A time-dependent multi-hazard framework for capacity-based assessment of bridges under hydraulic and seismic processes

This study presents a time-dependent multi-hazard evaluation framework for the Águila Norte Bridge in Maipo Province, Chile, that explicitly integrates unsteady hydraulic processes and seismic loading. Current bridge assessments commonly treat scour as a static or scenario-based condition, neglecting its temporal evolution and its interaction with structural response, thereby limiting the evaluation of structural capacity under interacting hazards. To overcome this limitation, the proposed framework integrates hydrological and hydraulic modelling with a time-dependent scour formulation based on an entropy-driven approach, in which erosion is governed by the cumulative hydraulic work exerted by unsteady flow conditions. Sediment redeposition within the scour hole is modelled using a complementary framework that enables simulation of erosion–redeposition cycles. The resulting scour time series captures the progressive evolution of the riverbed and is incorporated into a three-dimensional nonlinear finite-element model of the bridge, accounting for seismic loading, hydrodynamic drag forces, and partial soil reconsolidation. Structural response is evaluated through nonlinear seismic analysis, i.e., Nonlinear Static Pushover Analysis (NSPA) and Nonlinear Time-History Response Analysis (NTHRA), by examining displacement demands and the bridge's global structural behaviour under time-evolving scour conditions. The combined effects of hydraulic degradation and seismic loading are quantified using Engineering Demand Parameters (EDPs), including displacement-based response measures at abutments, elastomeric bearings, columns, and piles. These EDPs are subsequently used to evaluate damage states (DS), enabling a consistent assessment of how evolving hazard conditions translate into a progressive reduction of structural capacity. Preliminary results indicate that increasing scour depth leads to larger lateral displacements and a significant decrease (i.e., about ≈ 50%) in the lateral load-carrying capacity required to reach a moderate DS, reflecting a progressive degradation of structural capacity. Overall, this work provides a computationally explicit multi-hazard framework for capacity-based assessment of bridge structures under interacting hydraulic and seismic processes. The proposed approach provides a basis for supporting disaster risk reduction (DRR) strategies by improving understanding of how evolving hazards affect structural capacity, without requiring a full probabilistic risk formulation.