

Egress Provisions from Top of the World

Similar to the Windows on the World facilities on the 106th and 107th floors of WTC 1, there was a
public observation deck on the 107th floor of WTC 2 called Top of the World. The observation deck was
open to the public daily and was accessed by a dedicated, express elevator from the Concourse level after
paying an entrance fee. The facilities included several shops, food vendor, a small theater showing a
6 min film of a helicopter tour of New York City, exhibits depicting life in the city, and a perimeter
viewing area with telescopes and information on major landmarks visible along each face of the building.
Visitors could also ascend two escalators to an open, roof-top deck which was raised to provide
unobstructed views.

Observation decks are Assembly Use spaces (Group F in the NYC Building Code) like restaurants and
meeting spaces. Thus, the occupant load/egress capacity issues identified for Windows on the World also
existed for Top of the World. That is, the occupant load for the observation deck was calculated as the
net floor area times a load factor of 10 ft

per person. This clearly far exceeded the 390 people total

capacity for the three stairways. Since NIST did not find any documentation of the arrangement of the
space prior to 1995, it is unclear whether this deficiency existed from the original opening of the building
until it was addressed in 1996.

NIST has correspondence between Andrew Renter (STV/Silver & Ziskind, an Architectural/Engineering
Design firm) and Victor Weisberg (Ogden Series Corporation, the tenant and operator of the facility)
dated February 5, 1996, and referencing comments received from Port Authority on January 19, 1996.
This letter and the drawings referenced in it (which are also in NIST’s possession) detail a proposed space
arrangement that parallels the solution applied to Windows on the World the year before. The letter and
drawings are part of a Tenant Alteration submittal to Port Authority that was approved on January 5,
1996. STV proposes dividing the floor into three areas of refuge, each containing an existing stairway, by
2 h fire rated partitions. The drawings show the existing space arrangement of the floor and calculates the
occupant load for each using the load factors specified in the NYC Building Code. Their calculations
reveal that the occupant load of the 107th floor was 1,751, which before the subdivision of the space into
three areas of refuge, was 4½ times the maximum number of occupants permitted under the
NYC Building Code.

After the proposed subdivision, the floor had an area of refuge of 5,610 ft

net (incorporating Stairway A)

with an occupant load of 935 people, a second area of 2,430 net ft

(incorporating Stairway B) with an

occupant load of 343 people, and a third area of 2,940 net ft

(incorporating Stairway C) with an occupant

load of 473 people. STV observed that the occupant load of the perimeter gallery alone is 1,267 people,

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which exceeds the stairway capacity of 1,170 after subdivision. Under Sec. 27-367 of NYC Building
Code, the stair capacities are tripled with the provision of three areas of refuge, taking the capacities to
360 (Stairways A and C) and 450 (Stairway B) for a total of 1,170 people. Thus, even after the
subdivision, only the area incorporating Stairway B had an occupant load less than the maximum capacity
of the stair, and one area had an occupant load more than 2½ times the stair capacity.

These load calculations do not include the occupant load of the roof-top deck. Since these occupants
were required to return to the 107th floor to exit the facility, this load also needed to be accommodated by
the stairways and the refuge space provided on that floor. This fact is simply not addressed. As to the
fact that the proposed solution still does not provide sufficient egress capacity for the occupant load under
the NYC Building Code, STV’s position appears to be that this is an existing condition and the solution
(in their opinion) meets the intent of the Code, even though the problem existed from the time the
building first opened against the 1968 NYC Building Code. Taking advantage of a New York City
building code provision which permits a lower basis for occupant load, the PANYNJ permitted a
maximum occupant load of 1,170 on the floor, which was enforced by the lessee of the space with
periodic oversight by the PANYNJ.

NIST inquired of PANYNJ whether there was any means to limit the number of visitors to the
observation deck. The following response was received:

For controlling the number of occupants on the observation level in
WTC 2, there were turnstiles on the mezzanine before the entrance to the
elevators that were used to count the number of people going up, but
since the patrons exited via a different route & location, there was no
way to count the number of people leaving - and thereby calculate the
number actually on the deck. Since the turnstiles were not very effective,
their use was discontinued later and the number of ticket sales was used
for controlling the number of occupants. The length of the line waiting
for the elevators to take people down were constantly observed by staff.
If the crowds grew too large, ticket sales were halted until the crowd size
was reduced.

Fewer than ten people who were present on the observation level perished on September 11th. The
number of perople who were present and managed to evaculate is unknown.

10.2 ELEVATORS

Local Law (LL) 5 (1973) required that elevators be provided with an emergency recall system. This
requirement was incorporated subsequently into the American Society of Mechanical Engineers (ASME)
A17.1, Safety Code for Elevators and Escalators, that governs elevator design and operation in all
building codes. The ASME Code requires that:

•

All passenger elevators be marked with signs stating that they cannot be used during a fire;

Email from Saroj Bhol, PANYNJ, to Shyam Sunder, NIST, dated March 15, 2005.

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•

Fire detectors installed in every elevator lobby and machine room be arranged to initiate a
recall of the elevators to the ground floor where the doors open and the elevator is taken out
of service; and

•

Fire service personnel can use a special key to operate any individual car in a manual mode as
long as they feel it is safe to do so.

The elevator and building codes require that at least one elevator serving every floor be connected to
emergency power. Currently, there are no U.S. building codes that permit elevators to be used as a means
of occupant egress in emergencies, and ASME A17.1 (ASME 2000) requires signs at all elevators
warning that they shall not be used in fires. There are some recent exceptions to this general rule, but
these are limited to special cases. For example, NFPA 5000 permits protected elevators as a secondary
means of egress for air traffic control towers, and the City of Las Vegas accepted elevators as a primary
means of occupant egress from Stratosphere Tower based on a performance-based design
(Bukowski 2003).

The United States’ building codes (including New York City) require

accessible elevators

as part of a

means of egress that may be used by the fire service to evacuate people with disabilities. These elevators
must comply with the emergency operation requirements of ASME A17.1 (Phase II emergency operation
by the fire service), be provided with emergency power, be accessible from an area of refuge or a
horizontal exit (unless the building is fully sprinklered), and operate in a smoke protected hoistway.
Phase II operation involves the use of an elevator by a firefighter for fire service access or for rescue of
people with disabilities performed under manual control (with the use of a special key).

10.3

ACTIVE FIRE PROTECTION SYSTEMS

10.3.1 Fire

Alarm

Systems

At the time of design and construction of the WTC towers, most building codes did not require a fire
alarm or required only a manual fire alarm system in buildings where occupants do not sleep. Also,
concerns about smoke recirculation through heating, ventilating, and air conditioning (HVAC) systems
resulted in codes being amended to require smoke detectors positioned at return air grilles to stop fans and
prevent such recirculation.

In the 1970s (shortly after the adoption of LL 5/73), discussions of phased evacuation of tall buildings led
to the concept of high-rise emergency voice communications systems and fire command centers from
which incident commanders would manage fire incidents. The NFPA Committee on Protective Signaling
Systems developed a guide (later made a standard), NFPA 72F, for such systems that paralleled LL 5/73
requirements (see NIST NCSTAR 1-1G for a complete discussion of the requirements of LL 5/73).

10.3.2 Fire

Sprinklers

Neither the 1968 NYC Building Code nor any of the other contemporaneous codes that were examined
required sprinklers in tall buildings except for underground spaces. Thus, only the parking garage under
WTC 1 and WTC 2 was originally sprinklered. Although Local Law 16, adopted in 1984, required
sprinklers in new office occupancies, it was not retroactive. The incentive to retrofit for sprinklers (as
explained below) was the passage of Local Law 5 in 1973, which was retroactive.

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159

In the 1968 NYC Building Code, Class 1B construction for business occupancies had no limit on floor
area. Local Law 5 required compartmentation of large floor areas in existing business occupancies over
100 ft in height by the installation of fire rated partitions in accordance with the following:

•

Compartmentation to 7,500 ft

with 1 h partitions; or

•

Compartmentation to 10,000 ft

with 2 h partitions; or

•

Compartmentation to 15,000 ft

with 2 h partitions and smoke detectors.

Compartmentation was not required, however, if “complete sprinkler protection” was provided.
Compliance dates for these provisions were revised in 1979 by Local Law 84, so that one-third of the
total area of buildings had to be in compliance by December 13, 1981, two-third of the total area had to
comply by August 7, 1984, and full compliance was required by February 7, 1988.

