Testimony of Dr. W. Gene Corley
Senior Vice President
Construction Technology Laboratories, Inc.
Skokie, IL
On behalf of the
American Society of Civil Engineers
Before the
Subcommittee on Environment, Technology and Standards
&
Subcommittee on Research
Committee on Science
U.S. House of Representatives
May 1, 2002
Washington Office
1015 15
th
Street, N.W., Suite 600
Washington, D.C. 20005-2605
(202) 789-2200
Fax: (202) 289-6797
Web: http://www.asce.org
American Society of Civil Engineers – page 2
House Science Committee – May 1, 2002
Following the September 11, 2001, attacks on New York City's World Trade
Center, the Federal Emergency Management Agency (FEMA) and the Structural
Engineering Institute of the American Society of Civil Engineers
(SEI/ASCE),
in
association with New York City and several other federal agencies and professional
organizations, deployed a team of civil, structural, and fire protection engineers to study
the performance of buildings at the World Trade Center (WTC) site.
Founded in 1852, ASCE represents more than 125,000 civil engineers worldwide
and is the country’s oldest national engineering society. ASCE members represent the
profession most responsible for the nation’s built environment. Our members work in
consulting, contracting, industry, government and academia. In addition to developing
guideline documents, state-of-the-art reports, and a multitude of different journals,
ASCE, an American National Standards Institute (ANSI) approved standards developer,
establishes standards of practice such as the document known as ASCE 7 which
provides minimum design loads for buildings and other structures. ASCE 7 is used
internationally and is referenced in all of our nation’s major model building codes.
The events of following the attacks in New York City were among the worst
building disasters and resulted in the largest loss of life from any single building event in
the United States. Of the 58,000 people estimated to be at the WTC Complex, over
3,000 lives were lost that day, including 343 emergency responders. Two commercial
airliners were hijacked, and each was flown into one of the two 110-story towers. The
structural damage sustained by each tower from the impact, combined with the ensuing
fires, resulted in the total collapse of each building. As the towers collapsed, massive
debris clouds, consisting of crushed and broken building components, fell onto and blew
into surrounding structures, causing extensive collateral damage and, in some cases,
igniting additional fires and causing additional collapses. In total, 10 major buildings
experienced partial or total collapse and 30 million square feet of commercial office
space was removed from service, of which 12 million belonged to the WTC complex.
American Society of Civil Engineers – page 3
House Science Committee – May 1, 2002
Scope of the study
The purpose of the FEMA/ASCE was to see what could be learned to make
buildings safer in the future. Building performance studies are often done when there is
major structural damage due to events such as earthquakes or blasts. A better
understanding of how building respond to extreme forces can help us design safer
structures in the future.
Specifically, the scope of the FEMA/ASCE study was to:
•
review damage caused by the attack;
•
assess how each building performed under the attack;
•
determine how each building collapsed;
•
collect and preserve data that may aid in future studies; and
•
offer guidelines for additional study.
The team examined:
•
The immediate effects of the aircraft impact on each tower;
•
The spread of the fire following the crashes;
•
The reduction in structural strength caused by the fires;
•
The chain of events that led to the collapse of the towers; and
•
How falling debris and the effects of the fires impacted the other buildings
at the World Trade Center complex.
The team recommendations are presented for more detailed engineering studies,
to complete the assessments and produce improved guidance and tools for building
design and performance evaluation.
World Trade Center 1 and World Trade Center 2
As each tower was struck, extensive structural damage, including localized
collapse, occurred at the several floor levels directly impacted by the aircraft. Despite
this massive localized damage, each structure remained standing. However, as each
aircraft impacted a building, jet fuel on board ignited. Part of this fuel immediately
burned off in the large fireballs that erupted at the impact floors. Remaining fuel flowed
across the floors and down elevator and utility shafts, igniting intense fires throughout
upper portions of the buildings. As these fires spread, they further weakened the steel-
framed structures, eventually triggering total collapse.
The collapse of the twin towers astonished most observers, including
knowledgeable structural engineers, and, in the immediate aftermath, a wide range of
explanations were offered in an attempt to help the public understand these tragic and
unthinkable events. However, the collapse of these symbolic buildings entailed a
complex series of events that were not identical for each tower. To determine the
sequence of events, likely root causes, and methods or technologies that may improve
American Society of Civil Engineers – page 4
House Science Committee – May 1, 2002
or mitigate the building performance observed, FEMA and ASCE formed a Building
Performance Study (BPS) Team consisting of specialists in tall building design, steel
and connection technology, fire and blast engineering, and structural investigation and
analysis.
The SEI/ASCE team conducted field observations at the WTC site and steel
salvage yards, removed and tested samples of the collapsed structures, viewed
hundreds of images of video and still photography, conducted interviews with witnesses
and persons involved in the design, construction, and maintenance of each of the
affected buildings, reviewed available construction docume nts, and conducted
preliminary analyses of the damage to the WTC towers.
With the information and time available, the sequence of events leading to the
collapse of each tower could not be definitively determined. However, the following
observations and findings were made:
•
The structural damage sustained by each of the two buildings as a result of the
terrorist attacks was massive. The fact that the structures were able to sustain this
level of damage and remain standing for an extended period of time is remarkable
and is the reason that most building occupants were able to evacuate safely. Events
of this type, resulting in such substantial damage, are generally not considered in
building design, and the fact that these structures were able to successfully
withstand such damage is noteworthy.
