Study On Seismic Dissipation Mechanismof Longitudinal Elastic Cordto Cable-Stayed Bridge

As a lifeline engineering,large-span cable-stayed bridge(CSB)should have adequate seismic capacity to keep safe subjected to earthquake excitations.To reduce the tower,pier,pile foundation seismic internal force response,large span cable-stayed bridge usually adopts floating restraint system.However,due to the lack of effective longitudinal restraint,the longitudinal displacement of girder in floating system is huge,which may caused large deformation of tower and collisions between girders.Besides,the system has many other advantages,such as reliable performance,easy installation,less cost There is little literatures about seismic dissipation mechanism of longitudinal elastic cord to CSB.The contents and conclusions are as follows:(1)Study the damping effect of elastic cord to cable-stayed bridges by conducting shaking table test.Based on the principle of dynamic similarity,cable-stayed bridge 1:40 scale reduced physical test model is designed and produced.By setting longitudinal sliding bearing and elastic cord between tower and girder,floating system,elastic constraint system and elastic limit system of cable-stayed bridge are simulated respectively.Test results showed that under the same seismic excitation,longitudinal displacement of main girder in elastic constraint system is obviously lower than that in floating system,and bottom bending moment of main tower in elastic constraint system is also apparently lower than that in floating system.The results showed that,compared with floating system,elastic constraint system has less seismic response.(2)Base on the background bridge,the finite model is established to study seismic dissipation mechanism of longitudinal elastic cord to cable-stayed bridge.Dynamic characteristic analysis showed that:the differences between the two systems are mainly on frequency of the 1st order mode.The fundamental frequency ofelastic constraint systemis greater than that of floating system,as additional elastic cord increasing the longitudinal stiffness of CSB.Besides,the results of seismic response analysis showed that parts of girder inertia force transmits to tower directly,as the additional elastic cord.Therefore,it changes the distribution of the shear and bending moment of the main tower.Longitudinal bridge displacement of key no desin main tower and main girder are smaller than those in floating system.However,in elastic constraint system,shear force of tower under the connecting position is relatively larger.(3)Discussing the optimal stiffness of cable-stayed bridge longitudinal elastic cord in the last charpter.Based on single degree of freedom model,simplified model of cable-stayed bridge of elastic system is analyzed.Assuming bottom bending moments of main tower as critical factor,simplified estimation formula of optimal longitudinal stiffness for elastic cord is derived.Taking a double-tower cable-stayed bridge as example,optimal stiffness of elastic cordis calculated by simplified estimation formula and finite element method.The results showed that estimated value of that formula is close to result of finite element method.The FEA result of parametric stiffness of elastic cord showed that:the distance from bearings to bottom has the most influence to the optimal stiffness value.While the main span and heights of tower shas relatively smaller influence.