Following the February 13, 1975, fire in the lower stories of WTC 1 (Powers 1975), an independent
consultant was retained to review WTC life-safety provisions, including response to Local Law 5. It is
reported that the “consultant concluded that the existing structural fire retardants of the building are
sufficient to make the probability of serious structural damage extremely remote and the degree of
vertical compartmentation provided sufficiently limits the spread of fire in the structures but that the
spread of smoke requires attention from a life safety standpoint (PONYA 1976).” The consultant
reported that “…either of the two fire protection options provided for under Local Law 5 would provide a
good level of occupant life safety within the World Trade Center complex, provided that whichever is
selected is supplemented by certain additional measures.” The consultant provided a series of
recommendations to supplement either the compartmentation option or the sprinklering option.

The Port Authority initially decided to adopt the compartmentation option in response to Local Law 5.
The summary of the January 1976 report on the

Fire Safety of the World Trade Center

lists the following

actions to be implemented to enhance the fire safety of the WTC towers (PONYA 1976):

1. The openings between floors of telephone closets, which was a

source of fire spread during the February 13, 1975, fire should be
closed. This work has been accomplished to prevent any
reoccurrences of a similar condition.

2. In addition, the Port Authority will proceed with the

compartmentation option of Local Law 5, including all of its
requirements for fire alarm, communications, and stairway
pressurization.

3. Sprinklering of all storage rooms, janitor closets, mail rooms and file

rooms in the central core of each floor.

4. Building additional sprinkler capacity and provisions for extension

of a sprinkler system to any area of such usage requiring it in the
event of an occupancy change.

5. Equipping those doors which are normally kept open to the corridor

system, such as doors at consumer service areas, with

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electromagnetic ‘hold open’ devices which would be activated by
smoke detectors to close the doors.

6. Providing fail-safe automatic door closers, arranged to close upon

activation by smoke detectors, for the overhead rolling fire doors
separating the below-grade truck dock from the elevator lobby.

7. Developing an optimum mode of operation of the building air-

conditioning system to remove smoke from the central core
compartments without contaminating adjacent areas.

Thus, while the Port Authority initially chose to implement the compartmentation option, it also chose to
provide “for extension of sprinkler system to any area of such usage requiring it.” According to the
1993 joint report written by the NYC Fire Commissioner and Commissioner of Buildings, in the 1980s
the Port Authority began “a program to fully sprinkler the tower buildings (Rivera and Rinaldi 1993).”
The report goes on to state that by March 1993 sprinklering was “ nearly complete in tower 2 and
85 percent complete in tower 1.” The report also included a table that summarized “the major system
requirements of Local Laws 5/73 and 16/84 with conditions in place when the 1993 explosion occurred.”

The tenant alteration guidelines issued in 1998, contained the following requirement and information
(PANYNJ 1998):

All tenant spaces shall be sprinklered. Except for a few areas, most
tenant floors in The World Trade Center are provided with wet-pipe
sprinkler systems. New tenants normally require a new sprinkler system.
For renovations of existing spaces, modifications to the existing system
are normally needed to comply with any new partition configuration.

Because Local Law 16 required that business occupancies taller than 100 ft be sprinklered, WTC 7 was
sprinklered during the original construction.

Section 6 of Local Law 5 adopted by New York City in 1973, required the subdivision of unsprinklered
space in new office occupancies and in existing offices over 100 ft in height by fire rated partitions.
Local Law 5 was challenged in the courts and was eventually upheld, although the original compliance
dates were amended by Local Law 86 (1979) so that full compliance was required by February 7, 1988.

10.3.3 Smoke

Management

New York City has historically had fewer requirements for active smoke control than many other codes
and has required passive techniques such as venting of shafts and openable skylights in stairways in the
local laws amending the NYC Building Code. For high-rise buildings there is a requirement for a smoke
purge system to be used manually by the fire department to remove smoke after the fire is extinguished
and for the ability to pressurize corridors with 100 percent fresh air (NYC Building Code Sec. 504.15(c)).
Pressurized stairways are not required in sprinklered buildings. These features of WTC 1 and WTC 2
were confirmed in a March 1993 joint report from the fire and buildings departments on compliance with
LL 5/73 and LL 16/84 (Rivera and Rinaldi 1993). For details of the smoke management systems see
NIST NCSTAR 1-4D.

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161

10.4

DESIGN AND CONSTRUCTION OF FIRE SAFETY AND EGRESS
SYSTEMS

10.4.1 Construction

Classification

No contemporaneous documentation has been found that provides the rationale for the decision to select
Class 1B for the WTC towers. This decision, however, appears to have been made by the architect-of-
record on the basis of economics. In a 1987 memorandum on the subject of fire rating of the WTC
buildings, the following statement was included (Feld 1987):

For office buildings there is no [underline is in the original document]
economic advantage in using Class 1A Construction, and ER&S [Emery
Roth & Sons] used Class 1B Construction for the WTC Towers and
Plaza Buildings which are Occupancy Group “E” (Business) with a fire
index of 2 hours.

An interoffice memorandum between staff of the general contractor written in 1969 is the only
contemporaneous document found to date that refers to the classification of the WTC towers
(Bracco 1969). The following statement is included in that memorandum:

The WTC towers would be classified, by our interpretation of the code,
as occupancy Group E, Business; Construction Group 1, Non-
combustible; and Construction Classification 1-B (since there are no area
or height limitations applicable).

10.4.2 Occupancy

Group

As stated above, the primary occupancy group was Group B (Business) with the Windows on the World
space in WTC 1 being Group F (Assembly). While there was a Port Authority cafeteria on the 44th floor,
employee cafeterias not open to the public are specifically exempted from assembly classification because
they do not increase occupant load and are only used intermittently. Incidental mercantile spaces such as
news stands and coffee bars at the concourse level are also exempt from reclassification in most building
codes.

10.4.3

Compartmentation of WTC 1 and WTC 2

Due to their innovative structural design, WTC 1 and WTC 2 featured large, open office spaces devoid of
columns. Tenants could (and often did) utilize open plan office layouts that allowed impressive views of
the Manhattan skyline from the perimeter windows.

The NYC Building Code and Port Authority practice required partitions to separate tenant spaces from
each other and from common spaces, such as the corridors that served the elevators, stairs and other
common spaces in the building core. Fire rated partitions are intended to limit fire spread on a floor and
to prevent spread of fire in one tenant space to another. Partitions separating tenant space from exit
access corridors were permitted to be 1 h, although the PANYNJ specified them to be 2 h, allowing dead
ends to extend to 100 ft (rather than 50 ft with 1 h partitions), which allowed more flexibility in tenant
layouts. Partitions separating tenant spaces (so-called demising walls) were required to be 1 h (see
Sec. 9.2.5). Enclosures for vertical shafts, including stairways and transfer corridors, elevator hoistways,

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and mechanical or utility shafts were required to be of 2 h fire rated construction. Protection of vertical
shafts was intended to limit the spread of fire and smoke from floor to floor.

Another influence on compartmentation of the buildings was the adoption of Local Law 5 (1973)
amending the NYC Building Code. While it did not legally apply to the buildings, PANYNJ policy was
to follow the NYC Building Code requirements voluntarily. Local Law 5 (1973) required
compartmentation of unsprinklered spaces in existing office buildings over 100 ft in height “having air-
conditioning and/or mechanical ventilation systems that serve more than the floor on which the equipment
is located,” to be subdivided by 1 h fire separations into spaces or compartments not to exceed 7,500 ft

Floor areas could be increased up to 15,000 ft

if protected by 2 h fire resistive construction and smoke

detectors. Regardless of the floor area, compartmentation is not required when complete sprinkler
protection is provided (LL 5, Section 6)

Shortly after the adoption of LL 5 (1973), the PANYNJ began to add the required compartmentation as a
part of new tenant layouts as evidenced by several tenant alteration contracts at this time. Following the
1975 fire, a fire safety consultant report recommended to the PANYNJ that the buildings be retrofit with
sprinklers to address possible smoke problems, and the PANYNJ realized that this would also obviate the
need for compartmentation and permit the unobstructed views for which the buildings were known. The
decision to sprinkler left the arrangement again with the only required partitions being those separating
tenant spaces from each other and from exit access corridors or common spaces in the core, and with shaft
enclosures.

10.4.4

Construction of Partitions and Shaft Enclosures

Vertical shafts surrounding stairs, mechanical shafts
(carrying supply and return air), elevator hoistways,
and utility shafts were all contained within the
building core, and were enclosed by gypsum
planking similar to fire separations commonly used
today in single-family attached housing. While
similar to other gypsum shaft wall systems and
firewalls, this system was unique and innovative in
that it eliminated the need for any framing. The
gypsum planks were solid 2 in. thick (2½ in. on
floors with 16 ft ceiling heights) and 16 in. wide,
with metal tongue or groove channels attached to
the long sides that served as wall studs (see
Fig. 10–3). Where planks were cut to a narrower
width, the cut edge was covered with a 2 in. by
2 in. metal C channel fastened with drywall screws at the top and bottom. Each plank had a mesh layer at
its mid thickness and were likely custom fabricated for this job as NIST found no mention of similar
products in gypsum industry literature of the time or since. Planks were provided in 12 ft, 14 ft, and 16 ft
lengths to run full height. The planks were placed into metal L channels at the bottom and into metal top
channels of various shapes depending on the construction element with which it needed to interface (see
Fig. 10–4).