•
Preliminary analyses of the damaged structures, together with the fact the structures
remained standing for an extended period of time, suggest that, absent other severe
loading events, such as a windstorm or earthq uake, the buildings could have
remained standing in their damaged states until subjected to some significant
additional load. However, the structures were subjected to a second, simultaneous
severe loading event in the form of the fires caused by the aircraft impacts.
•
The large quantity of jet fuel carried by each aircraft ignited upon impact into each
building. A significant portion of this fuel was consumed immediately in the ensuing
fireballs. The remaining fuel is believed either to have flowed down through the
buildings or to have burned off within a few minutes of the aircraft impact. The heat
produced by this burning jet fuel does not by itself appear to have been sufficient to
initiate the structural collapses. However, as the burning jet fuel spread across
several floors of the buildings, it ignited much of the buildings' contents, permitting
fires to evolve across several floors of the buildings simultaneously. The heat output
from these fires is estimated to have been comparable to the power produced by a
large commercial generating station. Over a period of many minutes, this heat
induced additional stresses into the damaged structural frames while simultaneously
softening and weakening these frames. This additional loading and damage were
sufficient to induce the collapse of both structures.
American Society of Civil Engineers – page 5
House Science Committee – May 1, 2002
•
The ability of the two towers to withstand aircraft impacts without immediate collapse
was a direct function of their design and construction characteristics, as was the
vulnerability of the two towers to collapse as a result of the combined effects of the
impacts and ensuing fires. Many buildings with other design and construction
characteristics would have been more vulnerable to collapse in these events than
the two towers, and few may have been less vulnerable. It was not the purpose of
this study to assess the code-conformance of the building design and construction,
or to judge the adequacy of these features. However, during the course of this study,
the structural and fire protection features of the building were examined. The study
did not reveal any specific structural features that would be regarded as
substandard, and, in fact, many structural and fire protection features of the design
and construction were found to be superior to the minimum code requirements.
What caused the collapse of the towers?
Our analysis showed that the impact alone did not cause the collapse of the
towers, but instead, left the towers vulnerable to collapse from any significant additional
force, such as from high winds, an earthquake, or in the case of the Twin Towers, the
fires that engulfed both buildings. Without that second event, the team believes the
towers could have remained standing indefinitely.
Although steel is very strong, it loses some of its strength when heated. To
prevent that loss of strength, structural steel is protected with fireproofing and sprinkler
systems. In the towers, fires raged throughout several floors simultaneously, ignited by
the jet fuel and fed by a mixture of paper and furniture. The impact dislodged some
fireproofing on the structural beams and columns, which made them vulnerable to fire
damage. With the sprinkler systems disabled, the fires raged uncontrollably, weakening
the steel and leading to the collapse of the buildings.
Several building design features have been identified as key to the buildings' ability
to remain standing as long as they did and to allow the evacuation of most building
occupants. These included the following:
•
robustness and redundancy of the steel framing system;
•
presence of adequate egress stairways that were well marked and lighted; and
•
the conscientious implementation of emergency exiting training programs for
building tenants.
Similarly, several design features have been identified that may have played a role
in allowing the buildings to collapse in the manner that they did and in the inability of
victims at and above the impact floors to safely exit. These features should not be
regarded either as design deficiencies or as features that should be prohibited in future
building codes. Rather, these are features that should be subjected to more detailed
evaluation, in order to understand their contribution to the performance of these
buildings and how they may perform in other buildings. These include the following:
•
the type of steel floor truss system present in these buildings and their structural
robustness and redundancy when compared to other structural systems;
•
use of impact-resistant enclosures around egress paths;
American Society of Civil Engineers – page 6
House Science Committee – May 1, 2002
•
resistance of passive fire protection to blasts and impacts in buildings designed
to provide resistance to such hazards; and
•
grouping emergency egress stairways in the central building core as opposed to
dispersing them throughout the structure
Building Codes
During the course of this study, the question of whether building codes should be
changed in some way to make future buildings more resistant to such attacks was
frequently explored. Depending on the size of the aircraft, it may not be technically
feasible to develop design provisions that would enable structures to be designed and
constructed to resist the effects of impacts by rapidly moving aircraft, and the ensuing
fires, without collapse. In addition, the cost of constructing such structures might be so
large as to make this type of design intent practically infeasible.
Although the attacks on the World Trade Center are a reason to question design
philosophies, the BPS Team believes there are insufficient data to determine whether
there is a reasonable threat of attacks on specific buildings to recommend inclusion of
such requirements in building codes. Some believe the likelihood of such attacks on any
specific building is deemed sufficiently low to not be considered at all. However,
individual building developers may wish to consider design provisions for improving
redundancy and robustness for such unforeseen events, particularly for structures that,
by nature of their design or occupancy, may be especially susceptible to such incidents.
Although some conceptual changes to the building codes that could make buildings
more resistant to fire or impact damage or more conducive to occupant egress were
identified in the course of this study, the BPS Team felt that extensive technical, policy,
and economic study of these concepts should be performed before any specific code
change recommendations are developed. This report specifically recommends such
additional studies. Future building codes revisions may be considered after the technical
details of the collapses and other building responses to damage are better understood.
