A response-driven displacement-equivalent pulse-loading method for seismic pounding and fragility assessment of high-speed railway bridges

Seismic pounding between bridge girders and pier-top restraining components can alter the global seismic demand and fragility of high-speed railway (HSR) bridges. Conventional contact-element models require prescribed contact parameters, and their predictions may be sensitive to contact stiffness and numerical integration settings. To address this issue, this study develops a displacement-equivalent pulse-loading method, termed DEPS-Load, for representing pier-top pounding in OpenSees bridge-system analyses. Pounding events are identified from the instantaneous relative displacement and velocity at potential contact locations, and an equivalent rectangular pulse is generated and applied to the corresponding girder-side and pier-side node pair as equal and opposite dynamic loads. The implemented procedure is evaluated using shaking-table tests of a one-span scaled HSR bridge and is benchmarked against a conventional ElasticPPGap-based contact model. For the two pounding cases considered, the errors in peak pier-base moment are below 5%, and the errors in peak pounding force and maximum girder-pier relative displacement remain within approximately 16%. The ElasticPPGap-based model shows strong sensitivity to prescribed contact stiffness, with peak pounding-force errors ranging from −89.32% to 286.08% over the examined stiffness range. DEPS-Load is further applied to the fragility analysis of a representative five-span prototype HSR bridge using 100 ground-motion records. The results show that pier-top pounding increases pier curvature-ductility demand and bearing displacement demand, leading to higher damage probabilities for both individual components and the bridge system, especially for moderate and extensive damage states. The proposed method provides a practical peak-demand-oriented framework for incorporating pounding effects into repeated whole-bridge fragility analyses without solving a high-stiffness contact element at each impact step.