Pier material parameters' impact on seismic fragility of railway simply-supported girder bridges under scour

The structural safety of high-speed railway bridges in mountainous, seismic-prone regions is severely challenged by the concurrent hazards of earthquakes and foundation scour. This study presents a systematic investigation into how three key, design-controllable pier material parameters—concrete strength, longitudinal reinforcement strength, and reinforcement ratio—affect the system-level seismic fragility of a typical simply-supported girder bridge under both scoured and non-scoured conditions. A refined numerical model was developed in OpenSees and validated against dynamic response characteristics. Incremental Dynamic Analysis (IDA) was carried out using a suite of spectrally matched ground motions. A newly proposed "Synergistic Effect Index" quantifies the interaction between improvements in different material parameters. The results clearly identify the reinforcement ratio as the most influential parameter: increasing the reinforcement ratio from 0.5053% to 1.5097% reduces the probability of severe damage at PGA=0.4 g by approximately 25% under non-scoured conditions and 22% under scoured conditions. Although scour consistently increases fragility—reducing median seismic capacity by 16-18% across all material configurations—targeted material optimization still offers significant benefit. Notably, under high-intensity shaking (PGA=0.4 g) with scour present, pairwise combinations of material upgrades exhibit an antagonistic effect (Synergistic Effect Index=0.82-0.85), meaning their combined benefit is less than the sum of individual improvements. From these findings, the study proposes a practical, three-tiered design strategy: prioritizing reinforcement ratio optimization (target range 1.0%-1.3%), rationally upgrading material grades, and integrating mandatory scour protection measures. This integrated approach provides a clear and resilient design pathway for bridges facing combined seismic and scour threats.