Surrounding Buildings
Several other buildings, including the Marriott Hotel (WTC 3), the South Plaza
building (WTC 4), the U.S. Customs building (WTC 6), and the Winter Garden,
experienced nearly total collapse as a result of the massive quantities of debris that fell
on them when the two towers collapsed. The St. Nicholas Greek Orthodox Church just
south of WTC 2 was completely destroyed by the debris that fell on it.
WTC 5, WTC 7, 90 West Street, 130 Cedar Street, Bankers Trust, the Verizon
building, and World Financial Center 3 were impacted by large debris from the
collapsing twin towers and suffered structural damage, but arrested collapse to localized
areas. The performance of these buildings demonstrates the inherent ability of
redundant steel-framed structures to withstand extensive damage from earthquakes,
blasts, and other extreme events without progressive collapse.
American Society of Civil Engineers – page 7
House Science Committee – May 1, 2002
The debris from the collapses of the WTC towers also initiated fires in
surrounding buildings, including WTC 4, 5, 6, 7; 90 West Street; and 130 Cedar Street.
Many of the buildings suffered severe fire damage but remained standing. However, two
steel-framed structures experienced fire-induced collapse. WTC 7 collapsed completely
after burning unchecked for approximately 7 hours, and a partial collapse occurred in an
interior section of WTC 5. Studies of WTC 7 indicate that the collapse began in the
lower stories, either through failure of major load transfer members located above an
electrical substation structure or in columns in the stories above the transfer structure.
The collapse of WTC 7 caused damage to the Verizon building and 30 West Broadway.
The partial collapse of WTC 5 was not initiated by debris and is possibly a result of fire-
induced connection failures. The collapse of these structures is particularly significant in
that, prior to these events, no protected steel-frame structure, the most common form of
large commercial construction in the United States, had ever experienced a fire-induced
collapse. Thus, these events may highlight new building vulnerabilities, not previously
believed to exist.
In the study of the WTC towers and the surrounding buildings that were
subsequently damaged by falling debris and fire, several issues were found to be critical
to the observed building performance in one or more buildings.
General Observations Findings and Recommendations
These issues above fall into several broad topics that should be considered for
buildings that are being eva luated or designed for extreme events. It may be that some
of these issues should be considered for all buildings; however, additional studies are
required before general recommendations, if any, can be made for all buildings. The
issues identified from this study of damaged buildings in or near the WTC site have
been summarized into the following points:
a. Structural framing systems need redundancy and/or robustness, so that alternative
paths or additional capacity is available for transmitting loads when building
damage occurs.
b. Fireproofing needs to adhere under impact and fire conditions that deform steel
members, so that the coatings remain on the steel and provide the intended
protection.
c. Connection performance under impact loads and during fire loads needs to be
analytically understood and quantified for improved design capabilities and
performance as critical components in structural frames.
d. Fire protection ratings that include the use of sprinklers in buildings require a
reliable and redundant water supply. If the water supply is interrupted, the
assumed fire protection is greatly reduced.
American Society of Civil Engineers – page 8
House Science Committee – May 1, 2002
e. Egress systems currently in use should be evaluated for redundancy and
robustness in providing egress when building damage occurs, including the issues
of transfer floors, stair spacing and locations, and stairwell enclosure impact
resistance.
f. Fire protection ratings and safety factors for structural transfer systems should be
evaluated for their adequacy relative to the role of transfer systems in building
stability.
What significant recommendations does the team make in its report?
What may be most important is that the BPS Team does not recommend any
immediate changes to building codes. The Team believes that there are a number of
areas that need further study, and that there are some things that building designers
could do to improve safety for occupants in buildings that might be possible terrorist
targets.
In general terms, the FEMA/ASCE report suggests that critical building
components such as the structural frame, the sprinkler system or the exit stairwells be
designed to be more redundant, more robust, or both. Redundancy means, for example,
that if some structural columns were shattered, the building would be designed to
transfer the weight to other columns. Robustness means making the builder stronger
and better able to resist impact without collapse.
The team is also strongly urging additional study of the collapse of the buildings.
What key findings impact all existing buildings?
The team found that some connections between the structural steel beams failed
in the fire. This was most apparent in the collapse of World Trade Center Building 5,
where the fireproofing did not protect the connections, causing the structure to fail.
The team is calling for more research and analysis of the how the connections
weakened and how best to strengthen their resistance to future fires. Typically, fire
resistance tests are limited to steel members, not to the steel connections.
Furthermore, fireproofing is sprayed on the connections the same way it is applied to
the trusses, though the steel in the trusses and joints may be made of different alloys.
Specific Observations, Findings, and Recommendations
The following sections present observations, findings, and recommendations
specifically made in each chapter of the FEMA/ASCE report, including the discussion of
building codes and fire standards and the limited metallurgical examination of steel from
the WTC towers and WTC 7.