Figure 10–3. Gypsum plank shaft partition.

Source:

WTC 1&2 drawing A*A*209. Reproduced with

permission of The Port Authority of New York and
New Jersey.

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The 1978 edition of the Gypsum Association (GA) Fire Resistance Design Manual lists several, similar
shaft wall constructions utilizing 2 in. gypsum layers consisting of two 1 in. gypsum core board panels
with “metal channels on long edges.” The GA Manual lists shaft walls of a single 2 in. metal edged plank
(WP7015) having a 1 h fire rating, a single 2 in. metal edged plank with one layer of Type X gypsum
board on the unexposed side (WP7112) having a 2 h fire rating, and a single 2 in. metal edged plank with
two layers of Type X gypsum board on the unexposed side (WP 7575) having a 3 h fire rating.

Partitions separating tenant spaces from other tenant spaces on the same floor were constructed of two
layers of 5/8 in. Type X gypsum board on each side of steel studs, and ran slab to slab. This construction
is commonly recognized as a 2 h fire separation. Above the ceiling, penetrations for ducts or to allow for
return airflow were fitted with rated fire dampers to preserve the fire rating. This construction was not
used in the original design but was specified later by the PANYNJ as tenant spaces were altered.

Interior partitions not separating spaces occupied by different tenants were constructed of single or double
layers of 5/8 in. Type X gypsum board on each side of steel studs and ran from the slab to the suspended
ceiling but not above. Double layers of gypsum board were used when the tenant desired additional
sound attenuation. These partitions were not required to be fire rated and did not utilize fire rated doors.
However, a single layer of 5/8 in. Type X gypsum board on each side of steel studs (16 in. on center) is
generally considered to have a 1 h fire rating, and two layers of 5/8 in. Type X gypsum on each side of
steel studs (16 in. on center) is considered to have a 2 h fire rating. For a ceiling high partition to be
considered as having a fire rating, the ceiling itself would have to be rated as well. The ceiling system
used throughout these buildings was not fire rated.

10.4.5

Tenant Separation Walls

Section C26-504.3(a) of the 1968 NYC Building Code required that tenant spaces be separated “by fire
separations having at least the fire resistance rating prescribed in Table 5–1, but in no case less than 1 h,
and shall continue through any concealed spaces of the floor or roof construction above.” The Port
Authority chose to stop tenant (demising) partitions (walls separating spaces occupied by different
tenants) at the bottom of the suspended ceiling and use 10 ft strips of 1 h rated ceiling on either side of the
partition (Solomon 1969). The general contractor stated in a letter to the Port Authority “…we have been
unable to find any precedent for the fire rated ceiling 10’ on either side of the demising partitions beyond
the one you described from your construction experience on Port Authority hangers [sic] (Endler 1969).”

In a code compliance evaluation report written in 1997, it was stated “Tenant demising partitions,
including separations from the public corridor, do not in all cases meet the requirement of being built to
the slab above (Coty 1997).” The author of the report recommended that: “Generally, this condition has
been and will continue to be remediated as a requirement of new tenant alterations. However, it is
recommended that the Port Authority develop and implement a survey program to ensure that this
remediation process occurs as quickly as possible.”

The tenant alteration guidelines issued in 1998 required that tenant partitions have a 1 h fire rating, and
the standard details for fire rated partitions indicated a continuous fire barrier from top of floor to bottom
of slab (PANYNJ 1998). There were no requirements in the codes or in the PANYNJ guidelines for
partitions wholly within tenant spaces.

Draft for Public Comment

Influence of Codes and Standards

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165

10.4.6 Egress

Systems

net at 100 ft

per

person). The layout within the building core was consistent with the building code requirements for
maximum travel distance (200 ft unsprinklered, 300 ft sprinklered) and, while the separation was
consistent with New York City requirements (15 ft and later 30 ft), it was short of the more common
requirements found in all current building codes (one half the diagonal of the space served if
unsprinklered, or one-third the diagonal if sprinklered) on some of the floors where the transfer corridors
brought the stair access closer together.

The NYC Building Code uses the older “units of exit width” method for specifying exit capacity. Each
22 in. unit of exit width in an office stair provides the capacity for 60 people. Thus, each 44 in. stair
provides for 120 people and the 56 in. stair provides 2½ units, or 150 people, for a total occupant load per
floor of 390. Also, the PANYNJ made a design decision to use 2 h corridor walls to permit longer dead
ends (100 ft rather than the 50 ft limit if the walls had been the minimum 1 h rating) to provide additional
flexibility in tenant layouts. For a detailed description of the stairways, see NIST NCSTAR 1-7.

10.4.7 Elevators

There were 99 passenger elevators and 7 freight elevators in each tower, arranged in three vertical zones
to move occupants in stages to skylobbies on the 44th and 78th floors. The elevators were arranged as
express (generally larger cars that moved at higher speeds) and local elevators in an innovative system
first introduced in WTC 1 and WTC 2. There were eight express elevators from the concourse to 44 and
ten express elevators from the concourse to 78 as well as 24 local elevators per zone, which served groups
of floors in those zones. There were seven freight elevators, only one of which served all floors. All
elevators had been upgraded to incorporate firefighter emergency operation consistent with ASME A17.1
and Local Law 5 (1973). See also NIST NCSTAR 1-7.

10.4.8 Active

Systems

Fire Alarm Systems

Consistent with practice at the time, the original fire alarm system in WTC 1 and WTC 2 was a manual
system with four smoke detectors on each tenant floor, positioned to monitor smoke entering the HVAC
returns and arranged to stop the fans and prevent smoke circulation to non-fire areas. Local Law 5 (1973)
included retroactive requirements for fire alarm systems and emergency voice communication systems in
business occupancies over 100 ft in height. Subsequently, such systems were installed in WTC 1 and
WTC 2 with the required fire command center located in the underground parking garage where it was
destroyed by the blast in the 1993 bombing rendering most fire safety features inoperable. Following the
1993 bombing, the fire command stations were relocated to the tower building lobbies with a third
monitoring location in the Port Authority offices. The lobby location (within sight of the elevators) is
specified in the NYC Building Code for fire command centers required in high-rise buildings. There are
no code requirements for off-site monitoring of fire alarm systems in this occupancy. For a detailed

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description of the towers’ fire alarm system on September 11, 2001, and prior systems back to the
original, see NIST NCSTAR 1-4C.

Fire Sprinkler Systems

After the passage of Local Law 5, the Port Authority implemented a program to retrofit sprinklers and to
offer tenants the option of sprinklering or compartmentation consistent with Local Law 5 provisions.
Sprinklering of WTC 1 and WTC 2 was undertaken in three phases: Phase 1 was the sprinklering of
below grade spaces completed with the original construction. Phase 2 was begun after Local Law 5 was
adopted and included the installation of sprinkler risers and other infrastructure, and the installation of
sprinklers in corridors, storage rooms, lobbies, and smaller tenant spaces for tenants not selecting the
compartmentation option. Phase 3 involved sprinklering the remaining tenant spaces, initially as tenants
changed, and later on negotiated schedules. This process was underway when, in 1984, Local Law 16
was adopted, which required sprinklers in high-rise buildings, including new offices and new or existing
hotels. Following the settlement of legal challenges to LL 5 (1973), LL 84 (1979) changed the effective
date for compliance with LL 5 (1973) to February 8, 1988. By the new date, high-rise office buildings
had to either be subdivided in accordance with the compartmentation requirement or sprinklered. A 1997
report states that there were four floors and the skylobbies (all in WTC 1) left to be sprinklered, and that
the installation of sprinklers at these floors was underway (Coty 1997). In an October 1999 report, it is
stated that sprinklering of the tenant floors was completed and sprinklering of the skylobbies was
“currently underway” (PANYNJ 1999).

The sprinkler system in the towers was a high-quality, state-of-the-art system with a few features
following New York City practice that differed from practice in the rest of the country. An example of
the quality is the decision by the PANYNJ to install separate risers rather than to use the existing
standpipes as was permitted. An example of New York City practice is the use of manually operated fire
pumps and a so-called “standpipe telephone system” to communicate with the pump operator. Most
codes and standards require automatic fire pumps. On September 11, 2001, the fire department was
unable to deploy operators to the pumps, so they were not used. Since the risers were breached by the
aircraft impact, the lack of pumps may have been inconsequential. For a detailed description of the
towers sprinkler system see NIST NCSTAR 1-4B.

