SEISMIC PERFORMANCE ASSESSMENT OF AN ISOLATED BRIDGE-SOIL-FOUNDATION SYSTEM

Highway bridges are considered to be one of the most susceptible constituents of transportation networks when they are subjected to severe natural hazards such as earthquakes. The serviceability and functionality of these structures are a prerequisite to ensure continuous transport and emergency routes. To facilitate and enhance pre-hazard event mitigation and post-hazard emergency response strategies, probabilistic risk assessment methodologies have attracted increased attention. Seismic fragility assessment is one of the probabilistic techniques which predicts the damage probability of the structure for a given hazard level. However, bridge structures are composed of different vulnerable components that have a significant contribution to the failure probability of a bridge system. While a significant number of studies have focused on assessing the seismic performance of generic bridges, few have focused on developing fragility curves for existing ones. Moreover, the effect of soil-pile-structure interaction is commonly disregarded in order to alleviate the computational burden. This study aims to evaluate the seismic performance of an existing highway bridge located in Ottawa, Canada, and compare it with its rehabilitated counterpart. The employed rehabilitation technique is seismic isolation of the bridge superstructure utilizing Unbonded Fiber Reinforced Elastomeric Isolator (U-FREI), a novel type of elastomeric isolator with beneficial characteristics, including potentially low cost and light-weight. A comprehensive three-dimensional nonlinear finite element model of the bridge including deck, piers, bearings, abutments, and piles is modeled in OpenSees. The effect of Soil-Structure Interaction (SSI) is accounted for using the concept of a beam on a nonlinear Winkler foundation. A ground motion suite consisting of 45 ground motions has been selected, which reflects the seismicity of the bridge site. Incremental Dynamic Analysis (IDA) is conducted to monitor the seismic performance of the bridge from the elastic linear region up to collapse. Fragility curves, which serve as an important decision-support tool have been developed to identify the potential seismic risk of the bridge. Results show that utilizing U-FREI can effectively reduce the seismic demand on the bridge by decoupling the superstructure from the motion of the ground and mitigating the plastic hinge formation in the columns. Also, the importance of incorporating the effect of SSI in developing component fragility curves is highlighted.