Building Codes and Fire Standards
Observations and Findings
a. The decision to include aircraft impact as a design parameter for a building would
clearly result in a major change in the design, livability, usability, and cost of
American Society of Civil Engineers – page 9
House Science Committee – May 1, 2002
buildings. In addition, reliably designing a building to survive the impact of the largest
aircraft available now or in the future may not be possible. These types of loads and
analyses are not suitable for inclusion in minimum loads required for design of all
buildings. Just as the possibility of a Boeing 707 impact was a consideration in the
original design of WTC 1 and WTC 2, there may be situations where it is desirable to
evaluate building survival for impact of an airplane of a specific size traveling at a
specific speed. Although there is limited public information available on this topic,
interested building owners and design professionals would require further guidance
for application to buildings.
b. The ASTM E119 Standard Fire Test was developed as a comparative test, not a
predictive one. In effect, the Standard Fire Test is used to evaluate the relative
performance (fire endurance) of different construction assemblies under controlled
laboratory conditions, not to predict performance in real, uncontrolled fires.
World Trade Center 1 and World Trade Center 2
Observations and Findings
a. The structural damage sustained by each of the two buildings as aircraft impacted
them during the attacks was massive. The fact that the structures were able to
sustain this level of damage and remain standing for an extended period of time is
remarkable and is the reason that most building occupants were able to evacuate
safely. Events of this type, resulting in such substantial damage, are generally not
considered in building design, and the ability of these structures to successfully
withstand such damage is noteworthy.
b. Preliminary analyses of the damaged structures, together with the fact the structures
remained standing for an extended period of time, suggest that absent other severe
loading events, such as a windstorm or earthquake, the buildings could have
remained standing in their damaged states until subjected to some significant
additional load. However, the structures were subjected to a second, simultaneous
severe loading event in the form of the fires caused by the aircraft impacts.
c. The large quantity of jet fuel carried by each aircraft ignited upon impact into each
building. A significant portion of this fuel was consumed immediately in the ensuing
fireballs. The remaining fuel is believed either to have flowed down through the
buildings or to have burned off within a few minutes of the aircraft impact. The heat
produced by this burning jet fuel does not by itself appear to have been sufficient to
initiate the structural collapses. However, as the burning jet fuel spread across
several floors of the buildings, it ignited much of the buildings' contents, permitting
fires to evolve across several floors of the buildings simultaneously. The heat output
from these fires is estimated to have been comparable to the power produced by a
large commercial generating station. Over a period of many minutes, this heat
induced additional stresses into the damaged structural frames while simultaneously
softening and weakening these frames. This additional loading and dama ge were
sufficient to induce the collapse of both structures.
American Society of Civil Engineers – page 10
House Science Committee – May 1, 2002
d. Because the aircraft impacts into the two buildings are not believed to have been
sufficient to cause collapse without the ensuing fires, an obvious question exists as
to whether the fires alone, without the damage from the aircraft impact, would have
been sufficient to cause such a collapse. The capabilities of the building fire
protection systems make it extremely unlikely that such fires could develop without
some unusual triggering event like the aircraft impact. For all other cases, the fire
protection for the tower buildings provided in-depth protection. The first line of
defense was the automatic sprinkler protection. The sprinkler system was intended
to respond quickly and automatically to extinguish or confine a fire. The second line
of defense consisted of the manual (FDNY/Port Authority Fire Brigade) firefighting
capabilities, which were supported by the building standpipe system, emergency fire
department use elevators, smoke control system, and other features. Manual
suppression by FDNY was the principal fire protection mechanism that controlled a
large fire that occurred in the buildings in 1975. Finally, the last line of defense was
the structural fire resistance. The fire resistance capabilities would not be called
upon unless both the automatic and manual suppression systems previously
described failed. In the incident of September 11, not only did the aircraft impact
disable the first two lines of defense, they also are believed to have dislodged
fireproofing and imposed major additional stresses on the structural system.
e. Had some other event defeated both the automatic and manual suppression
capabilities and a fire of major proportions occurred while the structural framing
system and its fireproofing remained intact, the third line of defense, structural
fireproofing, would have become critical. The thickness and quality of the fireproofing
materials would have been key factors in the rate and extent of temperature rise in
the floor trusses and other structural members. In the preparation of this report,
there has not been sufficient analysis to predict the temperature and resulting
change in strength of the individual structural members in order to approximate the
overall respons e of the structure. Given the redundancy in the framing system and
the capability of that system to redistribute load from a weakened member to other
parts of the structural system, it is impossible without extensive modeling and other
analysis to make a credible prediction of how the building would have responded to
an extremely severe fire in a situation where there was no prior structural damage.
Such simulations have not been performed within the scope of this study, but should
be performed in the future.
f. Buildings are designed to withstand loading events that are deemed credible
hazards and to protect the public safety in the event such credible hazards are
experienced. Buildings are not designed to withstand any event that could ever
conceivably occur, and any building can collapse if subjected to a sufficiently
extreme loading event. Communities adopt building codes to help building designers
and regulators determine those loading events that should be considered as credible
hazards in the design process. These building codes are developed by the design
and regulation communities themselves, through a voluntary committee consensus
process. Prior to September 11, 2001, it was the consensus of these communities
American Society of Civil Engineers – page 11
House Science Committee – May 1, 2002
that aircraft impact was not a sufficiently credible hazard to warrant routine
consideration in the design of buildings and, therefore, the building codes did not
require that such events be considered in building design. Nevertheless, design of
WTC 1 and WTC 2 did include at least some consideration of the probable response
of the buildings to an aircraft impact, albeit a somewhat smaller and slower moving
aircraft than those actually involved in the September 11 events. Building codes do
regard fire as a credible hazard and include extensive requirements to control the
spread of fire throughout buildings, to delay the onset of fire-induced structural
collapse, and to facilitate the safe egress of building occupants in a fire event. For
fire-protected steel-frame buildings, like WTC 1 and WTC 2, these code
requirements had been deemed effective and, in fact, prior to September 11, there
was no record of the fire-induced-collapse of such structures, despite some very
large uncontrolled fires.
