Evaluation Method Of Residual Bearing Capacity Of In-service Steel Arch Bridges Considering Fatigue Damage

At present,considerable research work has been done on the calculation theory and evaluation method of the ultimate carrying capacities of steel arch bridges.However,most of them focused on the designed structural parameters,the most unfavorable load forms,the service environment of the bridge,etc.,on the carrying capacity of the structures.With the increase of traffic loads,the bridge spans increase and the plates in steel components become thinner.Fatigue damages caused by repeated vehicle loads are prone to accumulate in the steel components and important parts,leading to the degradation of mechanical properties in the material level,which further affects the mechanical behaviors in the structural level.Once fatigue cracks occur in the components or important parts,the bearing capacity of a structure is affected and the operation safety of the bridge is threatened during its service time.Therefore,the evaluation the carrying capacity of the in-service steel arch bridges is very important since the current performance of the structure can be timely obtained,and the bearing capacity in future can be predicted and thus,the safety of the bridge in its entire service life can be ensured.With the financial support of the National Natural Science Foundation of China(No.51708485),this paper studied the mechanical properties of steel material with consideration of fatigue damages and the ultimate carrying capacity of steel plates,steel piers and steel arch bridges in service by means of experimental and numerical analysis.The research contents are listed as follows:The influence of pre-fatigue damages on the mechanical properties of materials depends on the fatigue load forms and the material types.Taking the Q345qD structural steel,which is commonly used in structural construction,as the research object,the tensile tests,fatigue tests,pre-fatigue tests and subsequent tensile tests of the steel samples were carried out.The influence of pre-fatigue damages on the mechanical properties of steel was analyzed,and the elastic-plastic constitutive model was established according to the test results.The results show that the yield stress,ultimate stress,yield platform length,and elastic modulus of the material decrease with the increase of the pre-fatigue damage degree,while the shape parameters and initial tangential modulus change in a contrary way.When the pre-fatigue damage degree reaches 0.6,the yield platform disappears.When the pre-fatigue damage degree reaches 0.9,the material properties deteriorate rapidly and suddenly.In order to study the influence of pre-fatigue damage on the ultimate carrying capacity of steel plates,numerical analysis was conducted on the unstiffened steel plates and stiffened steel plates with different structural parameters.The effects of width-to-thickness ratio,diaphragm spacing ratio,initial defects and pre-fatigue damage degree on the carrying capacity of the steel plates were discussed.The results reveal that the ultimate carrying capacity of steel plate decreases with the increase of width-to-thickness ratio,initial defects and the pre-fatigue damage degree.The effect of diaphragm spacing ratio is very small.Based on the results of the parameter analysis,the ultimate carrying capacity prediction formulas of steel plates considering the influence of pre-fatigue damage were proposed.The effects of width-to-thickness ratio/radius-to-thickness ratio,diaphragm spacing ratio,axial compression ratio and pre-fatigue damage degree on the ultimate carrying capacity of steel piers with thin-walled box sections and circular sections were investigated.The results show that the ultimate carrying capacity of steel pier decreases with the increase of the paprameters including the width-to-thickness ratio/radius-to-thickness ratio,the axial ratio and the pre-fatigue damage degree.When the fatigue damage degree reaches 0.9,the ultimate carrying capacity of the steel piers decreases remarkably.The reason may be that the structure has suffered too much pre-fatigue damage and it is close to failure state.The diaphragm spacing ratio has little effect on the ultimate carrying capacity.Based on the results of the parameter analysis,predictive formulae for the residual ultimate carrying capacity evaluation of thin-walled steel piers with pre-fatigue damages were proposed.Due to the repeated actions of the vehicle loads,fatigue damages are prone to accumulate on the in-service steel bridges.In order to study the effect of cumulative fatigue damages on the residual bearing capacity of the in-service steel arch bridges,fine plate-shell element models were used for numerical analysis.The results show that the maximum cumulative fatigue damage degree of the main force components of the bridge is between 0.1 and 0.2 considering the influence of vehicle load only.Therefore,no direct fatigue failures occur on these components and joints during the 100-year service life.The ultimate carrying capacity of the steel arch bridge decreases due to fatigue damage,but the decrease is limited It can be found that the load coefficient calculated by the fiber element model is higher than that by the shell element model.The difference between the predicted results is larger when the span length is longer.It is indicated that,in comparison with the shell element model,the fiber element model is very conservative in evaluating the residual bearing capacity of steel arch bridges.