On the influence of multi-hazard correlation to bridge performance: Focusing on cascading earthquake and tsunamis

Earthquake and subsequent tsunami hazards pose serious threats to coastal bridges due to their cascading effects. This study examines the influence of their different correlation levels on isolated bridges. Hazard uncertainties are modeled via nested Copula method, and fragility assessed through vector-based functions, with a sampling-based method to assess bridge failure risk. A case study was conducted by considering multiple uncertainty sources including seismic through record-to-record variability, tsunami using hydraulic coefficient and layout-dependent factors, as well as structure in terms of geometry, mechanical, and dynamic properties. Results show distinct fragility patterns of isolated bridges compared to moment-resisting ones, where pier and girder display rise-fall trends as seismic intensity grows. System-level fragility is more sensitive to spectral acceleration ( S a ) and relative wave height ( H / d ) than to water depths, but remains consistent under higher tsunami levels. Vulnerability analysis indicates that increasing water depth and wave height can induce greater damage ratios, while the influence of spectrum level further depends on H / d values. Comparison across correlation levels reveals that failure risk increases proportionally under negative correlation but decreases for positive correlations, highlighting the necessity of incorporating joint hazard correlations for risk assessment. Further, correlation levels only affect post-cascading damage states, with negligible impacts on the damage state after seismic hazards alone. The presented study provides extensive guidance to the risk evaluation of coastal bridges under different correlations of cascading hazard events.