Smoke Management

The towers were originally constructed with vents in elevator and utility shafts in accordance with NYC
Reference Standard RS 18-1. In addition, smoke detectors were installed at each of the four return vents
on each floor to stop fans and prevent recirculation of smoke.

Later, LL 5 (1973) Section 7 (revised by LL 86, Sec. 2) added a requirement for smoke shafts (new) or
pressurized stairways (existing) with an exemption for fully sprinklered buildings. The 1976 decision to
sprinkler the towers relieved the need to add stair pressurization.

Local Law 16 (1984) Section 53 contained requirements for segregation of ventilation systems and a
smoke purge capability. These were addressed through the design and installation of an active system of
smoke management that provided a manually activated smoke purge and pressurization of corridors with
100 percent outside air. These systems are described in detail in NIST NCSTAR 1-4C.

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167

Chapter 11

AINTENANCE AND

ODIFICATIONS TO

IRE

AFETY

YSTEMS

The Port Authority of New York and New Jersey (PANYNJ or Port Authority) was very conscientious in
providing guidance to tenants about their use of space in the World Trade Center (WTC) buildings. The
Port Authority published tenant alteration manuals that detailed how tenant space could be fitted. There
were manuals for interfacing with the building fire alarm system, the building fire sprinkler system, and
other special systems installed in the buildings. National Institute of Standards and Technology (NIST)
located at least partial copies of most of these manuals.

When a tenant space was remodeled, such as to accommodate the needs of a new tenant, the process was
for the tenant to hire an architect or interior designer to design the space, following the tenant alteration
manual. If the tenant occupied less than an entire floor, they could opt to have the PANYNJ handle most
of the modifications through their existing contracts, or they could contract independently as the larger
tenants generally did. Creation or movement of interior partitions often required moving of sprinkler
heads or fire detectors. Also, whenever the suspended ceiling was pulled the PANYNJ required that the
spray applied fireproofing be inspected and upgraded (if needed) after the other trades had finished and
before the ceiling was reinstalled. The PANYNJ office reviewed and approved plans at the start of a
tenant project and conducted inspections prior to the tenant moving in.

Whenever work was done in the buildings, a project number was assigned by the PANYNJ under which
all contracts, drawings, and correspondence were filed. These numbers are of the format W(yy)-1234
(where yy is the year initiated and 1234 is a 4 digit number). The reports include these numbers as
reference for individual projects, and files retained by the PANYNJ are identified by these numbers.

The PANYNJ also conducted numerous inspections and condition surveys which were beyond any
requirements in New York City and other codes and practices, and generally implemented corrective
action to address problems identified.

11.1

LOCAL LAWS 5 (1973) AND 16 (1984)

In general, buildings are governed by the building code in force at the time the building permits are
issued, except in the rare case of the adoption of retroactive requirements. Local Laws (LLs) 5 (1973)
and 16 (1984) were adopted after completion of WTC 1 and WTC 2 but did contain some retroactive
provisions. However, the PANYNJ chose to implement virtually all of the provisions of LL 5 (1973) and
LL 16 (1984), which drove most of the modifications to the fire and life safety systems that occurred over
the life of the buildings. These modifications included the complete sprinklering of the buildings and
several upgrades to the fire alarm system.

Several requirements in LL 5 (1973) were retroactive to existing office buildings over 100 ft in height.
These included evacuation drills and planning, fire safety directors and wardens, and requirements for re-
entry from stairs every four floors with signs in the stairs identifying re-entry floors. Provisions regarding
compartmentation requirements for unsprinklered spaces, smoke and heat venting, sprinklers in

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NIST NCSTAR 1-1, WTC Investigation

showrooms, permitting standpipes to be used as sprinkler risers, fire alarm and voice communication
systems with a fire command center, and one elevator serving every floor supplied with emergency power
that can be used by the fire department were also included. LL 16 (1984) extended retroactive provisions
for sprinklers, fire alarm and communication systems (with fire command center), a fire service elevator
connected to emergency power, and exit lighting and signage, to most occupancy types. LL 16 (1984)
added construction class 1C (sprinklered high rise) and removed the compartmentation requirement added
by LL 5 (1973), since these buildings were now required to be sprinklered.

11.2

CODE COMPLIANCE SUMMARY FOLLOWING THE 1993 BOMBING

In the aftermath of the 1993 bombing, the exemption of PANYNJ facilities from regulation under the
NYC Building Code was once more being questioned. The Fire Commissioner and Commissioner of
Buildings co-authored a report on the state of various properties in New York City that were exempt from
City oversight (Rivera and Rinaldi 1993). They recommended that the States of New York and New
Jersey enact legislation making all Public Benefit Corporations, including the Urban Development
Corporation and The PANYNJ, subject to the New York City building regulations. The City had even
drafted such legislation and submitted it to Albany in 1975 (Rivera and Rinaldi 1993, Attachment I).
They report that as of the date of their report the legislation had not been enacted.

The Rivera and Rinaldi report includes a summary of code compliance at the WTC, including the history
of code compliance discussions between City departments and PANYNJ, and that “the trend in recent
years has been towards cooperation,” citing the sprinklering of the towers (Rivera and Rinaldi 1993,
page 6). The report goes on to say that, “since its compliance with fire code requirements was dependent
upon economic and design feasibility, the PA[NYNJ] agreed to comply with selected provisions of the
code, but has not fully done so. Moreover, it was difficult for the Fire Department to monitor code
compliance by the WTC because the WTC consistently asserted its legal exemption from local law. Fire
officials relied on persuasion and negotiation to gain compliance.” (Rivera and Rinaldi 1993, page 6).

Regarding compliance at the time of the explosion, Rivera and Rinaldi report that “a preliminary review
by the NYC Department of Buildings generally indicates that the WTC complies with the specific
provisions of Local Law 5/73 and Local Law 16/84, or provides acceptable equivalent systems.” (Rivera
and Rinaldi 1993, page 7) They go on to say that the WTC exceeds the requirements of these local laws
in several areas, including emergency power, smoke purge, and corridor pressurization. They cite the fire
alarm system as a “major departure” from the requirements of the local laws because each building does
not have its own fire command station, they have only one pull station per floor, and they do not provide
public address to all areas on all floors. These deficiencies were addressed by the PANYNJ as discussed
in NIST NCSTAR 1-4C. Several newly discovered deficiencies regarding occupant egress provisions are
also mentioned (Rivera and Rinaldi 1993, page 10).

The position of the PANYNJ was summarized in a statement by Stanly Brezenhoff, Executive Director
PANYNJ before the New York City Council, Committee on Housing and Buildings on March 26, 1993
(Rivera and Rinaldi 1993, Attachment F). On page 8 of his statement, Brezenhoff states that the
PANYNJ has a “tradition of designing for high standards of structural integrity, and our policy of

voluntarily meeting or even exceeding code requirements.” Brezenhoff goes on to give examples of
meeting or exceeding building code standards for structural integrity such as,

Draft for Public Comment

Maintenance and Modifications to Fire Safety Systems

NIST NCSTAR 1-1, WTC Investigation

169

•

The towers have three stairs for fire egress, rather than two required by code

•

The towers comply with or exceed code provisions controlling fire protection of structural
members, floors and partitions, and enclosure of shafts

•

The office floors can support 100 lb/ft

, twice the code requirement

•

The towers were designed for wind speeds approximately twice those in the code.

11.3

WTC DUE DILIGENCE STUDY OF NOVEMBER 22, 1996

In late 1996, the PANYNJ contracted with Rolf Jensen & Associates (RJA) and Jaros, Baum & Bolles
(JB&B) to conduct a study of code compliance at the WTC buildings. These reports, along with issues
identified by the World Trade Department of PANYNJ, were summarized in a report dated October 15,
1999 (PANYNJ 1999). This study appears to be related to the Memorandum of Understanding between
PANYNJ and the New York City Department of Buildings which provides for oversight by professionals
licensed to practice in New York State reporting to PANYNJ with these reports available for review by
the City.

Of particular interest is the division of the items identified in the report into categories:

•

Category A was non-conforming code items which will remain as such or for which no plans
will be prepared to accommodate the code,

•

Category B was non-conforming items which have been remedied, or are currently in
progress,

•

Category C was non-conforming code items whose remediation plans are currently being
prepared or will be prepared in the near future, and

•

Category D was items of policy, business, leases, repair, and operations (RJA report only).

