System Fragility Assessment of Tall-Pier Bridges Subjected to Near-Fault Ground Motions

Abstract

Numerous bridges with piers over 40 m are constructed in Southwest China, which is known as a region of high seismicity. In current research, this type of bridge is commonly simplified as a single-column system, and researchers mainly focused on how seismic performance is affected by the higher modes of columns. This study aims to investigate the seismic behavior of the entire system of tall-pier bridges subjected to near-fault ground motions, using probability-based fragility analysis. A numerical model of the entire bridge system is constructed incorporating the effects of higher-order modes of pier columns and the influence of various structural components as bearings, shear keys, and end abutments. Both pier columns and rubber bearings are considered as vulnerable components during fragility analysis. With probability seismic demand models (PSDMs) developed from extensive nonlinear time history analyses, both component-level and system-level fragility curves of the prototype bridge are constructed and compared. The results show that bridge performance is usually dominated by rubber bearings, rather than pier columns, and the relative vulnerability of components might change with the intensity of input motions. This fact implies that only focusing on the seismic demands of pier columns, as is usual in current investigations, is insufficient in estimating the damage states of tall-pier bridges; failure of bearings should be carefully considered in engineering practice as well. Furthermore, compared with fragility curves of individual components, higher seismic vulnerability is observed in those corresponding to the entire system, indicating that the seismic fragility of tall-pier bridges should be assessed at the system level rather than component level.