Time-Dependent Seismic Fragility of Corroded Bridge Piers Subjected to Sulfate–Chloride Attack Based on an Energy Dissipation Index

This study investigates the time-dependent seismic fragility of reinforced concrete (RC) bridge piers exposed to the harsh saline–alkali environments of Northwest China. A comprehensive analytical framework was developed by integrating quasi-static cyclic tests, nonlinear finite element modeling, and incremental dynamic analysis (IDA). Four pier specimens were subjected to accelerated corrosion in a composite sulfate–chloride solution for up to 90 days. Experimental results reveal a critical threshold: once the actual mass loss of the longitudinal reinforcement reaches approximately 12.05% (corresponding to a stirrup mass loss of approximately 21.45%), the severe loss of core confinement triggers a fundamental failure mode transition from ductile flexural yielding to brittle flexural-shear failure. Traditional displacement-based parameters are fundamentally inadequate for capturing this brittle shift; therefore, the Krätzig hysteretic energy dissipation index was adopted to rigorously quantify structural damage. Subsequently, a time-dependent Probabilistic Seismic Demand Model (PSDM) was constructed, explicitly incorporating the experimentally calibrated reinforcement degradation laws. The fragility analysis demonstrates a distinct biphasic degradation mechanism: while short-term sulfate attack temporarily enhances initial stiffness via a “pore-filling effect,” prolonged composite exposure drastically amplifies seismic vulnerability. Notably, under a severe earthquake intensity of 1.0 g (PGA), the exceedance probability for Severe Damage reaches 50.24% after 90 days of exposure, representing a 2.7-fold increase compared to the uncorroded baseline This research provides a robust, energy-based quantitative methodology for the lifecycle seismic evaluation and maintenance of transport infrastructure in aggressive composite environments.