Seismic-tsunami risk assessment for isolated coastal bridges using nested Copula method

Coastal bridges are highly endangered by the joint actions of earthquake and ensuing tsunami hazards, leading to the necessity to evaluate their potential risks. This study proposed a probabilistic method to assess the risk of isolated bridges under such scenarios. The proposed framework consists of joint probability estimation, component- and system-level fragility calculation, and loss assessment. The nested Copula method was employed to estimate the multi-variable joint probability of seismic and tsunami hazards, with the vector-valued fragility function used to account for the double intensity measures of tsunami hazards. The failure mode of girder floating, defined as its uplifting and swashing away, was considered for estimating the systematic fragility. The potential losses induced by earthquake and tsunami were assessed via a surrogate model method. Such framework was applied to a prototype bridge, with various seismic-tsunami correaltions incorporated by Kendall's factors to address their influences on hazard risks. It was found that the loss increases with seismic-tsunami correlations when they are relatively low but turns to decrease at high correlation levels. In addition, higher correlation between the double tsunami intensities results in larger losses in the weakly correlated seismic-tsunami events, whereas they also tend to undermine the risk levels as the seismic-tsunami correlation rises.