Seismic Fragility of Multispan Simply Supported Steel Highway Bridges in New York State. II: Fragility Analysis, Fragility Curves, and Fragility Surfaces

This paper presents the seismic fragility analysis of a typical multispan simply supported steel bridge in New York State. A detailed description of the bridge model including an analysis of parameter uncertainties was provided in the companion paper. The companion paper also describes a sensitivity analysis that was performed to determine the most critical parameters that control the seismic response of the bridge. A set of statistically independent bridge samples and earthquake samples were specified for the fragility analysis. Two alternative seismic retrofit designs were also presented in the companion paper. The results of the seismic fragility analysis performed in this paper based on the data assembled in the companion paper show that typical multispan simply supported steel bridges in New York State have more than 50% probability of exhibiting slight damage when subjected to earthquakes with peak ground accelerations (PGAs) of 0.51 g. A 50% probability of incurring moderate damage is observed for earthquakes with PGA=0.63 g and 50% probabilities of extensive damage and collapse are obtained for earthquakes with PGAs equal to 1.02 and 1.50 g, respectively. The detailed fragility analysis of the as-built bridge shows that the fixed steel bearings in the bridge are the most vulnerable components. Hence, the two most appropriate seismic retrofit measures consist of (i) steel bearing replacement by elastomeric bearings and (ii) deck/girder-splicing (continuity) with steel bearing replacement by elastomeric bearings. The seismic fragility analysis shows that although both retrofit strategies reduce the fragility of bridge piers drastically as compared to the as-built condition, the second retrofit strategy (i.e., the combination of steel bearing replacement and superstructure continuity) is overall more effective in reducing the seismic fragility of both piers and bearings. The results of the fragility analysis developed in this paper for both as-built and retrofitted bridges would help state engineers develop effective strategies for seismic retrofit prioritization and network seismic vulnerability assessment.