Scour-Dependent Fragility of Railway Bridges: From Component Response to System Reliability Under Seismic Loading

Flood-induced scour and earthquake loading jointly govern the seismic performance of river-crossing bridges. Existing conditional fragility assessment frameworks based on static dependence structures do not fully capture the evolving correlations between component failure modes under cumulative hydraulic degradation. This study develops a probabilistic conditional fragility assessment framework for continuous bridges and quantifies the scour-dependent fragility at both the bearing and pier levels, along with the resulting system fragility under series and parallel idealisations. A three-dimensional nonlinear finite element model with scour-dependent soil–structure interaction is constructed in OpenSees, and incremental dynamic analysis is conducted using spectrally compatible ground motions. The results indicate that scour primarily affects the bearing fragility in the moderate to complete regimes, whereas it has a negligible influence on the bearing under minor damage conditions. Unlike bearings, the fragility of piers decreases systematically toward lower PGA values with increasing scour depth, accompanied by a distinct threshold-like sensitivity shift within a specific scour depth range. At the system level, the series model is influenced by the early exceedance probability of the bearings at low PGA, whereas the parallel model is primarily governed by the exceedance probability of the piers at high PGA. Overall, the results demonstrate that scour affects system reliability not only by altering the PGA of the structural components but also by modifying the exceedance probability gap between the bearing and pier. These findings suggest that linear degradation-based management approaches can lead to biases in risk assessment and provide a practical extension and scientific basis for developing bridge system assessments under multi-hazard conditions.