g. The ability of the two towers to withstand aircraft impacts without immediate collapse
was a direct function of their design and construction characteristics, as was the
vulnerability of the two towers to collapse as a result of the combined effects of the
impacts and ensuing fires. Many buildings with other design and construction
characteristics would have been more vulnerable to collapse in these events than
the two towers, and few may have been less vulnerable. It was not the purpose of
this study to assess the code-conformance of the building design and construction,
or to judge the adequacy of these features. However, during the course of this study,
the structural and fire protection features of the building were examined. The study
did not reveal any specific structural features that would be regarded as
substandard, and, in fact, many structural and fire protection features of the design
and construction were found to be superior to the minimum code requirements.
h. Several building design features have been identified as key to the buildings’ ability
to remain standing as long as they did and to allow the evacuation of most building
occupants. These include the following:
•
robustness and redundancy of the steel framing system;
•
presence of adequate egress stairways that were; and
•
the conscientious implement ation of emergency exiting training programs for
building tenants.
i. Similarly, several design features have been identified that may have played a role
in allowing the buildings to collapse in the manner that they did and in the inability of
victims at and above the impact floors to safely exit. These features should not be
regarded either as design deficiencies or as features that should be prohibited in
future building codes. Rather, these are features that should be subjected to more
detailed evaluation, in order to understand their contribution to the performance of
these buildings and how they may perform in other buildings. These include the
following:
•
the type of steel floor truss system present in these buildings and their structural
robustness and redundancy when compared to other structural systems;
•
use of impact-resistant enclosures around egress paths;
American Society of Civil Engineers – page 12
House Science Committee – May 1, 2002
•
resistance of passive fire protection to blasts and impacts in buildings designed
to provide resistance to such hazards; and
•
grouping emergency egress stairways in the central building core, as opposed to
dispersing them throughout the structure.
j. During the course of this study, the question of whether building codes should be
changed in some way to make future buildings more resistant to such attacks was
frequently explored. Depending on the size of the aircraft, it may not be technically
feasible to develop design provisions that would enable structures to be designed
and constructed to resist the effects of impacts by rapidly moving aircraft, and the
ensuing fires, without collapse. In addition, the cost of constructing such structures
might be so large as to make this type of design intent practically infeasible.
Although the attacks on the World Trade Center are a reason to question design
philosophies, the BPS Team believes there are insufficient data to determine
whether there is a reasonable threat of attacks on specific buildings to recommend
inclusion of such requirements in building codes. Some believe the likelihood of such
attacks on any specific building is deemed sufficiently low to not be considered at all.
However, individual building developers may wish to consider design provisions for
improving redundancy and robustness for such unforeseen events, particularly for
structures that, by nature of their design or occupancy, may be especially
susceptible to such incidents. Although some conceptual changes to the building
codes that could make buildings more resistant to fire or impact damage or more
conducive to occupant egress were identified in the course of this study, the BPS
Team felt that extensive technical, policy, and economic study of these concepts
should be performed before any specific code change recommendations are
developed. This report specifically recommends such additional studies. Future
building codes revisions may be considered after the technical details of the
collapses and other building responses to damage are better understood.
Recommendations
The scope of this study was not intended to include in-depth analysis of many issues
that should be explored before final conclusions are reached. Additional studies of the
performance of WTC 1 and WTC 2 during the events of September 11, 2001, and of
related building performance issues should be conducted. These include the following:
a. During the course of this study, it was not possible to determine the condition of the
interior structure of the two towers, after aircraft impact and before collapse. Detailed
modeling of the aircraft impacts into the buildings should be conducted in order to
provide understanding of the probable damage state immediately following the
impacts.
b. Preliminary studies of the growth and heat flux produced by the fires were
conducted. Although these studies provided useful insight into the buildings'
behavior, they were not of sufficient detail to permit an understanding of the
probable distribution of temperatures in the buildings at various stages of the event
American Society of Civil Engineers – page 13
House Science Committee – May 1, 2002
and the resulting stress state of the structures as the fires progressed. Detailed
modeling of the fires should be continued and should be combined with structural
modeling to develop specific failure modes likely to have occurred.
c. The floor framing system for the two towers was very complex and substantially
more redundant than typical bar joist floor systems. Detailed modeling of these floor
systems and their connections should be conducted to understand the effects of
localized overloads and failures to determine ultimate failure modes. Other types of
common building framing should also be examined for these effects.
d. The fire-performance of steel trusses with spray-applied fire protection, and with end
restraint conditions similar to that present in the two towers, is not well understood,
but is likely critical to the building collapse. Studies of the fire-performance of this
structural system should be conducted.
e. Observations of the debris generated by the collapse and of damaged adjacent
structures suggests that spray-applied fireproofing may be vulnerable to mechanical
damage from blasts and impacts. This vulnerability is not well understood. Tests of
these materials should be conducted to understand how well they withstand such
mechanical damage and to determine whether it is appropriate and feasible to
improve their resistance to such damage.
f. In the past, tall buildings have occasionally been damaged, typically by earthquakes,
and experienced collapse within the damaged zones. Those structures were able to
arrest collapse before they progressed to a state of total collapse. The two WTC
towers were able to arrest collapse from the impact damage but not from the
resulting fire when combined with the impact effects of the aircraft attack. Studies
should be conducted to determine, given the great size and weight of the two
towers, whether there are feasible design and construction features that would
permit such buildings to arrest or limit a collapse, once it began.
