Influence Of Input Direction Of Near-fault Ground Motions On Seismic Response Of Rigid Frame Bridges

The faults and seismic belts are widely distributed in Western China,and the underground structures are active and the seismic risk is high.Since the seismic fault stretches for hundreds of kilometers or even thousands of kilometers,and bridges cannot be avoided to be constructed near or through the earthquake faults in the western traffic.Due to the obvious forward directivity effect of near fault ground motions,there will be obvious large pulse in the direction of vertical fault,and the seismic damages are significant.Continuous rigid frame bridge is a kind of bridge type widely adopted in the west mountainous area,and the trend of the bridge is not always parallel or vertical.Therefore,the influence of the angle between the continuous rigid frame bridge and the fault strike on the seismic response of the bridge is studied in this paper.The main work is as follows:(1)The near fault ground motion and its characteristics are introduced.The research status of bridge responses under near fault ground motions is analyzed from the aspects of bridge vulnerability analysis,seismic isolation design and seismic input direction effect.(2)The Midas Civil software is used to establish the calculation model of long span continuous rigid frame bridge with high piers of 126m,58m and 22m,respectively,and the natural vibration characteristics of the structure are analyzed.(3)The near fault seismic waves are selected from the PEER database and rotated to be the seismic input.The seismic responses of continuous rigid frame bridges subjected to bidirectional horizontal seismic excitation are compared when the angles between the longitudinal direction of the bridge and the fault strike are different.The result shows that the maximum response in the longitudinal direction of bridge is easy to occur between the angle of 60 degree to 120 degree,and the maximum response in the transverse direction of bridge is easy to occur between the angle of 0 degree to degree or between 150 and 180 degrees.According to the numerical results,the above conclusions are affected by the height of the bridge pier and the influence in the transverse direction of the bridge is greater than that in the longitudinal direction.(4)The effects of near fault vertical ground motions on the seismic responses of continuous rigid frame bridges are studied.By comparing with the numerical results that only considers the bidirectional horizontal seismic input,it can be concluded that:the effect of vertical earthquake component on the longitudinal seismic response of bridge pier is very limited,with an increase of less than 10%;but the vertical bending moment responses of girders increase caused by considering vertical earthquake component,the amplification coefficient could be 2-3 times or even more,especially at the junction of the side span and main pier.