Dynamic response and damage collapse of bridge subjected to debris flow impacts using coupled Eulerian Lagrangian method

The coupled Eulerian Lagrangian (CEL) fluid–structure interaction method was employed to study the bridge’s dynamic behaviour under debris flow. The accuracy of the numerical simulations using the CEL method was confirmed by comparing them with small-scale experimental results for debris flow impacting bridge piers. For a real continuous girder bridge susceptible to debris-flow disasters, a detailed finite-element model of the bridge was established, with an emphasis on the precise modelling of hollow thin-walled variable-section piers. Utilising the CEL method, the study explored the effects of debris flow with varying burst frequencies on the bridge. The process of debris flow impacting bridge pier can be summarized into four stages: ‘contact’, ‘flow around’, ‘closure’, and ‘stabilisation’. Prior to reaching the stabilisation stage, the stress near the flow-passing surface at the bottom of the pier gradually increases. The erosion effect of debris flow on the pier was further taken into consideration, distinguishing it from the simplified method of direct impact, as recommended in the standard. The CEL method provides a more precise representation of local damage and structural response in bridges under debris flow impact compared to the simplified method. During the impact under once-in-100-year debris-flow, the lower part of the pier experienced concrete damage and exposed steel bars, yet still maintained normal functionality. The study not only elucidated the effects of debris flow on the flow field distribution, stress, impact, displacement, and overall damage to the bridge but also highlighted vulnerable points within the bridge. The bridge appeared to meet operational requirements for a design lifespan of 100 years, but it may potentially collapse during once-in-200-year debris-flow. Under the once-in-200-year working condition, the weak point at the bottom of the pier, that is, the hollow solid conversion place, is first damaged, and further expands under the impact and erosion of the debris flow, resulting in the complete failure of the foundation support of the pier and the collapse of the bridge.