World Trade Center 3
Observations
WTC 3 was subjected to extraordinary loading from the impact and weight of debris
from the two adjacent 110-story towers. It is noteworthy that the building resisted both
horizontal and vertical progressive collapse after the collapse of WTC 2. The
overloaded portions were able to break away from the rest of the structure without
pulling it down and the remaining structural system was able to remain stable and
support the debris load. The structure was even capable of protecting occupants after
the collapse of WTC 1.
Recommendations
WTC 3 should be studied further to understand how it resisted progressive collapse.
American Society of Civil Engineers – page 14
House Science Committee – May 1, 2002
World Trade Center 7
Observations and Findings
a. This office building was built over an electrical substation and a power plant,
comparable in size to that operated by a small commercial utility. It also had a
significant amount of diesel oil storage and had a structural system with numerous
horizontal transfers for gravity and lateral loads.
b. The loss of the east penthouse on the videotape records suggests that the collapse
event was initiated by the loss of structural integrity in one of the transfer systems.
Loss of structural integrity was likely a result of weakening caused by fires on the 5th
to 7th floors. The specifics of the fires in WTC 7 and how they caused the building to
collapse remain unknown at this time. Although the total diesel fuel on the premises
contained massive potential energy, the best hypothesis has only a low probability of
occurrence. Further research, investigation, and analyses are needed to resolve this
issue.
c. The collapse of WTC 7 was different from that of WTC 1 and WTC 2. The towers
showered debris in a wide radius as their external frames essentially "peeled"
outward and fell from the top to the bottom. In contrast, the collapse of WTC 7 had a
relatively small debris field because the facade came straight down, suggesting an
internal collapse. Review of video footage indicates that the collapse began at the
lower floors on the east side. Studies of WTC 7 indicate that the collapse began in
the lower stories, either through failure of major load transfer members located
above an electrical substation structure or in columns in the stories above the
transfer structure. Loss of strength due to the transfer trusses could explain why the
building imploded, with collapse initiating at an interior location. The collapse may
have then spread to the west, causing interior members to continue collapsing. The
building at this point may have had extensive interior structural failures that then led
to the collapse of the overall building, including the cantilever transfer girders along
the north elevation, the strong diaphragms at the 5th and 7th floors, and the seat
connections between the interior beams and columns at the building perimeter.
Recommendations
The scope of this study was not intended to include in-depth analysis of issues. Certain
issues should be explored before final conclusions are reached and additional studies of
the performance of the WTC 7 building and related building performance issues should
be conducted. These include the following:
a. Additional data should be collected to confirm the extent of the damage to the south
face of the building caused by falling debris.
b. Determination of the specific fuel loads, especially at the lower levels, is important to
identify possible fuel supplied to sustain the fires for a substantial duration. Areas of
interest include storage rooms, file rooms, spaces with high-density combustible
materials, and locations of fuel lines. The control and operation of the emergency
power system, including generators and storage tanks, needs to be thoroughly
American Society of Civil Engineers – page 15
House Science Committee – May 1, 2002
understood. Specifically, the ability of the diesel fuel pumps to continue to operate
and send fuel to the upper floors after a fuel line is severed should be confirmed.
c. Modeling and analysis of the interaction between the fire and structure are
important. Specifically, the anticipated temperatures and duration of the fires and the
effects of the fires on the structure need to be examined with an emphasis on the
behavior of transfer systems and their connections.
d. Suggested mechanisms for a progressive collapse should be studied and confirmed.
How the collapse of an unknown number of gravity columns brought down the whole
building should be explained.
e. The role of the axial capacity between the beam-column connection and the
relatively strong structural diaphragms may have had in the progressive collapse
should be explained.
f. The level of fire resistance and the ratio of capacity-to-demand required for structural
members and connection deemed to be critical to the performance of the building
should be studied. The collapse of some structural members and connections may
be more detrimental to the overall performance of the building than other structural
members. The adequacy of current design provisions for members whose failure
could result in large-scale collapse should also be studied.
Recommendations for Future Study
The Building Performance Study Team has developed recommendations for
specific issues, based on the study of the performance of the WTC towers and
surrounding buildings in response to the impact and fire damage that occurred. These
recommendations have a broader scope than the important issue of building concepts
and design for mitigating damage from terrorist attacks, and also address the level at
which resources should be expended for aircraft security, how the fire protection and
structural engineering communities should increase their interaction in building design
and construction, possible considerations for improved egress in damaged structures,
the public understanding of typical building design capacities, issues related to the study
process and future activities, and issues for communities to consider if they are
developing emergency response plans that include engineering response.
National Response.
