Seismic Fragility Assessment of Shallow‐Foundation‐Supported Bridge Piers Subjected to Surge Wave‐Induced Scour

This study presents an integrated numerical framework for evaluating the seismic fragility of bridge piers subjected to surge wave‐induced scour. The methodology couples three‐dimensional hydrodynamic simulations of scour development with nonlinear finite element analyses of the pier‐foundation‐soil system, incorporating continuum soil representation and soil–structure interaction. Two shallow‐foundation configurations, representing uplift‐dominated and sinking‐dominated rocking behavior, are examined through quasi‐static and dynamic analyses, with damage‐state thresholds derived using principles of structural mechanics from pushover and pushdown results. Nonlinear time‐history analyses of the dynamic response of the deck‐pier‐scoured foundation assembly were conducted under 45 code‐compatible earthquake records. Results were used to develop drift‐ and settlement‐based fragility curves for scoured and unscoured conditions. It was found that scour reduces lateral capacity and stiffness in both systems, with greater degradation in the sinking‐dominated configuration, while vertical capacity loss is more pronounced in the uplift‐dominated system. Settlement‐based seismic fragility assessment shows that scour has negligible effect on the uplift‐dominated system but substantially increases settlement vulnerability for the sinking‐dominated system. Drift‐based fragility curves shifted markedly toward lower spectral accelerations after scouring, demonstrating that the governing damage mechanism depends strongly on the soil–foundation interaction mode. The proposed framework provides a mechanics‐based approach for multi‐parameter seismic fragility assessment of bridge foundations subjected to combined flood and earthquake hazards.