Earthquake Risk Assessment of Reinforced Concrete Bridges in Washington State Using Pushover Analysis

Abstract

The average age of reinforced concrete bridges in the state of Washington is 48 years, encompassing more than 100 years of design and construction techniques, which reflect evolving views on seismic risk and mitigation. As such, there is great uncertainty as to the actual seismic resistance of existing bridges. Having a simple and accurate process for assessing the seismic vulnerability of a bridge can help identify vulnerable design details and address them, either through retrofit or replacement. Using pushover analysis to determine the seismic capacity of structures can give an idea of the overall vulnerability of the structure while decreasing the computational power needed to perform analyses as compared to nonlinear time history (NLTH) analysis. Using the finite element program RUAUMOKO-2D, models of existing WSDOT bridges were created and pushover analyses were run. By defining damage states of interest and determining the associated displacement profiles, peak ground accelerations (PGAs) of seismic events were then correlated to the damage states. By combining fragility curves encompassing the probabilistic distribution of the PGA at which a damage state might occur with site hazard risks, Risk Indices were calculated for all bridges considered. The bridges were ranked in order to contextualize the Risk Index values. The results of the assessment process were then verified by comparing them to the results of NLTH analysis.