Estimation of Potential Seismic Damage in Urban Areas

Abstract

Large earthquakes are known to have significant damage potential in urban regions. Recently all over the world, efforts are made toward reduction of future probable damages. The first step in damage mitigation is the estimation of expected damage levels in buildings that are subjected to earthquakes with different intensities. Due to the inherent uncertainties involved in the analyses, estimation of seismic damage rates must be handled within a probabilistic framework. To assess the damage rates under different shaking levels, “seismic damage” needs to be quantified and measured in a standard manner. The most common approach to quantify seismic damage rates is to perform fragility analyses. Fragility is defined as the probability of a system reaching a limit state as a function of seismic intensity levels (Kafali and Grigouri 2004). Fragility analyses are generally carried out with one of the following five alternative approaches: empirical, analytical, subjective, experimental, and hybrid methods. The results of such damage probability assessment analyses can be expressed in terms of fragility curves or damage probability matrices. Fragility curves are continuous curves that state the cumulative damage rates whereas damage probability matrices express the same information in terms of discrete damage probabilities for a particular intensity level. Empirical method is based on the fundamental idea that similar type of structures experiences comparable damage rates under earthquakes. It involves analysis of empirical data collected in postearthquake surveys. In the analytical method, in order to estimate the limit states of the structure, the seismic performance under a given ground motion level is assessed through detailed time history analysis or other simplified methods. Subjective method includes expert opinions for the assessment of the damage probabilities under various levels of shaking intensities. Experts are requested to give their opinions on the level of the damage based on their relevant experience on seismic damage assessment. It is also possible to generate fragility curves based on the experimental data that has been obtained through a series of tests. Finally, hybrid method involves a combination of the previously described approaches for assessment of damage probabilities. An important problem is the comparison of damage probabilities obtained with these alternative methods. In this study, empirical and analytical approaches are compared in terms of damage data from a recent earthquake that occurred in Turkey. For this purpose, first, empirical damage probabilities are obtained from the damage database of the 17 August 1999 Kocaeli earthquake. Then, the results are compared with the damage rates from the analytical method. Results of this