Items in Category A included the issue of the discharge of Stairways A and C on the mezzanine level
when the Code required exit stair discharge to a level “opening onto a public way.” This was resolved by
an agreement between the PANYNJ and the NYC Department of Buildings that the Plaza was like a
public way, and the Concourse was an “underground street.” Also in this category is the issue of exit stair
venting.

Category B included structural fireproofing, which was “judged adequate” by RJA providing that all
floors in both towers were sprinklered and re-fireproofing “to the appropriate thickness for a 2 h rating”
was continued. The 1997 RJA report (which is Attachment A to PANYNJ 1999) actually states, “… the
protection provided by the automatic sprinkler systems will mitigate the fact that the towers’ structural
steel fireproofing fall somewhat short of that required to provide a 2 h rating.” The RJA report also states
that it is their understanding that the PANYNJ “has been currently been [sic] installing and will continue
to install 1½ in. thick steel fireproofing based on UL Design No. G508.”

NIST NCSTAR 1-1, WTC Investigation

171

Chapter 12

WTC

UEL

YSTEM

World Trade Center (WTC) 7 was constructed and owned by Silverstein Properties (Silverstein) on land
owned by the Port Authority of New York and New Jersey (Port Authority). It was built and operated by
Silverstein as a Port Authority tenant alteration. Many of the tenants conducted critical business
operations in the building and required uninterruptible power to prevent the loss of information or
operational continuity in the event of a power failure. Backup power was provided by diesel generators
located in the mechanical spaces of the building. These generators were designed to start automatically in
the event of an interruption of the utility supply. The total generator capacity and quantity of fuel stored
in the building was sized to tenant needs.

12.1 CODE

REQUIREMENTS

Design and installation of the WTC 7 emergency power and associated fuel systems was consistent with
the 1968 New York City (NYC) Building Code. The base system was installed in 1987 with
modifications occurring in 1990, 1994, and 1999. Over the period 1987 to 1999, the NYC Building Code
provisions discussed below were not changed, so all systems were installed to the same requirements.
Some of the key code provisions for the construction and location of fuel storage tanks, piping, and
controls are discussed here, and additional details are contained in NIST NCSTAR 1-1A.

12.1.1

Tanks (27-828 and 27-829)

All tanks must be fabricated of steel and coated to prevent corrosion. Minimum thicknesses are specified
by tank diameter for storage tanks and for so-called “day tanks” (60 gal or 275 gal). Large storage tanks
(up to 20,000 gal) may be buried inside or outside the building or on the lowers floor of the building with
protection related to the tank capacity. For example, tanks from 550 gal to 1,100 gal must be enclosed in
2 h fire rated, noncombustible construction and tanks larger than 1,100 gal in 3 h construction.

Tanks on floors above the lowest floor are limited to 275 gal and one such tank per story. These “day
tanks” must be surrounded by a concrete curb or steel pan with the capacity to hold twice the volume of
the tank in the event of a leak. The curb or pan must be provided with a float switch to sound an alarm
and shut off the transfer pump in case of tank failure. Appropriate controls (generally a float switch in the
day tank) must be provided to transfer fuel from the storage tanks to the day tank through a transfer pump
and piping, with only one such transfer pump and piping network per day tank.

12.1.2 Piping

(27-830)

Piping from transfer pumps to day tanks is required to be enclosed in a shaft of 4 in. thick concrete or
masonry with a 4 in. clearance to the fuel pipe. Horizontal offsets may be enclosed in a steel sleeve two

Sections of the NYC Building Code in which these requirements are found. These provisions are found in the subchapter on

“Heating and Combustion Equipment.”

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(pipe) sizes larger and enclosed in 2 h fire rated construction. The spaces between the fuel pipe and
sleeve or shaft must lead to an open sight drain or an open sump so leaks can be detected.

12.1.3

Power Systems Designs

National Institute of Standards and Technology (NIST) located and reviewed specifications and drawings
for each of the emergency power systems. Some of the fuel risers were installed in existing shafts
containing other utilities. The NYC Building Code requires that pipe shafts containing piping from the
transfer pump to storage tanks above the lower floors not be penetrated by or contain other piping or ducts
(27-830(f)(5)). Correspondence relating to the system for the Mayor’s Office of Emergency Management
shows that this system was reviewed and inspected by the New York City Fire Department (FDNY), a list
of needed corrections was produced, and each item was initialed as the corrections were verified.

12.2

BASE BUILDING SYSTEM

The initial base emergency power system was installed in 1987, and consisted of two 900 kW generators
and a 275 gal day tank located on floor 5. Main fuel storage was in two 12,000 gal tanks buried under the
loading dock on the south side of the building. The tanks were double wall fiberglass

with leak

detectors between the walls.

Fuel was transferred by one of the two pumps through a 2 in. supply line in an existing shaft containing
other utilities, near the west bank of passenger elevators. The transfer pump was controlled by a float
switch in the day tank with a low (pump on) and a high (pump off) position. An alarm would be sounded
if the fuel level in the day tank fell below the low level or went above the high level. The day tank was
located within a 550 gal pan fitted with an alarm and another pump cutoff. The vent for the day tank
terminated outside the south wall.

The 2 in. fuel lines were encased in a second pipe covered with 2 in. of calcium silicate to provide the
required 2 h fire rating. Pipe supports were located approximately 10 ft apart, and inspection plugs were
provided approximately 50 ft apart. Mechanical equipment rooms were sprinklered (ordinary hazard
group I), and the fuel pump room was sprinklered (ordinary hazard group III). The generator area on
floor 5 was not sprinklered.

12.2.1

Modifications to System

From 1990 to 1999, four major modifications (additions) were made to the base emergency power
system. These modifications are summarized in Table 12–1. Of significance are the 1990 modification
(Salomon Brothers) that required a pressurized fuel supply system, because a day tank already existed on
floor 5, and the 1999 modification (Mayors’ Office of Emergency Management) that required a separate
6,000 gal tank on the first floor. Figure 12–1 is a schematic of the locations of the various components of
the base system and the four major modifications.

While the NYC Building Code requires steel tanks, effective in November 1985 the U.S. Environmental Protection Agency

required (40CFR280) that all new underground fuel storage tanks be double wall fiberglass and that any steel tanks older than
20 years be replaced by double wall fiberglass.

Draft for Public Comment

WTC 7 Fuel System

NIST NCSTAR 1-1, WTC Investigation

173

For the Salomon Brothers system, the transfer pumps were powered from the output of the generators. In
the event of a failure of utility power, all nine generators were started automatically to ensure that if any
of the nine did not start there would be enough power. Once the generators were up to speed, the control
system would shut down those that were not needed, but these could be restarted later if power demand
increased. There was enough fuel and residual pressure in the lines to start the generators and to run them
for a few minutes, but once running, the fuel pumps were powered to supply fuel. As long as any one
generator was running, the pumps ran at full capacity.

Table 12–1. Summary of modifications to base emergency power system in WTC 7.

Year

Day Tank/Generator

Storage Tank

Piping

Comments

1990

No day tank permitted since
base design included one on
floor 5/nine generators on
floor 5, 1,750 kW combined
capacity

Two 6,000 gal next to
base tanks.

Two 2½ in. pipes in
separate rated shaft

50 psi pressurized
fuel system

1994

50 gal/125 kW on floor 9;
generator room sprinklered

Used existing base
tanks

1¼ in. in new 2 h
rated dedicated shaft

New transfer pump
connected to existing
storage tanks

1994

275 gal/350 kW on floor 8;
generator room sprinklered

Used existing base
tanks

2 in. in same
dedicated shaft as
above

New transfer pump
connected to existing
storage tanks

1999

275 gal/three 500 kW on
floor 7; smoke detectors in
generator room

6,000 gal on floor 1, in
4 h rated enclosure;
gaseous (clean) fire
suppression system;
space below tank
sprinklered

10 gauge conduit in
2 h rated enclosure

Storage tank kept
filled from base
storage tanks.

Draft for Public Comment

WTC 7 Fuel System

NIST NCSTAR 1-1, WTC Investigation

175

12.2.2 Ambassador

Modification

The Ambassador modification to the base system was performed in 1994. A new transfer pump set
(145 gph at 100 psi) was installed and connected to the existing main storage tanks. A new 1¼ in. supply
riser was located in a new 2 h shaft dedicated for the fuel distribution system, constructed of 4 in.
masonry and located at the south end of the center bank of passenger elevators. The line ran to a single
125 kW generator with a 50 gal day tank mounted in a 100 gal basin on the 9th floor. Controls and
alarms were the same as the base system. The transfer pipes were the same double wall design and,
outside the masonry shaft, were covered with a 2 h vermiculite. The area of the generator on the 9th floor
was sprinklered. No design criteria were located, but the pipe sizes for the entire 9th floor are consistent
with a light hazard pipe schedule design.

