IDA-based fragility curves for helical pile-supported bridges in cohesive soil

Fragility curves were developed for a three-span bridge structure supported by helical piles in homogeneous cohesive soil. To address uncertainties in material properties, Latin Hypercube Sampling (LHS) was used, while Incremental Dynamic Analysis (IDA) was employed to construct the seismic demand model. Fifteen bridge samples were subjected to 22 ground motion records, each scaled to 20 intensity levels, resulting in a total of 6600 three-dimensional nonlinear time history analyses. The resulting probabilistic seismic demand model estimated expected damage across a range of seismic intensities, using key engineering demand parameters, pier drift, pile ductility factor, and settlement ratio, to evaluate damage states from slight to complete. Regression results showed that total span length, rebar yield strength, and damping ratio significantly influence pier drift, with longer spans increasing drift while higher rebar strength and damping ratios decrease it. Furthermore, the ductility factor of piles is affected by damping ratio, the number of piles, and foundation area, while damping and pile spacing significantly impact the settlement ratio. Overall, the analysis indicated that helical piles are more vulnerable in terms of ductility than settlement, making them the most critical component in the bridge–soil–foundation system. • Developed fragility curves for a three-span bridge supported by helical piles in cohesive soil. •Conducted 6600 three-dimensional nonlinear time history analyses using Incremental Dynamic Analysis (IDA) and OpenSees, including soil modeling to move beyond spring-like simplifications commonly used in fragility studies. •Observed that longer spans increase pier drift, while higher rebar yield strength and damping ratios reduce it; and found that settlement ratio decreases with higher damping ratios and increased pile spacing. •Identified damping ratio, number of piles, and foundation area as key factors affecting pile ductility factor; highlighted the greater vulnerability of helical piles in ductility compared to settlement under seismic loading. •Determined that piles are the most vulnerable component within the bridge–soil–foundation system.