Seismic fragility analysis of bridge-isolator-foundation-soil systems in subfreezing temperatures

Several cold regions around the world are in seismic active zones. Seasonal freezing can significantly affect the seismic performance of complex bridge-isolator-foundation-soil systems by inducing brittle failure and premature collapse due to a combination of load reversals and cold temperature behavior of the bridge material. In cold conditions, constitutive material of bridge components, namely, concrete, steel reinforcement, rubber, and supporting soil undergo substantial stiffening which afflicts the ductility capacity of bridges. This study investigates the seismic performance of four bridge cases, namely, isolated bridge supported by pile groups, monolithic bridge supported by pile groups, isolated fixed-base bridge, and monolithic fixed-base bridge at room and subfreezing temperatures to unmask the effectiveness of using Fiber Reinforced Elastomeric Isolator (FREI) as a retrofit measure and to evaluate the performance of bridges at temperatures ranging from room (i.e. 20 degrees C) to -37 degrees C. To this end, an analysis matrix is developed based on the climatic condition of the bridge site along with the code provisions. A seismic fragility analysis is carried out in the context of Incremental Dynamic Analysis (IDA) using a set of synthetic ground motion records for longitudinal and transverse directions of the bridge, independently. It is shown that while the seismic isolation system can effectively mitigate the probability of damage to the bridge at room temperatures, isolated bridges demonstrated an acceptable behavior at subfreezing temperatures.