Analysis On Dynamic Characteristic And Seismic Response Of A Long-span Three-tower Self-anchored Suspension Bridge

Major bridge is usually a transportation hub of the regional economic development,and also the greenway of life in earthquake relief.As one of the bridges with strong spanning capacity,self-anchored suspension bridge has been developed and applied widely because of its elegant appearance,excellent adaptability of site and economic efficiency.At present,most self-anchored suspension bridges are used for twin towers.But it is more and more difficult to increase its span.So the idea of three-tower self-anchored suspension bridge has been put forward and gradually chosen as one of the preferred types of bridge construction in the cities.In this paper,the largest span three-tower self-anchored suspension bridge——the Binhe Yellow River Bridge in Yinchuan which is currently under construction is chosen as the study object.On the basis of previous research achievements,the dynamic characteristics and seismic response of the bridge are studied according to the analysis theory.The main study work of this paper is as follows:(1)A dynamic finite element model of the bridge is established to analyze its dynamic characteristics.The influence of concrete-deck on the vibration characteristics of the suspension bridge with composite beam is analyzed.The impact of several important design parameters(the rise-span ratio,the dead-load and the stiffness of members)on mode and frequency is also discussed.The results indicate that: the first-order frequency of the bridge is low with a vibration of longitudinal drifting.A leading role is played to the vertical bending of the stiffening girder and the longitudinal bending of the main tower in the vibration characteristics of the bridge.The concrete-deck can avoid premature lateral bending modes of the stiffening girder.As the rise-span ratio decreases,the frequency of the vertical bending of the stiffening girder which is related to the longitudinal bending of main tower reduce most.The increase of the dead-load will reduce vibration frequency of the longitudinal drift,vertical bending and torsional of the stiffening girder,and raise the frequency of the lateral bending of the main towers.Increasing the longitudinal bending stiffness of the side tower can effectively improve the longitudinal frequency drift of the stiffening girder.But the bending and torsional stiffness of the stiffening girder have no effect on its longitudinal vibration.The tensile stiffness of the boom has little influence on the vibration characteristics of the whole structure.(2)According to the site conditions of the bridge and related seismic code,the design response spectrum is determined.The effect of high-order modes on the seismic response of the bridge is analyzed by modal analysis method,in order to determine the number of mode required in response spectrum analysis.The internal force and displacement response of the bridge under seismic response spectrum are calculated and analyzed.The results show that: the number of modes should be sufficient to avoid missing some high modes with higher contribution rate.Under horizontal earthquake,the moment and shear response on the bottom of middle tower is greater than those on the side tower,more attention should be paid to the stress characteristics of the middle tower in seismic design.In addition,there is a stronger coupling effect between the vertical vibration and the longitudinal vibration to a three-tower self-anchored suspension bridge,and there is no contribution to the internal force and deformation of the transverse vibration.(3)Based on the site condition of the bridge location,the proper seismic time history curve is input.By using time history analysis method,the dynamic response under uniform excitation is studied,and a comparison is also made with the result of response spectrum analysis.The impact of traveling wave effect on the seismic response of the bridge under non-uniform excitation is also analyzed.The results show that the force responses of the time history analysis and the response spectrum analysis are quite consistent.The impact mechanism of traveling wave effect on seismic response analysis is very complicated.Not only is the influence degree of force response in different section different,but also may be the different force responses in the same section different.In addition,When the traveling wave velocity is between 1000m/s~1500m/s,it can cause the adverse internal force response at the cross section of the main span and the bottom section of the tower,so more attention should be taken to determine the wave velocity of the bridge proposed site while seismic design.