Influence Of Cable Breakage On Mechanical Behavior Of Cable-Stayed Bridge

Cable breakage accidents of cable-stayed bridges have occurred frequently around the world and more and more attention has been paid to the breakage of stay cables.As the main force-bearing component of cable-stayed bridges,once the cable breaks,large alternating stress will be born on the cable-stayed bridge,and the safety of pedestrians and traffic on the bridge will also be affected.In severe cases,the cables near the broken cable may break successively,which will eventually lead to the collapse of the cable-stayed bridge.At present,knowledge about the dynamic response of each component of cable-stayed bridge caused by cable breakage is poorly understood,and there is still lack of corresponding theoretical guidance in dealing with the cable fracture accidents and cable replacement project in an emergency.Therefore,it is necessary to study the effect of cable breakage on the mechanical behavior of cable-stayed bridges.The damage mechanism of stay cable was analyzed and based on this,the theory of dynamic response analysis of stay cable fracture was introduced by means of analytical method and numerical method.The theory of modal analysis of cable-stayed bridge under stress state was also introduced,which laid the theoretical foundation for the following study of the influence of cable fracture on the mechanical behavior on cable-stayed bridges.A three tower cable-stayed bridge was taken as an example,and the three-dimensional finite element model of the bridge was established by using the structural analysis software(ANSYS),and the following aspects were studied:In terms of natural vibration characteristics,the static calculation of the example bridge was performed first to ensure that it was in the state of stress.Based on this,the modal analysis of the example bridge before and after the cable breaking were carried out respectively to predict the degradation of the structure after the cable breaking according to the change of the dynamic characteristics.The results showed that:(1)The maximum variation of the natural frequency was only 1.19%after the single cable breaks,so single cable fracture had little influence on the natural vibration characteristics of cable-stayed bridges.(2)The maximum variation of natural frequency of cable-stayed bridge was 2.15%when two cables were broken on one side,and it was 3.58%when three cables were broken.With the increase of the number of broken cable at one side,the variation of the natural frequency of the structure increased,and the rate of structure degradation accelerated.In the aspect of simulation method of cable breaking process,the pseudo dynamic method proposed by PTI of the United States was used to simulate the cable fracture process,and the results were compared with the results of the dynamic analysis,then the applicability of the pseudo dynamic method in analyzing the cable fracture of the example bridge was validated.The results showed that the pseudo dynamic method can only ensure that the calculated data near the broken cable was larger than the data calculated by the dynamic method,and in some position,the results are smaller than those calculated by dynamic method.Therefore,the pseudo-dynamic method proposed by the American PTI was unsafe to calculate the cable breakage condition of the cable-stayed bridge in this example bridge.In terms of dynamic response,based on the state of the cable-stayed bridge with stress,a time-history curve model of sudden loading of the broken cable during the loading process was established,and the transient dynamic analysis of the fracture process of the cable-stayed cable was performed using the complete method.The dynamic response of a cable-stayed bridge caused by cable breakage was studied under four conditions including the cable breaking position,structural damping,cable breaking time and the cable breaking quantity.The results showed that:(1)For a single cable breaking at different locations,the dynamic response of the long cable fracture was more obvious than that of the short cable fracture for the same bridge span.The dynamic response of the structure caused by the broken cable at the main span location was greater than that of the side span.The dynamic response of the structure near the broken cable was very obvious,and the dynamic response of the structure far away from the broken cable location would rapidly decline with the increase of distance.(2)Compared with the single cable,the vertical displacement of the main girder in the span where two cables and three cables breaking simultaneously increased by28.6%and 59.8%respectively,and the amplification caused by two cables and three cables breaking successively were 32.6%and 66.5%.Therefor,when multiple cables broke,the dynamic response of the structure would amplify with the increase of the number of broken cables,and the effect on the structure caused by multiple cables breaking successively was greater than that of breaking simultaneously,which would cause the continuity failure of the main girder.(3)The influence of structural damping was 7.85%on the vertical displacement at the middle span where the broken cable was located,while the influence on the right side middle span of the middle tower was 15.52%.So the structural damping had a great influence on the dynamic response of the main girder far away from the broken cable position,which could effectively reduce the dynamic response of the structure far away from the position of broken cable.(4)In the process of simulating the cable breakage,the amplitude of the stiffening girder and the acceleration of the node would decrease with the increase of unloading time of cable force.When the unloading time of cable force exceeded one basic period,the vertical displacement amplitude of the node on the stiffening main girder was less than 1cm,and the acceleration of node was less than 0.036 m/s~2,and the structure would not vibrate obviously.