R. A. Khan - Research Scholar, Deptt. of Applied Mechanics, I.I.T.- Delhi, Hauz Khas, New Delhi-۱۶, India.
T.K. Datta - Professor, Deptt. Of Civil Engineering, I.I.T.- Delhi, Hauz Khas, New Delhi-۱۶, India.
S. Ahmad - Professor, Deptt. of Applied Mechanics, I.I.T.- Delhi, Hauz Khas, New Delhi-۱۶, India.

A fragility analysis of cable stayed bridge using Probabilistic Risk Analysis (PRA) procedure is presented. Seismic input to the bridge support is considered to be a response spectrum with a specified correlation function. The response spectrum at the free field is generated from the time history of ground acceleration which is obtained using one dimensional wave propagation analysis of the soil layer with white noise input at the bedrock. The response of the bridge deck is obtained by the response spectrum method of analysis for multi-degree of freedom system as proposed by Der Kiureghian. For the response analysis, the bridge deck is modeled as a beam supported on springs at different points. The stiffnesses of the springs are determined by a separate 2D static analysis of cable-tower-deck system. The analysis provides a coupled stiffness matrix for the spring system. The total dynamic response of the bridge deck is taken to be consisting of two components namely, the relative dynamic response and the quasi-static response at various points of the bridge deck. The PRA procedure includes uncertainties of response due to the variation in ground motion, material property,modeling and method of analysis. Uncertainties of the capacity of the structure are onsidered as the variation of ductility factor and damage concentration effect. Failure mode of the bridge is assumed to be bending failure of the bridge deck at the point of maximum bending moment. Assuming the variability factors to be log-normally distributed, probability of failure of the bridge deck is determined by First Order Second Moment (FOSM) reliability method. Probability of failure of the bridge deck for different values of the peak ground acceleration is plotted to obtain the fragility curves. A three span double plane symmetrical cable stayed bridge of total span 689.0 m, is used as an illustrative example. The fragility curves for the bridge deck failure are obtained under a number of parametric variations