Dynamic response and vulnerability assessment of mountain bridges under the combined action of seismic and debris flow

Mountain bridges are often required to cross high geologic hazard areas due to their engineering demands, and thus are inevitably affected by multiple geologic hazards such as earthquakes and debris flows, which often cause cumulative damages to the bridges. In order to quantitatively estimate the dynamic response and damage probability of bridges under the combined action of multiple hazards, this paper proposes a method for assessing the structural vulnerability of bridges under the multi-hazard combined action of earthquakes and debris flows by taking a four-span reinforced concrete bridge as the research object. The method first determines the intensity measures of the two hazards, then combines the finite element model to construct the vulnerability curves and surfaces under the combined action of single and multi-hazards, and finally analyzes the influence of the impact load parameters of the debris flow on the vulnerability of the bridge at the same time as revealing the dynamic response and damage probability of the bridge structure under the combined action of multi-hazards. The results show that the damage probability of the bridge under the combined action of earthquake and debris flow is significantly higher than that under the single-hazard action, and the structural vulnerability increases with the peak ground acceleration and the increase of the depth of the debris flow, the velocity, and the radius of the boulders. Under the action of rare earthquake, compared with the action of earthquake only, the probability of complete structural damage increases by 19.69% when the debris flow depth is 8 m, 91.75% when the flow velocity is 10 m/s, and 73.37% when the boulder radius is 0.5 m. The effect of debris flow velocity on the structural dynamic response is the most sensitive, followed by boulder radius and flow depth. This study provides a usable method and reference for the safety performance evaluation of mountain bridges under multi-hazard effects.