Resources should be directed primarily to aviation and
other security measures rather than to hardening buildings against airplane impact. The
relationship and cooperation between public and private organizations should be
evaluated to determine the most effective mechanisms and approaches in the response
of the nation to such disasters.
Interaction of Structural Elements and Fire.
The existing prescriptive fire
resistance rating method (ASTM E119) does not provide sufficient information to
American Society of Civil Engineers – page 16
House Science Committee – May 1, 2002
determine how long a building component can be expected to perform in an actual fire.
A method of assessing performance of structural members and connections as part of a
structural system in building fires is needed for designers and emergency personnel.
The behavior of the structural system under fire conditions should be considered
as an integral part of the structural design. Recommendations are to:
•
Develop design tools, including an integrated model that predicts heating
conditions produced by the fire, temperature rise of the structural component,
and structural response.
•
Provide interdisciplinary training in structures and fire protection for both
structural engineers and fire protection engineers.
Performance criteria and test methods of fireproofing materials relative to their
durability, adhesion, and cohesion when exposed to abrasion, shock, vibration, rapid
temperature rise, and high temperature exposures need further study.
Interaction of Structural and Fire Professionals in Design.
The structural,
mechanical, architectural, fire protection, blast, explosion, earthquake, and wind
engineering communities need to work together to develop guidance for vulnerability
assessment, retrofit, and design of concrete and steel structures to mitigate or reduce
the probability of progressive collapse under single- and multiple-hazard scenarios.
An improved level of interaction between structural and fire protection engineers
is encouraged. Specific recommendations are to:
•
Consider behavior of the structural system under fire as an integral part of the
design process.
•
Provide cross-training of fire protection and structural engineers in the
performance of structures and building fires.
Fire Protection and Engineering Discipline.
The continued development of a
system for performance- based design is encouraged. This involves the following:
•
Improve the existing models that simulate fire and spread in structures, as well
as the impact of fire and smoke on structures and people.
•
Improve the database on material burning behavior.
Building Evacuation.
The following topics were not explicitly examined during
this study, but are recognized as important aspects of designing buildings for impact
and fire events. Recommendations for further study are to:
•
Perform an analysis of occupant behavior during evacuation of the buildings at
WTC to improve the design of fire alarm and egress systems in high-rise
buildings.
American Society of Civil Engineers – page 17
House Science Committee – May 1, 2002
•
Perform an analysis of the design basis of evacuation systems in high-rise
buildings to assess the adequacy of the current design practice, which relies on
phased evacuation.
•
Evaluate the use of elevators as part of the means of egress for mobility impaired
people as well as the general building population for the evacuation of high-rise
buildings. In addition, the use of elevators for access by emergency personnel
needs to be evaluated.
Emergency Personnel.
One of the most serious dangers firefighters and other
emergency responders face is partial or total collapse of buildings. Recommended
steps to provide better protection to emergency personnel are:
•
Have fire protection and structural engineers assist emergency personnel in
developing broader pre- plans for buildings and structures to include more
detailed assessments of hazards and response of structural elements and
performance of buildings during fires, including identification of critical structural
elements.
•
Develop training materials and courses for emergency personnel with regards to
effects of fire on steel.
•
Review collaboration efforts between the emergency personnel and engineering
professions so that engineers may assist emergency personnel in assessments
during the time of the incident.
Education of Stakeholders.
Stakeholders (e.g., owners, operators, tenants,
aut horities, designers) should be further educated about building codes, the minimum
design loads typically addressed for building design, and the extreme events that are
not addressed by building codes. Should stakeholders desire to address events not
addressed by the building codes, they should have a basic understanding of developing
and implementing strategies to mitigate damage from extreme events.
Stakeholders should also be educated about the expected performance of their building
when renovations, or changes in use or occupancy, occur and the building is subjected
to different floor or fire loads. For instance, if the occupancy in a building changes to
one with a higher fire hazard, they should review the fire protection systems to ensure
there is adequate fire protection. Or, if the structural load is increased with a new
occupancy, the structural support system should be reviewed to ensure it can carry the
new load.
Study Process.
The report benefited from a tremendous amount of professional
volunteerism due to the unprecedented level of national disaster. Improvements can be
made that would aid the process for any future efforts. Recommendations are to:
•
Provide resources that are proportional to the required level of effort.
•
Provide better access to data, including building information, interviews, samples,
site photos, and documentation.
American Society of Civil Engineers – page 18
House Science Committee – May 1, 2002
Archival Information.
Archival information has been collected and provides the
groundwork for continued study. It is recommended that a coordinated effort for the
preservation of this and other relevant information be undertaken by a responsible
organization or agency, capable of maintaining and managing such information. This
effort would include:
•
cataloging all photographic data collected to date;
•
enhancing video data collected for both quality and timeline;
•
conducting interviews with building occupants, witnesses, rescue workers and
any others that may provide valuable information; and
•
initiating public requests for information.
Conclusion
ASCE is proud of the work done by the BPS Team, but strongly believes that the
follow up studies recommended by the FEMA/ASCE Report are critical to obtaining the
technical knowledge needed by engineers for future building design.
Thank you for the opportunity to express ASCE’s views. We offer you and all of
the agencies involved in the recovery efforts ASCE’s full resources to manage the
nation's critical infrastructure needs. We are ready to help in any way possible, and are
eager to hear from you regarding ways that ASCE’s Critical Infrastructure Response
Initiative can support you as you examine our infrastructure needs in the coming
months.
