Dynamic Response and Damage Assessment of Bridge Structures under Coupled Local Scour and Ship Collision

The local scour of pile foundations alters the dynamic boundaries and mechanical modes of bridges, and its coupling with ship collision exacerbates the risk of structural failure. However, existing research remains insufficient regarding the frequency-domain energy evolution characteristics and the impact effects on the superstructure. To address this, a systematic numerical analysis framework is proposed. First, a finite element model of ship-bridge collision is established using LS-DYNA, where nonlinear soil springs and the p-multiplier method are employed to simulate pile-soil boundary conditions and pile group effects. Second, the coupled influences of ship tonnage, impact velocity, impact height, and scour depth are systematically analyzed. Finally, the damage evolution mechanism is revealed from a time-frequency dual domain by integrating the wavelet packet analysis and displacement ductility ratio indices. Results indicate that scour weakens the dissipation capacity of high-frequency energy, while high-level impacts promote energy concentration in low-frequency bands. The structure exhibits greater sensitivity to impact velocity than to ship tonnage. Under conditions of deep scour and high-level impact, the pier displays significant residual displacement and unidirectional accumulation behavior, indicating the formation of irreversible plastic hinges. This physical damage phenomenon is corroborated by the abnormal aggregation of low-frequency energy in the frequency domain. Based on these findings, it is suggested that strict speed restrictions should be imposed in scour-prone waters, and that collision protection measures should be selected with consideration of impact energy.