Stress Analysis And Fatigue Characteristics Of Welding Details Of Orthotropic Steel Bridge Deck

Orthotropic steel bridge deck has been widely used in the construction of modern long-span steel bridges because of its light weight,high bearing capacity,installation conveniency and wide range of application.However,the fatigue problem of the structure is prominent due to the complexity of the structure.Among them,the most common fatigue cracks appear in the arc-shaped notch of the diaphragm and the welding joint of the plate and U-rib.The fatigue cracking of orthotropic steel bridge deck is becoming more and more serious,showing the characteristics of universality,early onset,multiple occurrence and repeatability,which has seriously affected the use and development of the structure in operation.Taking the steel box girder of one overpass as the background,systematic research was carried out on fatigue life assessment,structural details optimization and welding residual stress analysis using the methods of theoretical analysis,experimental research and numerical simulation.The main research work of this paper includes the following parts:(1)In order to study the fatigue performance of the arc-shape notch weld end of the diaphragm,the multiaxial fatigue effect evaluation index based on the deviation between the main stress component and the amplitude of the main stress was introduced,and a multiaxial fatigue life assessment method was established by combining zero-point structural stress and the bi-parametrical critical plane method.A full-scale model fatigue test was carried out,and the test results were compared with the calculation results of the proposed method in this paper and the calculation results based on the nominal stress method and the hot spot stress method.The results show that the arc-shape notch of the diaphragm is in obvious multiaxial stress state,and the nearer to the weld toe,the more prominent the multiaxial stress is.The fatigue crack first appeared at the welding toe of U-rib web.In the fatigue assessment of arc-shape notch weld details,bi-parametrical critical plane method based on multiaxial fatigue theory has the advantages of reasonable theoretical basis,and the results are more consistent with the experimental values.(2)In order to study the compatibility between the structural details of arc-shape notch and the type of diaphragm,a fine finite element model of the arc-shape notch was established,which adopts the construction form of "hyperbolic curve combined with straight line",based on the sub model technology.The sensitivity of the stress response of the vulnerable details on the diaphragm's notch and that on the transverse rib to the wheel load position was analyzed,and the relationship of the fatigue life of the details at the beginning of the notch and the free edge of the notch to the curvature ratio of the notch and the thickness of the diaphragm was discussed.Based on the principle of multi-objective constraint optimization,an optimization model was constructed,which take the overall life of the structure as the optimization objective and the curvature ratio of arc notch and the thickness of plate as the optimization variable.The constraint condition was constructed according to the cracking sequence of the details at the beginning of the arc-shape notch and the free edge.The weighting group method was used to solve the optimization model.Based on the analysis results,the optimized arc notch design parameters of the diaphragm and the transverse rib are proposed respectively.(3)In order to study the distribution and release law of welding residual stress at the joint of the plate and U-rib,the welding process of the weld was numerically simulated based on the thermoelastic-plastic method.On this basis,the initial stress method was used to realize the combination of welding residual stress and external load,and the local stress distribution model along the deck thickness was proposed.The cyclic plastic constitutive model Chaboche was introduced into the finite element model,and the effects of loading mode,loading cycle times,loading value and stress ratio on the relaxation of welding residual stress were discussed.Based on the analysis of the calculation data,a prediction formula of residual stress release rate under different loading modes was put forward,which taken the extreme value of loading stress as the parameter.(4)In order to study the fatigue performance of steel-UHPC light composite bridge deck,a full-scale two-span model with double U-ribs was fabricated and subjected to six million fatigue load cycles.The test results proved the excellent crack resistance of UHPC and the improvement of fatigue performance on the diaphragm arc-shape notch details by using UHPC pavement.Based on the reliability theory and cumulative damage model,the fatigue reliability of arc-shape notch welding details was evaluated.Referring to the common UHPC pavement design scheme,the influence of UHPC layer thickness,spacing of shear studs and steel mesh on the mechanical performance of the composite structure was studied.On this basis,the correlation between the fatigue life of notch details and the thickness of UHPC layer and the height of transverse rib was analyzed,and the reasonable thickness of UHPC and the height of transverse rib were suggested to meet the service life requirement of transverse rib notch.