Seismic Response Of Multispan Irregular Bridges In Mountain Area And Optimization Of Bearing Stiffness

Mountain bridges usually have high piers and the height of piers is quite different because of being limited by terrain.Using Simply-Supported or short unit could lead to uneven distribution of internal force among high and low piers,pounding of beam on each side of the expansion joint and poor integrity of bridge.All of these are adverse to resisting earthquake.In order to improve the seismic behavior of mountain irregular bridges,a method of using long unit and adjusting bearing stiffness was proposed in this paper.The main research contents include:(1)The influence of long or short unit on seismic behavior of bridge.Fundamental frequency is decreasing with the increase of pier height.natural frequency of long unit bridge is bigger than that of short ones,especially in high order frequencies.Most of bending moment at pier bottom of long unit bridges is smaller than that at short ones under seismic action.However,it is opposite in static conditions.Considering the bending moment under static load is smaller than that under seismic load,it is more advantageous to adopte the long unit form.(2)The influence of simplified simulation on seismic response of bridge.In order to facilitate the establishment of finite element model,the methods of simulating variable section piers,double-column piers and pile-soil interaction are studied to ensure the accuracy of calculation.Variable section can be replaced by equivalent section.The value of longitudinal moment at pier bottom of double-column of bridge can be acquired by using single-column pier finite element model.Equivalent fixed method can simulate pile-soil interaction as well as m method.(3)The influence of bearing stiffness on seismic response of bridge.Composite stiffness of bearing and pier is closely related to the height of pier.Composite stiffness of low pier and bearing can be adjusted in a large range while that of high pier and bearing only can be changed limitedly and depends on anti-push rigidity of pier.Changing composite stiffness of bearing and pier to a same value for all piers cannot improve the distribution of internal force in piers and even leads to larger moment at high piers.(4)Aimed to make the longitudinal moment at the bottom of piers well distributed by adjusting the stiffness of bearing,the method can be summarized as increasing it on high piers and decreasing it on low piers.The stiffness of bearing has a great influence on longitudinal moment while it is irregular on transverse moment.The value of transverse moment is smaller and force distribution is more uniform after stiffness adjustment.(5)In order to get reasonable bearing stiffness more conveniently and quickly,the program of “Bearing Stiffness Optimization” is developed on the basis of Visual Basic and APDL.Bearing stiffness can be obtained after inputting parameters of ground motion and bridge.Moreover,the result calculated by the program is better than manual adjustment,and is favorable to be applied to the practical projects.