Synergistic dynamic response of long-span ultrahigh-pier rigid-frame bridges under combined near-fault earthquake and rockfall impact

Long-span rigid-frame bridges carried on ultrahigh piers are increasingly built through high-intensity near-fault mountain corridors, where they face near-fault earthquakes and rockfall impact at the same time. The two hazards are almost always studied in isolation, and design codes treat them separately. This paper examines how such a bridge behaves when the two act together, taking a five-span continuous rigid-frame bridge with a hollow reinforced-concrete ultrahigh pier as the engineering prototype. A three-dimensional explicit finite-element model was built in LS-DYNA, with the Continuous Surface Cap Model (CSCM) used for the pier concrete. The bridge was subjected to a representative rockfall impact, with kinematics obtained from a probabilistic Monte Carlo trajectory analysis, acting concurrently with each of a suite of high-intensity near-fault ground-motion records spanning a wide range of pulse periods. Under the combined hazard the peak pier-top displacement is markedly larger than under either hazard alone, and the algebraic sum of the two single-hazard responses underestimates it appreciably, showing that simply adding the separate demands is unsafe for global displacement and that the interaction is genuinely synergistic. The rockfall contact force and the localized concrete loss at the impact zone stay almost unchanged from record to record, governed by the rockfall's own kinematics, whereas the pier-top displacement varies strongly with the spectral content of the motion. Local damage and global displacement are therefore governed by different hazards, a distinction with direct consequences for the multi-hazard design of ultrahigh-pier bridges in near-fault mountain regions.