Fatigue Performance And Fatigue Life Study On Cable-Girder Anchorage Of Rail-Cum-Road Cable-Stayed Bridge

In recent years,the rail-cum-road cable-stayed bridge is widely used in order to satisfy the requirement of multi-use for railway and highway.Under the combination action of vehicle and train loads,the long-span rail-cum-road cable-stayed bridge is subjected to a considerable loading effect.As a type of connection component between cable and girder,the cable-girder anchorage contains a complicated local welding structure.Moreover,the stress concentration on cable-girder anchorage is very serious during loading effects are transferred.To solve the problem,a new type of cable-girder anchorage structure,which is called as composite anchor-box with double pulling-plates connecting the cable and steel truss is used in present rail-cum-road cable-stayed bridge.For this new cable-girder anchorage,double pulling-plates are the vertical plates for transferring.The upper part of double pulling-plates is formed to be anchor box by welding seams,and it fixes cable.The lower part of double pulling-plates is connected with the top plate of girder.Under the combination action of vehicle and train loads,the loads applying on the cable-girder anchorage are transferred by welding seams,thus the fatigue problem of welding details is very serious.The systematic research on composite anchor-box with double pulling-plates is carried out in this dissertation.Based on the fatigue test with equivalent boundary,fatigue performance of the cable-girder anchor structure has been put into study by the analytical methods of nominal stress,hot spot stress and fracture mechanics,respectively.Fatigue life of the cable-girder anchor is analyzed by formulated hybrid reliability model with consideration of the parameter characteristics of accumulated damage model and crack propagation model.The main contents of the dissertation are as follows:(1)The development of rail-cum-road cable-stayed bridge is reviewed,structural characteristics of cable-girder anchorage is analyzed.Combined with the fatigue disease of present steel bridge,the necessity to research the fatigue problem of rail-cum-road cable-stayed bridge's cable-girder anchorage is discussed.The latest research and existing problems on fatigue strength and fatigue life analysis of cable-girder anchorages are summarized.The significance and research basis of this doctoral dissertation are introduced.(2)Depends on the present standard fatigue vehicle and typical fatigue trains,the fatigue loading effects and its combination of the cable-girder anchorage are analyzed by numerical simulation method.Stress distribution characteristics and structural geometric parameters sensitivity of cable-girder anchorage are also analyzed.Then a basic structure of composite anchor-box with double pulling-plates is proposed.Three types of fatigue sensitive details are obtained,which contains uniaxial tension details in the end of connection of anchor bearing plate and transferring vertical plate,tension shear details in the side of connection of anchor bearing plate and transferring vertical plate,and compression shear details in the end of anchor bearing plate.Computational formulas of nominal stress amplitudes of fatigue sensitive details are also given.(3)Based on the similarity theory,fatigue test of the basic structure of composite anchor-box with double pulling-plates has been carried out with the consideration of equivalent stress and equivalent boundary.The results of the static test show that,test data are agreement with calculated values of the basic structure by finite element simulation.Calculated values of the basic structure and the original cable-girder anchorage are in good consistency.The test specimen is suitable to be used to analyzed fatigue performance of the cable-girder anchorage.The results of the fatigue test show that,fatigue cracks has been observed in uniaxial tension details and compression shear details.The general failure of the cable-girder anchorage structure is induced by crack fracture of uniaxial tension details.Fatigue strength of uniaxial tension detail has relation to the width-to-height ratio of anchor bearing plate.With the consideration of related S-N curves in the codes,computational formula of nominal stress amplitude of uniaxial tension detail is corrected by parameter related to the width-to-height ratio of anchor bearing plate.Recommended categories for uniaxial tension detail and compression shear detail are also selected.(4)The hot spot stresses of fatigue details of the cable-girder anchorage with different stress states are identified and obtained by extrapolation method.The influence on hot spot stress of element types,meshing sizes,and extrapolation methods has been analyzed by finite element modelling.The feasible finite element simulation method is given by comparing to the measure values of hot spot stresses.And FAT90 curve has been verified with reasonability for fatigue life analysis.With the considerations of sensitivity and rule of the geometric parameters,hot spot stress concentration factors of uniaxial tension details and tension shear details are formulated by nonlinear regression expression.(5)Based on linear elastic fracture theory,numerical simulation of the stress intensity factor crack propagation regulation is analyzed by the finite element software of ANSYS and FRANC3D.The validation of the Finite Element Model is verified by measure values of the penetrating crack and semi-elliptic surface crack in literatures.The propagation of the cracks at connection end of anchor bearing plate and transferring vertical plate is simulated by the maximum circumferential stress criterion.The influence on stress intensity factor and crack propagation of initial crack sizes and shapes is discussed based on fatigue fracture morphology.With the considerations of sensitivity and rule of the geometric parameters,correction factors of semi-elliptical surface crack stress intensity factor are formulated by nonlinear regression expression.(6)Generally,the error in the prediction of fatigue for traditional steel structure is always considerable.Because the characteristics of the variables in the linear accumulative fatigue and crack propagation model are distinguished as uncertain and random.Based on the interval-probability theory,the consistency relation of the interval model and the probability model distributed normally and log-normally is derived.Hybrid reliability simplified model in theory and solution method is given considering the interval and stochastic variables simultaneously.A category of fatigue detail in a real bridge is chosen as case study,and the fatigue life analysis is taken to prove the validity and adaptive range of the method in this paper.The fatigue probability analysis towards the cable-girder anchorage is carried out based on the linear accumulative fatigue and crack propagation model.The suggestion of setting reasonable cycle for detection is given by fatigue life analysis of the cable-girder anchorage.