American Society of Civil Engineers – page 19
House Science Committee – May 1, 2002
o
W. Gene
Corley
Senior Vice President
gcorley @ c-t-l.com
Educational Background •
University of Illinois
B.S. Civil Engineering, 1958
M.S. Structural Engineering, 1960
Ph.D. Structural Engineering, 1961
Registration •
Licensed Structural Engineer - Illinois
Licensed Professional Engineer - Illinois
Registered Civil Engineer - California, Hawaii
Registered Professional Engineer - Alabama,
Florida, Kansas, Louisiana, Michigan,
Mississippi, Missouri, Pennsylvania,
South Dakota, Tennessee, Texas, Utah,
Virginia, Washington
Chartered Engineer, FI Struct E, UK
CTL Experience
• Dr. Corley has served as CTL
Vice President since 1987. In this position, he
serves as CTL’s managing agent for professional
and structural engineering and leads structural
evaluation projects related to industrial,
transportation and parking facilities, bridges and
buildings. He also is active in projects related to
earthquake engineering. His wide range of
experience includes evaluation of earthquake
and blast damaged buildings and bridges;
investigation of distress in prestressed concrete
structures; repair of parking garages damaged
by corrosion; evaluation and repair of high rise
buildings, stadiums, silos and bridges; design
and construction of repairs for prestressed and
conventionally-reinforced, precast and cast-in-
place concrete and structural steel facilities. In
1995, Dr. Corley was selected by ASCE to lead
a Building Performance Assessment Team
investigating the bombing of the Murrah Federal
Building in Oklahoma City.
Prior Experience
• After receiving his B.S.
degree, Dr. Corley worked for the Shelby
County, Illinois highway department where he
designed highways and bridges. He then
returned to the University of Illinois as a research
assistant and National Science Foundation
teaching fellow while pursuing his graduate
studies.
Upon completion of his Ph.D., he served as a
commissioned officer in the U.S. Army from 1961
until 1964. During this period, Dr. Corley was a
research and development coordinator with the
U.S. Army Corps of Engineers at Fort Belvoir,
Virginia. His duties included bridge design,
acceptance testing of mobile floating assault
bridge equipment, design of tank launched
bridges and fatigue testing of bridges fabricated
from high strength steel, aircraft aluminum and
titanium alloys.
In 1964, Dr. Corley began work as a
development engineer with the Portland Cement
Association. While serving in successively more
responsible positions, he was directly involved in
the development of improved design procedures
for structural concrete, concrete pavement,
railroads and structures subjected to fire loads.
In addition, he served on an earthquake damage
investigation team, carried out investigations of
damaged or deteriorated structures and
developed repair procedures for numerous
buildings and bridges.
American Society of Civil Engineers – page 20
House Science Committee – May 1, 2002
Publications and Professional Activities
•
W. Gene Corley has authored more than 150
technical papers and books. He frequently
lectures to technical and non-technical groups on
the subjects of prevention of failures, effects of
earthquakes and design and repair of structures.
He regularly presents training courses on
reinforced concrete design and teaches the
seismic design portion of a refresher course to
candidates for the Illinois Structural Engineering
License examination.
Dr. Corley chaired ACI Committee 318 for six
years as the committee developed the 1995
Building Code Requirements for Structural
Concrete. He also serves on several other
national and international committees that
prepare recommendations for structural design
and for design of earthquake resistant buildings
and bridges. His professional activities resulted
in his receiving 11 national awards including the
Best Structural Publication Award from NCSEA,
Outstanding Paper from the ASCE Journal of
Performance of Constructed Facilities, the
Wason Award for research from ACI, the T.Y. Lin
Award from ASCE and the Martin Korn Award for
PCI. He also has received several regional
awards, including the UIUC Civil Engineering
Alumni Association's Distinguished Alumnus
Award, the SEAOI Service Award, Illinois ASCE
Structural Division's Lifetime Achievement
Award, the Henry Crown Award, and the SEAOI
John Parmer Award.
Dr. Corley serves or has served in leadership
roles for numerous professional organizations,
both national and international, including the
following:
American Society of Civil Engineers (Fellow)
National Society of Professional Engineers
(Member)
National Council of Structural Engineers
Associations (Founding Member, Board of
Direction, Former President)
American Concrete Institute (Fellow) Former
Chairman, Committee on Standard
Building Code
American Railway Engineering Association
(Member)
Building Seismic Safety Council (Former
Vice-Chairman and Founding Member,
Board of Direction)
Chicago Committee on High Rise Buildings
(Member and Former Chairman)
Earthquake Engineering Research Institute
(Member and Former President, Great
Lakes Chapter)
Institution of Structural Engineers, UK
(Fellow)
International Association for Bridge and
Structural Engineering (Member)
National Academy of Engineering (Member)
National Association of Railroad Safety
Consultants and Investigators (Member)
NACE International (Member)
Prestressed Concrete Institute (Member)
RILEM (Member)
Post Tensioning Institute (Member)
Transportation Research Board (Member)
Structural Engineers Association of Illinois
(Member, Former President)
Governor’s Earthquake Preparedness Task
Force (Illinois)