Taking the long-span three-pylon cable-stayed bridge with the main span of 720m in the sea-crossing project as the object, the influence of the different scour depths on the seismic responses of the structure was analyzed. The research shows that with the increase of the scour depth, the overall stiffness of the structure and the frequency of the main mode shape gradually decrease, but the change is small; the scour of pile foundation has the greatest impact on the force of the pile foundation, and has a greater impact on the internal force of the bottom of pylon; The pile foundation scour depth has different influence rules on different parts of the structure, and the seismic effect does not necessarily increase with the increase of scour depth.
The bridge foundation is directly affected by water scour. Scour leads to the exposure of the foundation of the bridge pier or the reduction of the soil covering, which leads to weakening of the restriction of the soil on the structure. This will lead to a reduction in structural rigidity, which will definitely affect the dynamic characteristics and seismic capacity of the bridge structure, especially for flexible structures like cable bridge. The scour disease of the basic structure is one of the most important reasons for the failure of the function of the bridge structure and the loss of its safety performance. From 1966 to 2005, the collapsed bridges in the United States caused by the scour of the bridge infrastructure accounted for 60% of the total collapsed bridges(Kattell and Eriksson,1998),and the scour has made a quarter of the bridges in the United States unsafe. In the coastal waters of my country, the scour of the foundation of cross-sea bridges is one of the important factors affecting the safety of the bridge (Yang, Zhang and Li, 2021). Therefore, it is particularly important to study the seismic performance of structures under the combined action of scour and earthquake.
The general scour depth is usually less than the local scour depth. Melville and Coleman (2000) believe that the general scour depth is only about 10% of the local scour depth. China's "Highway Engineering Hydrological Survey and Design Code" stipulates the general scour calculation formula of the bridge foundation. Chen and Li.(2019) predicted the general scour of the bridge after the change of water and sediment in the lower reaches of the Yangtze River, and the results are closer to the actual measurement. Ye and Zhang (2007) conducted a preliminary exploratory study on the influence of changes in riverbed scour depth on the seismic response of large piled bridges, and the results showed that changes in riverbed scour depth had a great impact on the seismic response of large piled bridges, with the increase in scour depth, the seismic response of the bridge structure is not monotonously decreasing, but a peak may appear. Yang and Ma (2015) studied the effect of scour on the seismic response of long-span self-anchored suspension bridge pylons, and provided a reference for the seismic design of such bridges under scour conditions. Liang and Liu (2017) used probabilistic seismic demand analysis methods to establish seismic vulnerability models of bridge pile foundations under different scour depths. The results showed that with the increase of scour depth, the failure probability of bridge pile foundations under earthquake increased significantly. Li and Gong (2019) studied the influence of pile foundation scour on the seismic response of offshore long-span cable-stayed bridges, the adverse effect of scour on the bridge increases with the increase in the intensity of ground motions, the local scour of the pile foundation will further aggravate the danger of the bridge under the action of earthquake. Chen (2019) took the PC cable-stayed bridge as the research object, and showed that scour has a certain influence on the seismic response of the key section bending moment of the main pylon. None of the above studies involved the three- pylon cable-stayed bridge.
