Mountainous Valley Trerrain Effect On Seismic Response Of Long-span Bridges With High Piers

The mountainous terrain is changeable,especially in the valley site.The bridge design across the mountain valley often chooses the structure form of long span and combination of high and low piers.The difference of ground motion at the foundation of each pier caused by seismic traveling wave effect and terrain effect will make the structural response of this kind of bridge become very complex.However,the seismic wave propagation from different directions and the valley topography with different shapes have differences on the ground motion at the pier bottom foundation,which leads to the bridge structure response becoming more complex.Therefore,it is necessary to analyze the difference of seismic response of high pier and long-span bridges.This paper introduces the characteristics of earthquake disaster of bridge engineering in China,summarizes the forms of bridge seismic damage,briefly reviews the influence of seismic traveling wave effect and terrain effect on the site ground motion characteristics and seismic response of bridge structure,as well as the research results of multi-point excitation seismic response analysis method of high pier and long-span bridge.The research purpose and content of this paper are put forward.Based on the symmetrical and asymmetric valley terrain and the corresponding high pier and long-span concrete continuous rigid frame bridge structure model,the influence of seismic traveling wave effect and terrain effect on bridge structure response characteristics is studied.(1)In this paper,the symmetrical and asymmetric two-dimensional valley terrain sites are taken as the research objects,and the two viscoelastic site terrain finite element calculation models are established.The explicit finite element finite difference method is used to calculate and analyze the seismic response of the site terrain,so as to improve the ground motion input for the subsequent Research on the seismic impact of high pier and long-span bridge structure.In this paper,two strong earthquake records el with different spectral characteristics are considered Centro seismic records and Wolong seismic records are used as seismic incident waves,and three incident angles of 0 °,30 ° and 60 ° are selected to consider the oblique incidence effect of seismic waves.Then,the ground motion acceleration time histories of six different observation points(pier foundation position)on the ground surface of the site terrain model are calculated,and the variation characteristics and laws of ground motion of the two sites are obtained through comparative analysis,including the change of peak ground motion acceleration PGA.(2)In this paper,the structural model of high pier and long-span concrete continuous rigid frame bridge is constructed.Considering the symmetrical and asymmetric structure models of pier height and the seismic input of different spectrum characteristics,the large mass method is used to carry out the numerical simulation of bridge structure response.The effects of traveling wave effect and both traveling wave effect and terrain effect on the structural response of symmetric and asymmetric high pier long-span bridges are calculated respectively,and the sensitivity of symmetric and asymmetric bridge structures to traveling wave effect and terrain effect is compared.The results show that the seismic traveling wave effect and terrain effect have significant influence on the seismic response of the bridge structure,which leads to the asymmetry of the response of the symmetrical structure bridge,and increases the complexity of the response of the asymmetric structure bridge;The seismic traveling wave effect and terrain effect lead to the uneven distribution of the internal force of the bridge structure response,which is closely related to the incident direction of the seismic wave;he shear force of the middle span response of the high pier in the main beam response of the bridge is most affected,while the maximum shear force and bending moment of the high pier bottom in the middle of the bridge pier response are greatly affected.The influence of nonuniform ground motion input should be fully considered in seismic design of high pier and longspan bridge across river valley,and enough attention should be paid to the influence of seismic wave incident direction and its uncertainty.(3)In this paper,the trend analysis of seismic response of main joints of bridge structure considering only traveling wave effect and terrain effect is carried out.When considering the terrain effect,the incident angle of seismic wave has an important influence on the influence of terrain effect on the seismic response of bridge structure.The seismic response of bridge structure under vertical input is more sensitive to the influence of terrain effect.Traveling wave effect and terrain effect alone can not completely control the most unfavorable response of the structure.It is found that the influence of traveling wave effect and terrain effect on the seismic response of main beam joints is quite different;the incident angle of seismic wave has great influence on the seismic response of bridge structure considering traveling wave effect and terrain effect;In the middle span of high pier of bridge girder,the increase range of shear force considering terrain effect is obviously greater than that considering only traveling wave effect.In engineering practice,shear force variation in main span of main girder should gain more attention;hen considering terrain effect,seismic response of each node is more complex than that considering only traveling wave effect.The input direction of seismic wave should be further considered when considering the influence of terrain effect on seismic response of bridge structure.