Influence Of Local Scour On The Seismic Performance Of Cable-stayed Bridge Under Near-field Earthquakes

Earthquake is an inevitable natural disaster,once it happens,it will bring a huge threat to the safety of human life and property.China is located between the Pacific seismic belt and the Eurasian seismic belt in the world’s two major seismic belts.Under the compression of the Pacific plate,the Indian plate and the Philippine Sea plate,the seismic fault zone is very active.Many cities in China are located near the fault or in the seismic zone.With the rapid development of economy and society,the construction of bridge across river and sea becomes necessary.For the cable-stayed bridge built in deep water near fault,in addition to the strong earthquake action,the scouring action of riverbed / sea bed will also weaken the rigidity of the whole bridge,thus further aggravating the risk of bridge under earthquake.This paper studies the influence of local scour on the seismic performance of cable-stayed bridge under the action of near-field earthquake.The research results can provide reference for the seismic analysis,design and possible reinforcement treatment of similar projects.The main research work and conclusions are as follows:(1)First of all,taking a cable-stayed bridge with two towers and single cable plane as an example,SAP2000 software is used to establish a three-dimensional finite element model,in which the pile foundation part uses curve method to simulate the pile-soil interaction.Through the existing classical theoretical formula,the maximum local scour depth of background engineering is calculated,and it is divided into five different scour depths for comparative analysis.The results show that: scour will increase the natural vibration period of the bridge,and increase with the increase of scour depth;the change of scour depth has no obvious impact on the low-order vibration mode,and the impact on the high-order vibration mode mainly lies in the pile foundation;the impact of scour on the quality participation coefficient of vibration mode is mainly reflected in the first-order vibration mode.(2)Secondly,the near-field seismic wave with peak acceleration PGA of 0.3g is used to analyze the nonlinear time history response of the whole bridge analysis model under different scour depths,focusing on the comparative study of the impact of scour depth on the seismic response of the main tower,pier,main beam and pile foundation under the Near-Field Seismic Action.The results show that: because the scour only occurs in the pile foundation under the main tower,the impact of scour on the seismic response of the main tower is significantly greater than that of the pier,and the impact of scour on the bending moment and shear force in the longitudinal direction of the bridge is mostly adverse,while it is partially beneficial to the transverse direction of the bridge;under the near-field earthquake with PGA of 0.3g,the pile foundation is in the linear elastic stage without scour and the maximum scour depth,so the scour is in the linear elastic stage The influence of seismic response of main tower and pier is still small,but the influence on the displacement of main tower and main beam can not be ignored.(3)Then,the PGA of the typical near-field seismic wave is adjusted to 0.1g ～ 1.0g respectively.The incremental dynamic analysis IDA method is used to study the joint effect of the near-field vibration intensity and scour depth on the seismic response of the bridge.The results show that: with the increasing of seismic intensity,the seismic response of the bridge is increasing,and the scour will increase the longitudinal seismic response of the bridge,but the impact on the lateral seismic response is favorable under many working conditions;the adverse impact of scour on the seismic internal force and displacement response of the pile foundation of the main tower can not be ignored,especially at the intersection of different soil layers on the pile side,the larger scour depth will increase significantly The moment response of pile foundation should be paid special attention to in the design,especially under the action of medium strong earthquake.(4)Finally,based on the results of IDA analysis,the seismic performance of cablestayed bridges under different scour depth is evaluated by using the method of capacity demand ratio,and the specific location and damage sequence of various damages of bridges under different intensity earthquakes are studied.The results show that: under the increasing earthquake intensity,with the increase of scour depth,the seismic surplus of each key section of the bridge decreases;the increase of scour depth mainly reduces the bending surplus at the bottom of the tower,but there is almost no damage at the bottom of the tower;the impact of scour on the bending capacity of the pier is mainly in the level I damage state,and the greater the scour depth is,the earlier the pier will enter the yield state,However,the influence of scour on the seismic performance of pile foundation is relatively weak,especially in the case of level I damage,the greater the scour depth is,the more prone the longitudinal bridge direction of pile foundation to bending damage.