On the influence of earthquakes and soil characteristics on seismic response and performance of isolated bridges

Abstract

Seismic isolation technology is an effective means of reducing seismic risk and enhancing the structural seismic performance. However, some parameters, such as earthquake inputs and soil characteristics, affect and mitigate the efficiency of this technology. The main purpose of this study is to investigate the simultaneous effects of different records and flexible soils on isolated bridges. To this end, an isolated bridge is assumed to be at different distances from the ruptured fault (Rrup) in order to represent near-fault (NF) and far-field (FF) situations. These records are extracted on different soils, which are categorized based on their shear velocity, to represent different soil behaviours and characteristics. Nonlinear time history analysis (NTHA) is carried out on a typical isolated bridge model using the SAP2000 software. Responses in terms of deck acceleration, base shear, displacement of the isolation system, and the performance of the isolation units are studied. Results demonstrate that for NF zones, the soil effects must absolutely be taken into account. In soft soils, all seismic responses are amplified, leading to higher forces and displacement demands. In such zones, failing to consider this fact during the design process results in largely underestimated seismic displacement and force demands for the isolated bridge system, seriously harming its seismic performance. Likewise, the amplification of responses in soft soils is observed for FF records and it should be considered for these prone areas. However, despite their importance, these records are not as destructive as NF records for isolated bridges on soft soils.