Probabilistic Risk Assessment of Fan Type Cable Stayed Bridges Against Earthquake Forces

A fragility analysis of a fan type cable stayed bridge using a probabilistic risk analysis (PRA) procedure is presented to determine its probability of failure under random ground motion. Seismic input to the bridge support is considered to be a band-limited white noise at the bedrock. 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 the cable—tower—deck system. A continuum method of analysis using dynamic stiffness is used to determine the dynamic properties of the bridges.The responses of the bridge deck are obtained by the response spectrum method of analysis for multiple-degrees-of-freedom system. The fragility analysis includes uncertainties of responses due to the variation in ground motion, material properties, modeling and method of analysis. Uncertainties of the capacity of the structure are considered, such as ductility factor variation and damage concentration effects. Failure of the bridge is assumed to take place when the bridge deck moment at any section within a supported span reaches the yield value. The probability of failure of the bridge deck is determined by the first order second moment (FOSM) reliability method. A three-span double-plane symmetrical cable stayed bridge is used as an illustrative example. The fragility curves for the bridge deck failure are obtained under a number of parametric variations which include ratio of the components of ground motion, spatial correlation of ground motion, angle of incidence of earthquake, ductility ratio and coefficients of variation of the uncertainty factors. The results of the study show that these factors have considerable effect on the probability of failure of the bridge.