Seismic-tsunami fragility analysis for box-girder simple-support bridge with transverse RC constrainers

In recent decades, earthquakes and subsequent tsunamis have increasingly affected coastal bridges. The sequential occurrence of these hazards has frequently led to numerous bridge destructions owing to the separated girder-pier configuration and limited constraints on girders. The failure mechanisms of these bridges are complicated because of the separate configuration and complex loads induced by tsunami waves. Reinforced concrete (RC) constrainers are widely used to provide additional constraints on bridge girders. These components typically exhibit monotonic behavior when subjected to lateral impacts, such as earthquakes and tsunamis, leading to more uncertainties in the failure modes of these bridges. To this end, this study examined the performance of coastal bridges with box-girder under successive seismic-tsunami impacts using the fragility method. A framework was proposed to quantify the fragility of various bridge components subjected to seismic and tsunami effects. Demand and capacity analyses were performed for each event, and capacity parameters were identified for the considered components. The nonlinear analysis was conducted using OpenSees, with sequential seismic-tsunami loads incorporated using natural ground motions and solitary wave theory. Increment dynamic analysis (IDA) was employed to derive fragility curves, with the peak spectrum acceleration and relative wave height selected as the intensity measures for each event. The analysis results indicated that high-strength RC constrainers can effectively protect bridge girders from both seismic and tsunami impacts. The tsunami fragility showed significant sensitivity to the relative wave height. A comparison of the fragility at different water depths showed that a lower water depth could escalate bridge fragility owing to intensified downward tsunami impacts involving significant P−Δ effects. •Limit states were identified to critical bridge components by separately addressing them under seismic and tsunami impacts.•A framework was developed for the fragility of bridges with separated configuration under earthquakes and tsunamis.•Seismic fragility exhibits distinctions for bridge components, while tsunami fragility indicates transient changes.•Tsunami fragility becomes higher at relatively lower water depth due to significant P−Δ effects.•Floating failure is more prone to lower water depths due to significant uplifting wave effects compared to the dominant flow effects at higher water leve