Performance of vector-valued fragility for coastal bridge under earthquake and tsunami hazards

Extreme hazards such as earthquake and ensuing tsunamis can pose significant threats to offshore infrastructures, among which bridges are particularly vulnerable due to their locations. Accurate assessment of bridge performance under such events is crucial to enhance structural safety. In this study, the fragility method was employed to evaluate bridge capability against combined hazard effects, with three variables introduced to capture multi-hazard intensity. The vector-valued method was used to quantify bivariate tsunami intensities, with different fragility functions compared in their fitting capability. A new fragility form was proposed for earthquake-tsunami scenarios, with the system-level fragility also examined via multiple bridge components. A case study was conducted to compare the effectiveness of various functions to isolated bridges. The component-level fragility shows an inconsistent development with increasing seismic magnitudes but consistent trends with tsunami intensity. The comparison analysis implies the highest fitness of log-sum model, while the proposed method yields consistent outcomes despite the unified factor. System-level fragility results indicate that isolated bridges have notable vulnerability due to multi-component contributions. Further, the expected damage ratio was assessed and shows notable sensitivity to spectral acceleration and relative wave height, as opposed to the limited influences from water depths. This study provides preliminary guidance for estimating the seismic-tsunami fragility of isolated bridges using complex intensity sets. • Three-variable intensity measure was introduced for random sampling and fragility analysis. • Bivariate tsunami intensity was used for the joint effects of water depth and wave height. • Vector-valued functions were compared in terms of seismic-tsunami fragility fitting capability. • System-level fragility was assessed by combining SOLM with multi-mode failures of girders. • Log-sum model has the best fitting performance in seismic-tsunami hazard fragility.