12.2.3

American Express Modification

At about the same time in 1994, American Express installed a system to supply their operations. Another
new pump set rated 170 gph (at 100 psi) was installed on the first floor and was tapped into the existing
base system pipes and tanks. Another 2 in. supply pipe ran in the same masonry shaft used for the
Ambassador system to a 275 gal day tank and a single 350 kW generator on the 8th floor. Controls and
alarms were the same as the base system. The 8th floor generator room was protected with sprinklers
designed to light hazard criteria.

12.2.4

Mayor’s Office of Emergency Management (OEM) Modification

In 1999, the Mayor’s Office of Emergency Management was constructed on the 7th floor. This system
differed from the others because the specifications were to provide an independent source of power for
full operations for at least one week, requiring the installation of a new, 6,000 gal storage tank and three
500 kW generators fed from a single 275 gal day tank on the 7th floor. The main storage tank was
located on the 1st floor of the building in an existing storage room adjacent to the elevators. The room
was modified by installing a raised structure on which the tank was installed, enclosed in 4 h masonry
(8 in. concrete masonry unit) construction.

A new fill pump set rated 2,000 gph at a design pressure of 125 psi was located in the 1st floor pump
room along with a transfer pump set rated 700 gpm at a design pressure of 125 psi. The 6,000 gal OEM
tank was kept filled from the two 12,000 gal base system tanks by means of the fill pump. The 1st floor
tank room was protected with an Intergen suppression system with the space below the tank still
sprinklered (high hazard). The 7th floor generator room was not sprinklered but was protected by smoke
detectors connected to the building alarm system.

12.2.5

Salomon Brothers Emergency Power System

In 1990 Salomon Brothers installed a system to provide emergency power to their trading floor that was
independent from the other systems in the building. The Salomon Brothers system involved two
6,000 gal tanks identical to and buried adjacent to the base system tanks under the loading dock on the
south side of the building. Salomon Brothers had a contract with a fuel delivery service who always

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maintained the tanks full.

Therefore, both tanks likely contained 6,000 gal of fuel on

September 11, 2001.

The system utilized nine generators on the 5th floor with a combined capacity of 1,750 kW. Seven
cooling fan sets (four fans per set) were installed to provide cooling and combustion air to the generators.
Three fan sets were installed on the north end of the east wall and four fan sets on the north end of the
west wall. There were exhaust louvers on the south end of the west wall. These fans were arranged to
come on when the generators were running.

Since there was already a 275 gal day tank on the 5th floor associated with the base system, the New
York City (NYC) Building Code did not permit another tank on that floor. The Salomon Brothers system
was designed with a pressurized fuel system without a storage tank near the generators. Two 70 gpm at
50 psi total head pumps were located in a separate enclosure in the existing fire pump room (not in the
fuel pump room with the other transfer pumps). A double supply and return pipe (each 2½ in. covered in
2 in. of calcium silicate) were run in a separate shaft to the 5th floor where the pipes ran outside the
mechanical room to the generators in three groups. At the end of the pipe run, where the fuel supply pipe
ended and the fuel return pipe began, there was a valve box containing a backpressure regulator, gauges,
and a by-pass line. This liquid tight valve box was mounted to the underside of the floor slab for the
6th floor near generator #1.

The transfer pumps were powered from the output of the generators. In the event of a failure of utility
power, all nine generators were started automatically. This is to ensure that if any did not start there
would be enough power. Once up to speed, the control system would shut down generators that were not
needed, but they could be restarted later if demand increased. There was enough fuel and residual
pressure in the lines to start the generators and to run them for a few minutes, but once running, the fuel
pumps were powered to supply fuel. As long as any one generator was running, the pumps ran at full
capacity.

The system also included cooling fan units (each consisting of four fans) with three units (rated
30,000 cfm per fan, 12 fans) installed in the northeast corner of the 5th floor near generators 1 through 4,
six units (rated 38,000 cfm per fan, 24 fans) in the northwest corner near generators 8 and 9, and exhaust
louvers in the southwest corner near generators 5 through 7. The fans were powered from the generators
and ran whenever the generators were running. They brought outside air into the building and across the
generators.

12.3 POSSIBLE

FAILURE

MODES

Fuel oil piping systems like these are fairly common and are used to operate diesel generators and oil
fired furnaces in many applications. The systems generally use day tanks at the appliance kept filled from
storage remote tanks through transfer pumps and piping. The pipe-in-pipe design used in WTC 7 is quite
robust and reliable in preventing leaking fuel from escaping the system.

At the time WTC 7 was designed and built there were no seismic design requirements for buildings in
New York City much less for piping systems. More recent research into the failure of fire sprinkler

Interview with Mike Catalano, maintenance person for Salomon Brothers, who was responsible for these systems.

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177

systems in earthquakes has resulted in seismic design requirements for critical piping systems in seismic
zones. The research on sprinkler systems has shown the need for lateral bracing to prevent the failure of
the piping systems due to differential movement between the pipes and the building in an earthquake.

A working hypothesis is that the impact sustained by WTC 7 from the collapse of WTC 1 resulted in
fractures in the fuel piping system (both the fuel pipe and the containment pipe) especially at the point
where the pipes entered the valve box, which was rigidly mounted to the underside of the floor slab. With
the base system and all of the modifications thereto, such a fracture would result in a small leak of
residual fuel in the pipes at the point of the fracture. A fracture of the pipe at the valve box would release
fuel under pressure that, if ignited, could produce a spray fire and/or a pool fire very near column 79.

Rupture of a day tank would release more fuel, but it would be contained by the overflow pan. Not until
the generators ran for long enough to drain the day tank to its low fuel level and bring on the transfer
pumps would additional fuel and pressure in the transfer lines cause a more significant fuel leak.
Depending on the number of generators connected to the day tank, this would require several hours.

The Salomon Brothers pressurized system is different. If the supply or return pipes were fractured along
with the containment pipe and the generators started, the fuel pipes would be continuously pressurized,
and any leak would continue until the storage tanks were empty as long as any one generator was running.

NIST reviewed the report of an environmental contractor (Langan 2002) hired in the months after the
collapse of WTC 7 to recover remaining fuel and to mitigate any environmental damage from the
Salomon Brothers tanks. The Salomon Brothers tanks were damaged and appeared to be empty, “ …
Neither the UST’s (underground storage tanks) nor their associated piping contained any residual
petroleum product. No residual free product or sludge was observed in either UST.”

The tanks were installed on a concrete slab over existing silty sand. A layer of bedding gravel on the slab
provided a foundation for the tank. Examination of the gravel below the tanks and the sand below the
slab showed some fuel contamination but none was observed in the organic marine silt/clay layer below.
Also, the sand and soil below the slab was continuous below the adjacent base system tanks, which
contained a total of 24,000 gal of fuel. Thus, it is likely that a fuel leak in any of the tanks would result in
fuel contamination in this soil.

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179

Chapter 13

INDINGS

13.1 FINDINGS

13.1.1 General

Finding 1:

The NYC Department of Buildings reviewed the WTC tower drawings in 1968 and provided

Finding 2:

In 1993, the PANYNJ and the NYC Department of Buildings entered into a memorandum of

understanding that restated the PANYNJ’s long-standing stated policy to ensure that its facilities in the
City of New York meet and, where appropriate, exceed the requirements of the NYC Building Code. The
agreement also provided specific commitments to the NYC Department of Buildings regarding
procedures to be undertaken by the PANYNJ to ensure that buildings owned or operated by the PANYNJ
are in conformance with the Building Standards contained in the NYC Building Code. Some salient
points included in this agreement and the 1995 enhancement to the agreement are:

•

Each project would be reviewed and examined for compliance with the Code.

•

All plans would be prepared, sealed, and reviewed by New York State licensed professional
engineers or architects.

•

The PANYNJ engineer or architect approving the plans would be licensed in the State of
New York and would not have assisted in the preparation of the plans.

•

The person or firm performing the review and certification of plans for WTC tenants may be
the same person or firm providing certification that the project had been constructed in
accordance with the plans and specifications unless the proposed alteration would “change
the character of the occupancy group under paragraph 27-237 of the New York City Building
Code which would have been applicable to such space had such space been located in a
privately owned building.”

•

Variances from the Code, acceptable to the PANYNJ, would be submitted to the NYC
Department of Buildings for review and concurrence. Disagreements between the PANYNJ
and the NYC Department of Buildings over such variances from the Code would be referred
to the Port Authority Board of Commissioners for resolution.

