Optimal Intensity Measure Selection and Probabilistic Seismic Demand Model for Vehicles Running on Bridges During Earthquakes

High-speed railways are expanding into seismic zones, where earthquakes severely threaten the safety of vehicles on bridges. Existing seismic studies of high-speed railway bridges mainly address the structural fragility of the overall bridge, piers, and bearings. However, field observations show vehicles often derail before major bridge damage during earthquakes. Fragility analysis based on vehicle derailment as the failure criterion is urgently needed. A core task in fragility analysis of vehicle derailment is to select an optimal ground motion intensity measure (IM). In this study, the dynamic analysis of the vehicle–bridge interaction system under earthquakes was conducted using the incremental dynamic analysis (IDA) method. The characteristics of the IDA curves of the derailment coefficient and the wheel unloading were investigated. Ground motion IMs are classified into time-history-related and spectrum-related categories. The derailment coefficient and wheel unloading are adopted as engineering demand parameters (EDPs). Probabilistic seismic demand models (PSDMs) are established, and 22 candidate IMs are evaluated based on efficiency, practicality, and proficiency criteria. The results revealed that the proficiency of spectrum-related IMs is better than that of time-history-related IMs. The Sa(T1) performs the best among the 22 candidate IMs for the derailment coefficient, and ASI performs the best for the wheel unloading.