Study On Fatigue Fracture Of Steel Box Girder Of Long-span Cable-stayed Bridge Under Complex Stress Coupling

In recent years,orthotropic steel deck has been widely used in long-span cable-stayed bridges and suspension bridges.Geometric discontinuity and welding residual stress at weld joints lead to complex stress state near weld toe of orthotropic steel deck.Repeating action of vehicle causes fatigue cracking,and fatigue crack propagating the service life of bridge seriously.Therefore,it is important to study the crack propagation and fatigue life of weld joints of orthotropic steel deck under the coupled effects of random vehicle,wind and welding residual stresses.Sutong Bridge was applied as engineering example to study the questions mentioned above.The main contents to be studied are as follows:(1)Based on the overall bridge finite element model and the local refined finite element model of steel box girder,Monte-Carlo method was applied to analyze the mechanical behavior of the welding details of the orthotropic steel deck under random vehicle loads was analyzed based on the Monte-Carlo method.(2)Mean wind is calculated with Static Three Component Coefficient,and fluctuating wind is simulated with Harmonic Synthesis based on Kaimal wind speed spectrum.The mechanical behavior of steel bridge deck welding details in the filed combining the two winds above are simulated and analyzed numerically.(3)By establishing local refined finite element model of top plate U-rib,the distribution of residual stress near the welding toe is analyzed,and the effect of high temperature hammering treatment on the residual stress at the weld toe is analyzed.The stress peak value of the welding details of the steel bridge deck under the coupling of random vehicle and welding residual stress is much larger than that of the random vehicle.Compared with the residual fatigue life of welded details under random vehicle,the residual stress reduces the residual fatigue life by 30-46%;the residual fatigue life decreased by 13%due to the high temperature hammering treatment.(4)Based on the dual-parameter fatigue crack propagation model,the fatigue crack propagation of the welding details of the orthotropic steel deck was studied under the coupling of random vehicle,and wind and welding residual stress,and then the fatigue crack propagation life was evaluated.Compared with the residual fatigue life of the welded details of the steel bridge deck under random vehicle,the coupling of complex stress reduces the residual fatigue life by 36-53%.Compared with the wind load,the welding residual stress has a greater influence on the fatigue life,so it should be considered when evaluating fatigue crack growth life.(5)Based on the theory of linear elastic fracture mechanics,the fatigue crack propagation of steel-asphalt composite bridge deck and steel-UHPC composite bridge deck were studied respectively,and the influence of bridge deck pavement on the fatigue life of welding details was analyzed.Steel-asphalt composite deck and steel-UHPC composite deck can prolong the fatigue life to varying degrees.The increase of reinforcement ratio of UHPC pavement section can prolong the fatigue life of welding details.Increasing the thickness of the UHPC pavement can improve the residual fatigue life,but it leads to more complicated mechanical properties of the composite bridge deck.