Finding 3:

While the PANYNJ entered into agreements with the NYC Department of Buildings in the

1990s with regard to conformance of PANYNJ buildings constructed in New York City to the NYC

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Building Code and sought review and concurrence as required by the agreements, the PANYNJ was not
required to yield, and appears not have yielded, approval authority to New York City. The PANYNJ was
created as an interstate entity “body corporate and politic,” under its charter, pursuant to Article 1
Section 10 of the U.S. Constitution permitting compacts between states, and like many other
nongovernmental and quasi-governmental entities in the United States is not subject to building and fire
safety code requirements of any governmental jurisdiction.

Finding 4:

State and local jurisdictions do not require retention of documents related to the design,

construction, operation, maintenance, and modifications of buildings, with few exceptions. These
documents are in the possession of building owners, contractors, architects, engineers, and consultants.
Such documents are not archived for more than about 6 to 7 years, and there are no requirements that they
be kept in safe custody physically remote from the building throughout its service life. In the case of the
WTC towers, the PANYNJ and its contractors and consultants maintained an unusually comprehensive
set of documents, a significant portion of which had not been destroyed in the collapse of the buildings
but could be assembled and provided to the investigation. In the case of WTC 7, several key documents
could not be reviewed since they were lost in the collapse of the building.

Finding 5:

Consistent with the practice at the time the (code) architect was responsible for specifying the

fire protection and designing the egress system in accordance with the prescriptive provisions of the
building code. The architect and owner engaged the services of structural engineers to perform the
structural design and to ensure that his/her design was properly implemented. At that time the fire
protection engineering profession was not sufficiently mature to require the same standard of care
employed with the structural design. There is no reason to believe that the involvement of a fire
protection engineer at that time would have resulted in any differences in the design or performance of the
fire protection systems. However, the technical base and sophistication of the practice of fire protection
engineering today is well advanced of where it was then. Today, particularly when designing a building
employing innovative features, the involvement of a fire protection engineer in a role similar to the
structural engineer, and under the overall coordination of the Design Professional in Responsible Charge
is central to the standard of care. Further, when designing the structure of selected tall buildings or
selected other buildings to resist fires, or evaluating the fire resistance of such structures, it is essential for
the structural engineer and the fire protection engineer to jointly provide the needed standard of care.

13.1.2 Structural

Safety

Applicable Building Codes

Finding 6:

Although not required to conform to NYC codes, the PANYNJ adopted the provisions of the

proposed 1968 edition of the NYC Building Code, more than three years before it went into effect. The
proposed 1968 edition allowed the PANYNJ to take advantage of less restrictive provisions and of
technological advances compared with the 1938 edition, which was in effect when design began for the
WTC towers in 1962. The 1968 code:

•

Changed partition loads from 20 psf to one based on weight of partitions per unit length (that
reduced such loads for many buildings including the WTC buildings); and

•

Permitted wind tunnel tests using models to establish design values for the wind load.

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181

Many of these newer requirements, instituted in the 1968 NYC Building Code, are contained in current
model codes and building regulations.

Structural Integrity

Finding 7: Building codes lack

explicit structural integrity provisions to mitigate progressive collapse.

Federal agencies have developed guidelines to mitigate progressive collapse and routinely incorporate
such requirements in the construction of new federal buildings. The United Kingdom incorporates such
code requirements for all buildings. New York City adopted by rule in 1973 a requirement for buildings
to resist progressive collapse under extreme local loads. The rules, which were adopted after the WTC
towers were built but before WTC 7 was built, applied specifically to buildings that used precast concrete
wall panels and not to other types of buildings.

Finding 8:

Building Codes lack minimum structural integrity provisions for the means of egress

Finding 9:

Standards and code provisions for conducting wind tunnel tests and for the methods used in

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13.1.3 Fire

Safety

Applicable Building Codes

Finding 10:

Although not required to conform to NYC codes, the PANYNJ adopted the provisions of the

proposed 1968 edition of the NYC Building Code, more than three years before it went into effect. The
1968 edition allowed the PANYNJ to take advantage of less restrictive provisions compared
with the 1938 edition that was in effect when design began for the WTC towers in 1962. The 1968 code:

•

Eliminated a fire tower

as a required means of fire department access;

•

Reduced the number of required stairwells from 6 to 3 and the size of doors leading to the
stairs from 44 in. to 36 in. (by increasing stairway and door capacity allowances);

•

Reduced the required fire rating of the shaft walls in the building core from 3 h to 2 h; and

•

Many of these newer requirements, instituted in the 1968 NYC Building Code, are contained in current
codes.

Finding 11:

In 1993, the PANYNJ adopted a policy providing for implementation of fire safety

recommendations made by local government fire departments after a fire safety inspection of a PANYNJ
facility and for the prior review by local fire safety agencies of fire safety systems to be introduced or
added to a facility. Later that year, the PANYNJ entered into an agreement with FDNY which reiterated
the policy adopted by the PANYNJ, recognized the right of FDNY to conduct fire safety inspections of
PANYNJ properties in the City of New York, provided guidelines for FDNY to communicate needed
corrective actions to the PANYNJ, ensured that new or modified fire safety systems are in compliance
with local codes and regulations, and required third-party review of such systems by a New York State
licensed architect or engineer.

Standard Fire-Resistance Tests

Finding 12:

Code provisions with detailed procedures to analyze and evaluate data from fire resistance

tests of other building components and assemblies to qualify an untested building element do not exist.
Based on available data and records, no technical basis has been found for selecting the spray-applied fire
resistive material (SFRM) used (two competing materials were under evaluation) or its thickness for the
large-span open-web floor trusses of the WTC towers. The assessment of the fireproofing thickness
needed to meet the 2 h fire rating requirement for the untested WTC floor system evolved over time:

•

In October 1969, the PANYNJ directed the fireproofing contractor to apply ½ in. of
fireproofing to the floor trusses.

A fire tower (also called a smoke-proof stair) is a stairway that is accessed through an enclosed vestibule that is open to the

outside or to an open ventilation shaft providing natural ventilation that prevents any accumulation of smoke without the need
for mechanical pressurization.

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183

•

In 1999, the PANYNJ issued guidelines requiring that fireproofing be upgraded to 1½ in. for
full floors undergoing alterations.

•

Unrelated to the WTC buildings, an International Conference of Building Officials (ICBO)
Evaluation Service report (ER-1244), re-issued June 1, 2001, using the same SFRM
recommends a minimum thickness of 2 in. for “unrestrained steel joists” with “lightweight
concrete” slab.

Finding 13:

Code provisions that require the conduct of a fire resistance test if adequate data do not exist

Finding 14:

Use of the “structural frame” approach, in conjunction with the prescriptive fire rating,

would have required the floor trusses, the core floor framing, and perimeter spandrels in the WTC towers
to be 3 h fire-rated, like the columns for Class 1B construction in the 1968 NYC Building Code. Neither
the 1968 edition of the NYC Building Code which was used in the design of the WTC towers, nor the
2001 edition of the code, adopted the “structural frame” requirement. The “structural frame” approach to
fire resistance ratings requires structural members, other than columns, that are essential to the stability of
the building as a whole to be fire protected to the same rating as columns. This approach, which appeared
in the Uniform Building Code (a model building code) as early as 1953, was carried into the 2000
International Building Code (one of two current model codes) which states: “The structural frame shall be
considered to be the columns and the girders, beams, trusses and spandrels having direct connections to
the columns and bracing members designed to carry gravity loads.” The WTC floor system was essential
to the stability of the building as a whole since it provided lateral stability to the columns and diaphragm
action to distribute wind loads to the columns of the frame-tube system.

Finding 15:

A technical basis to establish whether the construction classification and fire rating

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limitations on the height of elevator shafts. The 110-story WTC towers, initially classified as Class IA
based on the 1938 NYC Building Code, were classified as Class 1B before being built to take advantage
of the provisions in the 1968 edition of the code. This re-classification permitted a reduction of 1 h in the
fire rating of the components (columns from 4 h to 3 h and floor framing members from 3 h to 2 h).

Fire Performance of Structures

Finding 16:

Rigorous field application and inspection provisions and regulatory requirements to ensure

Finding 17:

Structural design does not consider fire as a design condition, as it does the effects of dead

loads, live loads, wind loads, and earthquake loads. Current prescriptive code provisions for determining
fire resistance of structures—used in the design of the WTC towers and WTC 7— are based on tests using
a standard fire that may be adequate for many simple structures and for comparing the relative
performance of structural components in more complex structures. A building system with 3 h rated
columns and 2 h rated girders and floors could last longer than 3 h or shorter than 2 h depending upon the
performance of the structure as a 3-dimensional system in a real fire. The standard tests cannot be used to
evaluate the actual performance (i.e., load carrying capacity) in a real fire of the structural component, or
the structure as a whole system, including the connections between components. Performance-based code
provisions and standards are not available for use by engineers, as an alternative to the current
prescriptive fire rating approach, to (1) evaluate the system performance of tall-building structures under
real fire scenarios, and (2) enable risk consistent design with appropriate thickness of passive protection
being provided where it is needed on the structure. Standards development organizations, including the
American Institute of Steel Construction, have initiated development of performance-based provisions to
consider fire effects in structural design.

