Mohammad Hossein Ahmadi - Department of Civil Engineering, Sharif University of Technology, Tehran, Iran
Babak Ghods - Department of Civil Engineering, Sharif University of Technology, Tehran, Iran

A lot of structures have been damaged during destructive historical earthquakes in the past. To mitigate casualties in future earthquakes, it is necessary to evaluate the current status ofthese buildings and propose effective methods to retrofit them. One method of evaluating the performance of buildings in earthquakes is through the use offragility curves. These diagrams show the probability of exceeding a specific state of damage versus seismic intensity parameters. Much work has been done on bridges, concrete and steel structures and also masonry buildingsbut stochastic processing of fragility curves as an important matter should be evaluatedbecause Seismic vulnerability analysis by means of fragility curves, obviously, needs a probabilistic approach to consider the uncertainties involved in structures and earthquakes characteristics. In this paper is focused on an analytical method for constructing fragility curves of a givenclass of existing structures, based on a stochastic approach.In stochastic analysis the problem is related to the well-known threshold crossings theory connected to the evaluation of structural reliability. In many structural problems the mostused reliability definition is the probability that system will not have a failure associated to a bilateral threshold crossing of a given level in the time interval. Thus, some frames which are most widely used in Iran are evaluated in this research. Thesemodels are analyzed under various earthquake records using the IDARC2D (ver7.0) nonlinear analysis program.To develop fragility curves, a displacement based damage index is adopted.Fragility curves are obtained in terms of probability of exceeding a given damage level by using an approximate crossings theory of stochastic processes and compare with most used probabilistic approach that system will not have a failure associated to a bilateral threshold crossing of a given level in the time interval.

Vulnerability; fragility curve; stochastic process; dynamic analysis