Fragility curves for isolated bridges in eastern Canada using experimental results

One option to mitigate the seismic risk of highway bridges in Quebec is to replace typical elastomeric bearings used there with natural-rubber seismic-isolator devices. To support this alternative, this paper assesses the seismic vulnerability of typical bridge classes retrofitted with seismic-isolator devices through the development of fragility curves. Retrofitted-bridge fragility curves provide a powerful tool to evaluate the impact of a retrofit measure on the performance of different bridge classes. The analytical fragility approach uses nonlinear time-history analysis with 3-D detailed models for typical configurations of highway bridges. Experimental results of square bearings with different sizes and shape factors are used to account for uncertainties related to the mechanical properties of seismic-isolators. Critical load tests are conducted on slender seismic isolation bearings and a finite-element model is calibrated with the test results to define the seismic-isolator limit states. The fragility curves of different key components of the bridge system are compared and the results reveal that seismic isolation is effective significantly in reducing the probability of damage to columns and foundations. Due to insufficient clearance between the superstructure and abutment wing walls, however, the probability of damage in wing walls is increased and the fragility of this component controls the bridge-system fragility for all bridge classes evaluated. Concrete-girder bridges are found to be more vulnerable than steel-girder bridges due to the larger superstructure mass involved in the seismic response. The results from this work can be used to evaluate and select bridge retrofits, and can form the basis for cost-benefit retrofit studies.