Research On Fatigue Damage Mechanism And Disposal Technology Of Truss Type Longitudinal Diaphragm Of Steel Box Girder Cable-Stayed Bridge

This paper relies on the scientific and technological research and development project of China Communications Construction Company Limited,"Research on fatigue cracking disposal technology and device for hollow web longitudinal spacer of orthogonal anisotropic steel box girder of cable-stayed bridge(2021-ZJKJ-QNCX08)" and a large-span steel box girder cable-stayed bridge maintenance project in China,in view of the widespread end cracking of steel box girder truss longitudinal spacer of large-span cable-stayed bridge and the lack of disposal measures.Based on the problem of end cracking and lack of disposal measures,we proposed a modification method of elastic damping support for rigid connection structure of truss longitudinal spacer based on the study of its cracking mechanism by numerical simulation,and carried out corresponding theoretical analysis,device development and experimental research.The main research contents and conclusions are as follows:(1)The basic structure of flat steel box girders of cable-stayed bridges and the characteristics of current fatigue assessment methods of steel bridges are analyzed,the typical phenomenon of fatigue damage of truss-type longitudinal spacer is described,and the applicability of fatigue crack treatment methods for steel bridges is discussed.(2)The finite element model of steel box girder truss longitudinal spacer is established and the end cracking mechanism of truss longitudinal spacer is analyzed.The results show that the alternating positive stress along the length direction of the circular tube at the intersection of the circular tube and the nodal plate under the action of the vehicle is the main cause of its fatigue cracking.And accordingly,a new idea of using end-articulated elastic damping support to transform the original rigid jointed steel tube diagonal brace is proposed.(3)Develop a kind of elastic damping support device with large initial stiffness and carry out the mechanical performance test of the full-scale specimen(device).The static load test results show that the stiffness is linear in the loading process,and the stiffness is non-linear in the unloading process,which is larger then smaller.The results of dynamic load test show that the loading frequency has little effect on the mechanical properties and can be ignored;the equivalent stiffness decreases with the increase of loading displacement amplitude;the energy dissipation capacity and stiffness both decrease with the increase of cycle number.(4)The refined finite element model of steel box girder section is established.Firstly,the proportional relationship between the load bearing of the longitudinal spacer at different positions under the action of vehicle wheel load was studied.The results show that the vertical load sharing ratio of the longitudinal spacer at different positions shows a symmetrical distribution;and the vertical load sharing ratio of the longitudinal spacer is obviously reduced after replacing the elastic damping support.Secondly,the stress time course of the truss type longitudinal partition under the vehicle load at each key fatigue location was studied,and the differences before and after the installation of the elastic damping support were compared,and the stress spectrum was calculated based on the stress time course.The fatigue life of the trussed longitudinal partition before and after the installation of elastic damping support is derived based on the S-N curve.It is shown that the stress amplitude of the circular tube diagonal brace with elastic damping support installed is less than the cut-off stress amplitude and there is no fatigue problem.(5)A study on the selection of parameters and their effects on the fatigue life of adjacent longitudinal diaphragms and the stress performance of bridge deck slabs was carried out with the disc-type spring-type elastic damping brace as an example.The results show that increasing the number of spring stacked pieces and friction coefficient can increase the fatigue life of the adjacent circular tube diagonal brace,and the effect of increasing the number of spring stacked pieces is better,both have little effect on the force of the bridge deck.(6)Comparative analysis of the deployment scheme of the elastic damping support device to replace the circular tube diagonal brace and its effect on the fatigue life of the longitudinal partition and the force of the bridge deck,and the proposed replacement of the elastic damping support device.