The performance of two real-life California bridges is assessed under a possible regional multihazard condition involving floods and earthquakes. For flood events with varied frequency, expected scour depths at bridge piers are calculated and incorporated in finite-element analyses (FEAs) of the bridges under earthquakes that represent regional seismic hazards. Based on FEA results, fragility curves of bridges are developed at component and system levels. Fragility surfaces are generated to acquire comprehensive knowledge on bridge failure probability for the combined effect of earthquake and flood events of varying frequency. Quantified bridge vulnerability is applied to a risk evaluation framework that combines hazard probability with bridge failure consequences. Obtained results depict that the occurrence of flood events can increase the seismic vulnerability and risk of bridges, although the amount of increase depends on bridge attributes. Bigger (large-diameter) foundations tend to reduce the impact of flood hazard on bridge seismic performance. Moreover, the multihazard response of the bridges indicates that the seismic design philosophy used for bridges can play a role in attaining the seismic safety of bridges with flood-induced scour at the foundations.NS
