Analysis of the bridge failure direction and the increased probability of collapse due to multi-hazard effects: scour and earthquakes

Abstract

The southwestern region of Mexico is highly susceptible to the concurrent threats of river scour from frequent hurricanes and intense seismic activity. This study aims to evaluate the risks to simply supported reinforced concrete bridges from scour and earthquakes, incorporating the uncertainties of both natural hazards within a probabilistic framework. While multi-hazard risks to infrastructure are widely recognized, existing joint scour-earthquake frameworks typically use seismic records from transform faults. These cannot be reliably extrapolated to regions governed by interplate and intraplate subduction zones, which exhibit distinct frequency contents and energy radiation patterns. To bridge this gap, this study presents a probabilistic multi-hazard framework evaluating the system reliability of typical medium-span, simply supported reinforced concrete (RC) bridges along Mexico’s Pacific Coast. A Multi-Hazard Probabilistic Seismic Demand Model (MH-PSDM) was developed to generate component and system fragility surfaces based on curvature ductility demands under various scour depths. The findings reveal that combined hazards significantly modify the structural failure mechanism, driving a critical transition of damage from pier columns to the foundation piles as scour depth increases. The manuscript emphasizes that a bridge’s likelihood of collapsing is heavily influenced by the relationship between pier height and scour depth. Specifically, bridges with a low height of 4 m are particularly vulnerable to longitudinal collapse under extreme conditions. In contrast, taller bridges, measuring 12 m, are more prone to transverse collapse. Under maximum scour conditions, the reliability indices of the system decreased significantly across all scenarios, from a baseline range of 2.9 to 3.5 without scour to 1.9 to 3.1 with maximum scour depth. The study also shows that tall-pier bridges, unlike short-pier bridges, exhibit less pronounced reductions in the reliability index with increasing scour depth. The results show significant regional differences in vulnerability, with bridges in Oaxaca being the most critically affected. This indicates that macro-level seismic zonations need to be finely detailed to effectively address localized scour hazards. Ultimately, the quantified reliability indices provide a solid basis for adjusting multi-hazard load factors and prioritizing bridge maintenance programs across regions.