Seismic Response Analysis Of Fault-Crossing Suspension Bridges Based On Soil-Structure Interaction

Fault-crossing bridges may suffer more serious damages during an earthquake compared to ordinary bridges,even resulting in the collapse of the bridges.Up to date,a rational seismic design philosophy for bridges crossing active faults has not yet been established in some seismic design codes around the world.Forbiddance of structure crossing fault or keeping a certain avoidance distance from fault rupture zone is outlined in the codes.Due to the rapid development of China’s transportation construction,some bridges may across the fault rupture zones inevitably in the regions with dense traffic networks of active faults.Previous studies mainly focus on the seismic responses of small-span fault-crossing bridges.Therefore,it is of great necessity to investigate the seismic responses of long-span bridges.A long-span suspension bridge is used as the case study in this thesis to carry out the systematic research on the seismic responses of fault-crossing suspension bridges.The main research work and conclusions are listed as following:(1)The baseline correction method for raw ground motion records was utilized to remove the background noise and recover the permanent ground displacement.Ground motions with different target final displacement were obtained by scaling the corrected ground motion records.The response of the overlying soils was analyzed using a 1D equivalent-linear site response program.The results indicate that the overlying soils at the fault have an obvious amplification effect on the bedrock ground motions and cause a difference of the spectral characteristics of ground motions on the two sides of the fault rupture.(2)Four models of soil-structure interaction(SSI),fix base model,linear spring model,nonlinear spring model,and effective pile length model,were modeled respectively using ANSYS and the effect of SSI and the simplification methods on the seismic responses of faultcrossing suspension bridges was explored.Furthermore,the seismic performance of bridge pylons was evaluated by using XTRACT.The results indicate that the bridge pylons are more likely to yield when subjected to across-fault seismic ground motions and considering SSI in analysis of fault-crossing suspension bridges exhibits an opposite trend of the two sides of fault in the internal force demands but both a significant increase in displacement demands of pylons.The SSI model considering soil nonlinearity can better reflect the actual seismic responses of the bridges.(3)An analysis was performed to study the impact of uncertainty in the prediction of parameters of SSI on the seismic responses of fault-crossing suspension bridges,including the radiation damping coefficient and the vibration quality of soil mass.The results indicate that the radiation damping has a great influence on the internal force of the bridge structures.Compared with the undamped model,the nonlinear spring model has a maximum decrease of 21.1% and83.4% respectively in the internal force of the pylons and piles.The increase in the vibration quality of soil mass gives rise to negligible influence on the seismic responses of pylons,but leads to an increase in the internal force demands of piles.(4)The bridge model was reanalyzed using different ground motions to systematically study the influence of pulse periods,permanent ground displacement and fault crossing location on the seismic responses of fault-crossing suspension bridges.The seismic response analysis of suspension bridges crossing reverse fault was carried out to evaluate the influence of the hanging wall effect and it might be of use to researchers in the seismic design of fault-crossing suspension bridges.