Study On Fatigue Mechanism Of Orthotropic Deck Of Steel Bridge With Box Girder

Orthotropic steel bridge deck has the advantages of light weight,high bearing capacity,wide application.It has become the preferred bridge deck structure for large-span bridges and has been widely used.However,due to the defects of the structure itself,there are many fatigue-sensitive parts,which are prone to fatigue cracks.At present,fatigue cracking has become a control problem that hinders the development of bridge projects.Fatigue cracks are characterized by universality,early occurrence,multiple occurrences,and repetitiveness,which severely affect the performance of the structure,leading to major economic losses and adverse social impacts.The fatigue details of the orthotropic steel bridge deck are mainly sensitive to the position of the vehicle wheel load,and the weakest part between the two adjacent openings of the diaphragm in the heavy traffic lane is poor capacity to fatigue resistance.Statistics indicate that the floorbeam cutouts and U rib to diaphragm connection end were fatigue crack prone area,the fatigue crack accounted for more than 1/3,or even more than 80%.The reasons why cracks in the diaphragm are premature and frequent occurred are not yet clear enough to-date.At present,this problem has become a challenging problem in the world.For this reason,this paper proposed that under the action of load,elastic-plastic buckling occurs locally at the opening of the diaphragm,causing out-of-plane deformation,thereby inducing cracking.The combination of theoretical analysis,model testing and modern numerical simulation techniques to simulation analysis of fatigue of orthotropic steel bridge decks,focusing on the exploration of the mechanism of out-of-deformation and crack generation between two adjacent cutouts of floorbeam in the range of truck lanes base on the stability theory of bridge structures.The main contents and conclusions of this study are as follows:(1)The study on crack plasticity and wake plasticity in the wake field.In order to solve the problem of early crack closure and delayed crack propagation under overload conditions,this paper studies the plasticity of cracks and the cyclic plasticity of the wake field by numerical methods,reveals the causes of early crack closure,and clarifies the mechanism of crack propagation delay under overload conditions.(2)Study on the causes of out-of-plane deformation at the opening of the diaphragmStarting from the stability theory of bridge structures,investigation on the reasons of out-of-deformation of the detail of floorbeam in the range of truck lanes base on local buckling theory using numerical methods.It was revealed that the out-of-plane deformation at this location was caused by local buckling and confirmed by tests.Based on the analysis results,the relationship between fatigue damage and local buckling was clarified.(3)The mechanism of cracks at the openings of diaphragmsBased on the latest advances in fatigue research of orthotropic steel decks,starting from the basic theories of fracture mechanics and fatigue cracks,taking into account the effects of out-of-plane deformation caused by loads,the finite element model was used for numerical analysis of the deck.The principal stress distribution of the peripheral elements between the two adjacent openings of the diaphragm shows that a high stress region appears in this region,considering the superposition of the hot spot stress,the principal stress exceeds the constant fatigue limit of the fatigue detail and results in crack.It is revealed that the out-of-plane deformation caused by the load is the main reason for the occurrence of fatigue cracks in this area.The discontinuous Galerkin-extended finite element method(DG-XFEM)was used to crack growth analysis,which is in accordance with the basic theory of crack propagation.It is also consistent with the test results and similar cracks in real bridges.(4)Study of the principal stress history of the deckBased on two typical fatigue vehicle checking models,the deck is loaded longitudinally and laterally.The finite element method is used to analyze the change of the principal stress history line.The results show that the superposition effect of the front and rear axles load and the vehicles in adjacent lanes can be ignored analyzing the orthotropic steel deck fatigue.(5)The method to improve the anti-fatigue performance of the diaphragmAccording to the results of local buckling analysis,the weakest part of the diaphragm is the cutout,which is the main location where local buckling is likely to occur.To this end,two feasible solutions to improve the fatigue resistance are proposed,that is,vertical stiffening or local thickening of two adjacent cutouts in the ranger of heavy traffic lane,can greatly reduce the out-of-plane deformation caused by local buckling,and the local thickening method is better.