Finding 18:

Detailed procedures to select appropriate design-basis fire scenarios to be considered in the

performance-based design of the sprinkler system, compartmentation, and passive protection of the
structure are needed. The standard fire in current prescriptive fire resistance tests is not adequate for use
in performance-based design. While the NFPA 5000 model building code contains general guidance on
design fire scenarios (the IBC Performance Code contains no such guidance), the details of the scenarios
are left to the fire engineer and regulatory official. The three major scenarios that are not considered
adequately are: frequent but low severity events (for design of sprinkler system), moderate but less
frequent events (for design of compartmentation), and a maximum credible fire (for design of passive fire
protection on the structure). The maximum credible fire scenario for passive protection of structures
would assume that the sprinkler system is compromised or overwhelmed and that there is no active
firefighting, as is explicitly considered for US Department of Energy facilities. These building-specific
representative fire scenarios are similar in concept, though not identical, to the approach used in building
design where the performance objectives and design-basis of the hazard are better defined (e.g., a two-

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185

level design that includes an operational event with a 10 percent probability of occurrence in 50 years and
a life safety event with a 2 percent probability of occurrence in 50 years). The design-basis fire hazards
for the WTC towers and WTC 7 are unknown, and it is difficult to evaluate the performance of the fire
protection systems in these buildings under specific fire scenarios.

Finding 19:

Code provisions to ensure that structural connections are provided the same degree of fire

Finding 20:

A technical basis to establish whether the minimum mechanical and durability related

Finding 21:

Validated and verified tools for use in performance-based design practice to analyze the

Compartmentation and Sprinklers

Finding 22

: Building fire protection is based on a four-level hierarchical strategy comprising detection,

suppression (sprinklers and firefighting), compartmentation, and passive protection of the structure.

•

Detectors are typically used to activate fire alarms and notify building occupants and
emergency services.

•

Sprinklers are designed to control small and medium fires and to prevent fire spread beyond
the typical water supply design area of about 1,500 ft

•

Compartmentation mitigates the horizontal spread of more severe but less frequent fires and
typically requires fire-rated partitions for areas of about 7,500 ft

. Active firefighting

measures also cover up to about 5,000 ft

to 7,500 ft

•

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NIST NCSTAR 1-1, WTC Investigation

local structural collapse until occupants can escape and the fire service can complete search
and rescue operations.

Finding 23:

Building codes typically require 1 h fire-rated tenant separations but do not impose minimum

compartmentation requirements (e.g., 13,000 ft

) for buildings with large open floor plans to mitigate the

horizontal spread of fire. This is the case with both the 1968 NYC Building Code, which did not require
sprinklers in occupied spaces on or above the ground floor, and the 2001 NYC Building Code, which
requires sprinklers in Group E (Business) buildings over 100 feet in height. The sprinkler option was
chosen for the WTC towers in preference to the compartmentation option in meeting the subsequent
requirements of Local Law 5 adopted by New York City in 1973. Thus, if there was only one tenant on a
WTC floor there would be no horizontal compartmentation requirement. Conversely, if there were a
large number of tenants on a WTC floor, it would be highly compartmented with separation walls. The
affected floors in the WTC towers were mostly open—with a modest number of perimeter offices and
conference rooms and an occasional special purpose area. Some floors had two tenants and those spaces,
like the core areas, were partitioned (slab to slab). Photographic and videographic evidence confirms that
even non-tenant space partitions (such as those that divided spaces to provide corner conference rooms)
provided substantial resistance to fire spread in the affected floors. For the duration of about 50 min to
100 min prior to collapse of the WTC towers that the fires were active, the presence of undamaged 1 h
fire-rated compartments may have assisted in mitigating fire spread and consequent thermal weakening of
structural components.

Finding 24:

State and local building regulations are needed that require installation of sprinklers in

Finding 25:

Modern building codes allow a lower fire rating for structural elements when a building is

sprinklered. This trade-off provides an economic incentive to encourage installation of sprinklers.

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187

Sprinklers provide better intervention against small and medium fires, fires which are more likely to occur
than a WTC disaster, as long as the water supply is not compromised and there is redundant technology in
place. The required technical basis is not available to establish whether the “sprinkler trade-off” in
current codes adequately considers fire safety risk factors such as: (1) the complementary functions of
sprinklers and fire-protected structural elements, (2) the different fire scenarios for which each system is
designed to provide protection, and (3) the need for redundancy should one system fail. It is noteworthy
that the British Standards Institution has established a group to review all the sprinkler trade-offs
contained in their standards. No such formal review has yet been initiated in the United States. Although
the classification and fire rating of the WTC towers did not take advantage of the sprinkler-tradeoff since
such provisions were not contained in the 1968 NYC Building Code, had such provisions existed, they
would have permitted a lower fire rating for many WTC building elements.

Use of Elevators in Emergencies

Finding 26:

With a few special exceptions, building codes in the United States do not permit the use of

fire-protected elevators for routine emergency access by first responders or as a secondary method (after
stairwells) for emergency evacuation of building occupants. The use of elevators by first responders
would additionally mitigate counterflow problems in stairwells. While the United States conducted
research on specially protected elevators in the late 1970s, the United Kingdom along with several other
countries that typically utilize British standards have required such “firefighter lifts,” located in protected
shafts, for a number of years. Without functioning elevators (e.g., due to a power failure or major water
leakage), first responders carrying gear typically require about a minute per floor to reach an incident
using the stairs. While it is difficult to maintain this pace for more than about the first 20 stories, it would
take a first responder about an hour to reach, for example, the 60th floor of a tall building if that pace
could be maintained. Such a delay, combined with the resulting fatigue and physical effects on first
responders that were reported on September 11, 2001, would make firefighting and rescue efforts difficult
even in tall building emergencies not involving a terrorist attack. Each of the WTC towers had
106 elevators, and WTC 7 had 38 elevators. By code, the elevators could not be used for fire service
access or occupant egress during an emergency since they were not fire-protected, nor were they located
in protected shafts. The elevators were equipped through normal modernization with fire service recall.
Most were damaged by the aircraft impacts; though prior to the impact in WTC 2 the elevators were
functioning and contributed greatly to the much faster initial evacuation rate in WTC 2.

NIST NCSTAR 1-1, WTC Investigation

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Chapter 14

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Appendix B

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January 25, 1968

Mr. Malcolm P. Levy, Chief

Planning and Construction Division

The World Trade Center

111 Eighth Avenue

New York, New York

Re: The World Trade Center Building Department Review

Dear Mal,

We have reviewed the comments submitted by Commissioner Ferro with regard to conformance of the World
Trade Center with the Building Code. The Tower plans adhere to the Proposed Building Code with respect to
all five points noted.

The Proposed Code provides for a stair capacity of 60 persons per 22 inch unit of exit width. Each
Tower floor has two 3’ 8” stairs (two units each) and one 4’ 8” stair (two and one half units) for a total
of 6 ½ units. The stairs, therefore, have a capacity of 390 persons per floor. The largest floor area is
about 36,500 square feet net on the 106

floor. At one person per hundred square feet, there will be

365 persons per floor, well within the permissible maximum.

The maximum distance ---corner of the --- is about 140---. The Proposed Code permits a maximum
travel distance of 200 feet in an unsprinklered building in occupancy group classification E (business).

No fire tower is required under the Proposed Code.

All cellar stairs are completely enclosed in two hour fire-rated masonry construction as required by the
Proposed Code. Each cellar stair is contained in a continuous enclosure leading directly to the street,
the street floor lobby, or the Concourse floor landing to the street. The Concourse floor is considered
as an underground street in accordance with the interpretation made by the Port Authority.

The cellar stairs that are to be used for tenant storage are less than ten percent of the total area of the
building. In accordance with the Proposed Code, therefore, they are treated the same as office
building with regard to egress requirements.

The garage spaces are limited to storage of cars with a maximum tank capacity of 26 gallons. There will be

servicing of cars or dispensing of fuel. All garage areas will be sprinklered. The Proposed Code limits the
maximum distance to stairs for this type of garage to 150 feet without distinction to garages above or below
grade. The World Trade Center is in conformance with these requirements.

Cordially,

Signed by Joseph H. Solomon

Emory Roth & Sons