Prediction Of Failure Modes And Ultralow Cycle Fatigue Damage Of Thin-walled Steel Bridge Piers Under Cyclic Loading

Buckling and ultralow cycle fatigue failures are the main failure modes of steel bridge piers under strong ground motions.At present,buckling failure of steel bridges has attracted many attentions,and the ultimate strength and ductility evaluation formulas of thin-walled steel bridge piers have been established.However,the ultra low cycle fatigue failure prediction formulas haven't been established yet.The mechanism of ultra low cycle fatigue failure of steel bridge piers remains unclear.Moreover,the occurrence of buckling or ultra low cycle fatigue failures needs to be clarified.In this paper,based on the research about the ultralow cycle fatigue of steel bridges under strong ground motions in the National Natural Science Foundation of China(No.5178485),steel bridge piers with pipe and rectangular sections are taken as the research object,and their hysteretic mechanical behaviors under cyclic loads are investigated.The influence of structural parameters on the ultra low cycle fatigue damage evolution is studied.Practical formulas of damage indexes to predict the ultra low cycle fatigue damage of steel bridge piers under different critical state are proposed and validated.Finally,the occurrence order of the two failure modes is discussed.In order to study the influence of different design parameters on the damage indicators of steel bridge piers under three critical states,22 piers with pipe sections and 24 piers with rectangular sections are analyzed.The user subroutine UVARM coded by Fortran is utilized to introduce the ultra low cycle fatigue damage index to the commercial software ABAQUS.Firstly,the ultralow cycle fatigue damages of the base metal,the welded metal and the heat-affected zone near the pier bottom are analyzed.The weld and the heat affected zones at the bottom are more prone to ultra low cycle fatigue than the base metal.Then,by changing the structural parameters the effects of structural parameters,e.g.the axial force ratio,slenderness ratio,width-to-thickness ratio,diameter-to-thickness ratio,and diaphragm spacing ratio,on the ultra low cycle fatigue damage of the pier under repeated loading are analyzed and compared.The axial force ratio and diameter-to-thickness ratio of a circular steel bridge pier have a significant influence on the ultra low cycle fatigue,while the ULCF damage indices gradually decreases as the axial force ratio and diameter-to-thickness ratio increases.The diaphragm spacing ratio and slenderness ratio affects little on the failure modes of circular steel bridge piers.The axial force ratio,slenderness ratio,diaphragm spacing ratio and width-to-thickness ratio of rectangular steel bridge piers have a significant impact on the ultra low cycle fatigue damage.The damage index decreases with the increase of these parameters under the same critical state.Finally,empirical formulas of the damage index under different critical states are proposed,which can be used to identify the occurrence of 2 failure modes(namely buckling and ultra low cycle fatigue failures).These formulas provide a reference for the seismic design of steel bridge piers.In order to validate the proposed empirical formulas,non-linear analysis of 3 thin-walled rectangular steel bridge piers are performed,and the predicted crack initiation location and time are compared with the experimental results.The results show that the predicted crack initiation locations are consistent with the test results,and they are all located in the heat affected zone.By comparing the predicted and the testeded results,it can be concluded that the difference of crack initiation time is less than 2 load cycles,and the error is 8.1%?14.2%.Therefore,the damage index prediction model and the proposed formulas can predict the ultra low cycle fatigue fracture very well.Finally,elastoplastic seismic response analysis of a complex steel arch bridge is carried out.The occurrence order between different failure modes is studied using the displacement-time history curve of the arch bridge.The cracking characteristics are analyzed by selecting the joints of arch ribs and columns that are prone to ultra low cycle fatigue failure.The results show that the ultra low cycle fatigue damage index in the heat affected zone is obviously larger than that in the welded and the base metals.Ultra low cycle fatigue occurs first in the affected zone,followed by the weld zone,and there is no crack on the base metal.In addition,the boundary conditions of the arch bridge have little influence on the damage index of ultra low cycle fatigue.Through comparing the bridge structural displacement history and the ultra low cycle fatigue damage evolution,it is found that the ultra low cycle fatigue failure mode of the structure under earthquake action is occurs earlier than the buckling failure mode.