Developing a Seismic Damage Evaluation Model for Tehran’s Buildings in a GIS Environment

A. Bazgard, B. Omidvar
1,3 : Faculty of environment Eng., University of Tehran, Tehran, Iran.
a.bazgard@gmail.com, bomidvar@ut.ac.ir

A. Mansourian
2: Faculty of Geodesy & Geomatics Eng., K.N.Toosi University of Technology, Tehran, Iran.
mansourian@kntu.ac.ir

Abstract
In an earthquake disaster, having proper estimation about destructed buildings and the degree of destruction, can considerably facilitate decision-making and planning for disaster managers. By use of this information, the managers can estimate disaster area and number of victims to determine required resources and how to allocate them.

Scientific studies and historical data show that the faults around Tehran, the capital of Iran, are capable to create strong earthquakes which would bring the largest damages in the world history to Tehran. So it is necessary to be prepared for a rapid and knowledge-based response to such an earthquake. One of the aspects of such preparedness relates to developing a proper model to estimate destruction of buildings. Since, no practical model has been proposed for Tehran yet, so devising such a model seems to be absolutely essential. In the context of a research project, development of a model to estimate destruction of buildings is ongoing. The model is going to be developed by using the data obtained from the buildings and the fragility curves in Tehran as well as probabilistic seismic hazard analysis. Since mainly spatial data is used in this project, the model is developing in a GIS environment. This paper outlines the results of the project till now.

Introduction
Due to time-sensitive nature of disasters, rapid and reliable decision-making has a significant impact on managing disasters. Such decision-making highly requires proper information on current emergency situation. Meanwhile, estimation on disaster area and number of victims is initial and essential information that a manager may need for a proper decision-making. As a result, a map of destructions and their severity is indeed of a high value to managers.

Tehran, the capital of Iran with a population of more than ten millions, is built on a network of faults with high risk to earthquake. Historically, Tehran has an earthquake return period of 150 years and no earthquake has occurred since 175 years ago. Having mentioned that, Tehran is threatened by a strong earthquake which will bring large damages and casualties to the city. So, disaster management community should prepare itself for responding to such huge disaster. One of the aspects of the mentioned preparedness relates to having proper tools and models to estimate disaster area and number of victims for preparing initial plans.

Attenuation relationships and fragility curves are two basic concepts/models that are used for estimating damage level of buildings in the case of earthquake. In these models, different data such as the earthquake center and magnitude, distance of building from the center, soil type and geological characteristics of the ground between the center and building, structural characteristics of the building and the age of the building are used. As most of these data have spatial component and location, GIS is a proper tool for their integration and analysis to get the result.

In the context of a research project, which is conducted by K.N.Toosi University of Technology and Tehran University, a general model is developing to estimate destruction levels of buildings in Tehran, in the case of occurrence of earthquake. This model is being developed in GIS environment. This paper outlines the basic concepts which are being used to develop the model as well as the results of the research till now.

Attenuation Relationships
After occurrence of an earthquake, the center and the magnitude of the earthquake are two initial data that are available. The magnitude of earthquake in any location is different from the magnitude at the center. It differs and generally decreases when the distance between a certain point and the earthquake center increases. Different factors such as soil type and distance from the center have effect on the value of magnitude on a certain point. To obtain this effect and the magnitude in any place, some formulas called attenuation relationships are used. Common parameters that can be calculated by these formulas are PGA (Peak Ground Acceleration), PGV (Peak Ground Velocity) and PGD (Peak Ground Displacement). These formulas are developed based on empirical or theoretical studies. In this research Zare's attenuation relationship (Zare, 1999) that is based on empirical method is used.

Fragility Curves
Fragility curves describe the destruction excess probability (from a predefined percentage) with respect to the earthquake physical parameters (Jalalian, 2006). To develop fragility curve for an area, buildings are classified to different groups according to their structural system, the design code and number of floors. A fragility curve is offered for each group. For example, in U.S emergency management software (Hazus99 Technical Manual, 1999), buildings have been classified into 36 groups according to structural system and number of floors. In this category, number of floors have been classified into Low-Rise(1-3), Mid-Rise(4-7) and High-Rise(8+). With respect to the design code, the 36 groups are presented in four code groups: Pre-code, Low-code, Moderate-code, and High-code.

There are four methods to develop a fragility curve namely empirical, judgmental, and analytical and hybrid. In figures 1, 2, 3 some fragility curves for Masonry buildings are illustrated (Jalalian, 2006).

Figure 1 – Fragility curve for 0-30% Destruction (Jalalian, 2006)

Figure 2 – Fragility curve for 30-60% Destruction (Jalalian, 2006)

Figure 3 – Fragility curve for 60-100% Destruction (Jalalian, 2006)
How to Use These Models
To estimate destruction for an individual building, firstly, the earthquake magnitude at the location of building should be calculated by use of attenuation relationships in term of PGA, PGV or PGD. In the second step, by use of building properties, respective fragility curve should be identified and then by use of that curve the probability of excess of destruction from a predefined percentage should be obtained. For example, to calculate the 0-30% destruction probability for a masonry building when the PGA=0.2, by use of figure 1 (which is for masonry building) the probability would be 74%.

Research Attitude
Different efforts have been conducted to develop the mentioned models for Tehran. Japan International Cooperation Agency (JICA, 2000) has developed a fragility curve for Tehran, using available data from Manjil earthquake (1990). Jalalian (2006) has also developed fragility curves for Tehran. These curves are good for masonry buildings however not accurate for other types of building due to data shortage during model development.

With these in mind, it is essential to develop a more proper fragility curves for Tehran. In this research, it is intended to develop a fragility curve for Tehran by adopting the offered fragility curve in Hazus software (Hazus Technical Manual) to Tehran, with respect to the two above mentioned curves. The developed model will then be implemented in GIS environment.

Conclusion
In this paper, the scientific basic applied in seismic destruction of buildings as well as role of GIS in developing an appropriate model for this purpose were studied.

In addition, the method of developing a primary model for damage evaluation of buildings in Tehran, which is still underway, was defined. The primary model is being completed on the basis of the aforementioned method.

More accurate studies to reach a more developed model to replace the primary model will be done in our future study.

References:

Zare, M., P.-Y. Bard, M. Ghafory-Ashtiany, Attenuation Law for the Strong Motions in Iran, 3rd Int. Conf. on Seismology and Earthquake Engineering (SEE3), Tehran, 1999
Jalalian, M. (2006). ” Deriving of Empirical Vulnerability Functions for Iran”, a MSc Thesis, University of Technology, Tehran, Iran.
Hazus99 Technical Manual (1999), Federal Emergency Management Agency, Washington, USA.
JICA (2000). “The Study on Seismic Microzoning of the Greater Tehran Area in the Islamic Republic of Iran", a report from Center for Earthquake an Environmental Studies of Tehran (CEST) and Japan International Cooperation Agency (JICA), Tehran Municipality, Tehran, Iran.