SEISMIC FRAGILITY ASSESSMENT OF ISOLATED BRIDGES IN COLD REGIONS: A CASE STUDY FOR EASTERN CANADA

Seismic isolation systems are typically used in bridge structures to mitigate the damage risk of bridge components against natural hazards including earthquakes. The objective of base isolation is to provide an intermediate layer with high vertical stiffness to accommodate the gravitational forces and low horizontal stiffness to decouple the superstructure from the motion of the ground. Seismic isolation of bridges is usually achieved through utilizing elastomeric isolators made by alternating layers of elastomeric material (e.g. neoprene, natural rubber, high damping rubber) and reinforcing material. Steel Reinforced Elastomeric Isolators (SREI), which use steel shims as their reinforcement layer, are the most commonly used type of isolators whereas recent research has revealed superior seismic behavior of Fiber Reinforced Elastomeric Isolators (FREI), (e.g. reduced manufacturing costs, and lower weights) which use fiber fabrics as their reinforcement layer, instead. The structural response of isolated systems is highly dependent on the mechanical properties of the isolating layer. However, ambient conditions, such as temperature can affect the properties of the isolation layer and the overall structural response, consequently. Rubber, as the main constituent of the elastomeric layer, is highly sensitive to negative temperatures and related duration of exposure. At low temperatures, rubber undergoes two successive phases of stiffening, that is, instantaneous thermal stiffening and crystallization, which substantially afflicts the horizontal behavior of isolators. The objective of this study is to quantify the seismic performance of an existing bridge in Ottawa, Canada where the historical mean daily temperature fluctuates in the range of -35°C to +31°C. The bridge is seismically isolated by means of FREI and its performance is evaluated both in the room and subfreezing temperatures. A comprehensive three-dimensional model of the bridge is modeled in OpenSees. A ground motion suite containing 45 synthetic ground motions generated for eastern Canada corresponding to the soil type of the bridge site is selected. Incremental Dynamic Analysis (IDA) is performed to monitor the seismic performance of the bridge and its components including the deck, columns, isolators, abutments, and backfill soil from the linear region up to collapse. Fragility curves are developed in order to compare the probability of damage for bridge components under different thermal conditions and earthquake intensities. Results show that utilizing FREI as the isolation system can effectively reduce the force demand in the substructure, deck acceleration, and column drifts. However, the stiffening occurred in the isolators as a result of crystallization decreases the efficiency of the isolation system resulting in higher demand on the bridge components.