Impact Factor And Fatigue Study Of Steel Truss Joint For Parallel-type Combined Highway And Railway Bridge

Steel truss bridge,with its lightness,high stiffness and good crossing ability,has been widely used in combined highway and railway bridges.As the important component of combined highway and railway bridge,steel truss joint has complex stress and obvious spatial effect,which makes it more prone to fatigue damage during repeated loading of vehicles.In addition,environmental corrosion will further aggravate the fatigue damage of steel truss joint,and even lead to fatigue failure of the component.Therefore,it is of great theoretical significance and engineering application value to study the dynamic impact effect and fatigue performance of combined highway and railway steel truss bridge joints due to vehicle bridge coupling vibration.This thesis takes a parallel-type combined highway and railway steel truss bridge as the research object.In consideration of the asymmetric layout of railway and highway lines,the vehicle bridge coupling vibration effect is analyzed.Then the impact factor and the fatigue damage of the paralleltype combined highway and railway steel truss bridge joints are sophisticated studied.The main contents are as follows:(1)The finite element model of the parallel-type combined highway and railway steel truss bridge and the multi-body dynamic model of train and vehicle are established,and on this basis,the vehicle bridge coupling vibration analysis system is constructed.According to the measured data of highway traffic flow of the same line,the random traffic flow model is generated by Monte Carlo method.The transverse and vertical acceleration and displacement responses of the steel truss joints due to the train and vehicle load separately or simultaneously are calculated based on Craig Bampton method.The element stress of the joint is further obtained based on the elastic finite element theory.(2)According to the vehicle bridge coupling vibration effect and the definition of impact factor,the stress impact factor of the steel truss joint at mid span is calculated.The differences of the impact factor values between the joints near the railway side and the highway side when the train and vehicles load separately or simultaneously are compared.The effects of train speed,carriage load and unevenness on the value of the stress impact factor are further discussed.(3)According to the stress impact factor of the parallel-type combined highway and railway steel truss joints,the joints near the railway side are taken as the research object of fatigue damage.The refined models of the steel truss joints with or without diagonals are established.The stress history of the fatigue details according to vehicle induced vibration is counted by rain flow counting method and the fatigue stress spectrums of two types of joints are obtained.(4)Referring to the total traffic volume of the combined highway and railway bridge and the recommended S-N curve of the fatigue detail by Chinese specifications,the annual damage of the steel truss joints caused by vehicle load is calculated.The effects of train speed,carriage load and unevenness on the fatigue damage are studied,and the fatigue life of different steel truss joints is further predicted.(5)In consideration of the environmental corrosion effect,the stiffness degradation model of the steel truss joint during service period is introduced.The fatigue cumulative damage development curve after the gradual degradation of structural performance is obtained,and the fatigue life of the parallel-type combined highway and railway steel truss joints under different environmental corrosion levels is finally predicted.