Soil-pile-structure interaction effects on seismic demands and fragility estimates of a typical Ontario highway bridge retrofitted with fiber reinforced elastomeric isolator

This study investigates the seismic performance of a seismically isolated soil-pile-structure system incorporating the effects of isolation system, soil layers of low stiffness and strength, pile inclination, pile-soil-pile interactions, and bridge-embankment interaction. A coupled three-dimensional nonlinear finite element model of an existing bridge in Ottawa, Ontario is developed as a representative of the 44% of the total bridges in Ontario that have been built prior to 1970. A seismic retrofit technique which involves isolating the bridge superstructure utilizing a novel type of elastomeric isolator called Fiber Reinforced Elastomeric Isolator (FREI) is adopted. Two bridge conditions, namely, monolithic (pre-retrofit) and isolated (retrofitted) are considered to investigate the effect of Soil-Structure Interaction (SSI) on the seismic performance of the bridge prior to and after the retrofit in its longitudinal and transverse directions, independently. Analytical fragility curves are developed for the bridge components based on the outputs of an Incremental Dynamic Analysis (IDA) using 45 synthetic ground motion records. It is shown that seismic isolation can effectively reduce the superstructure acceleration by allowing the deck to experience large lateral displacements. Therefore, the transferred shear forces to the columns and their supporting piles are decreased significantly. While seismic isolation of the bridge is found to be beneficial for bridge components, it has a detrimental effect on the extensive and collapse damage states of abutment piles in the longitudinal direction. This conclusion is limited to the conventional abutment models where the abutment backwall damage has not been taken into account. Using more complex physics-based abutment spring systems may reveal that using the seismic isolation systems can mitigate the probability of extensive damage and collapse to abutment piles by allowing the abutment to serve as a fuse and dissipate the imposed energy by its fracture mechanism.