Elastoplastic Seismic Performance Evaluation Of Steel Bridges Considering Fatigue Damages During Service

Steel bridges are prone to seismic damage or even collapse directly due to the excitation of a strong earthquake.In order to improve the elastoplastic seismic design methodology of steel bridges,many researchers have conducted a lot of studies on the seismic damage and destruction of steel bridge structures.However,the elasto-plastic seismic performance evaluation of a steel bridge is usually calculated using a newly-built bridge without any initial damages,indicating the fatigue damage caused by high-frequency vibrations such as vehicle loads during its service life is not considered.This may leads to an overestimation of the structural seismic resistance.At present,an effective hysteretic constitutive model which is suitable for the seismic performance evaluation of a steel structure considering its service history has not been established.Further researches are needed to evaluate the seismic performance of an in-service steel bridge.To this end,based on the combined fatigue experiments of high-low cycle for steel bridges under strong earthquakes in the National Natural Science Foundation of China(No.51708485),an elastoplastic constitutive model considering the initial fatigue damage was proposed in this study.The elastoplastic hysteretic performance of the steel bridge piers during service was analyzed,and the relevant ultimate bearing capacity prediction formula was proposed.On this basis,nonlinear seismic response analysis of the in-service structures,including steel piers and a steel arch bridge,were carried out,and the influence of the initial fatigue damage on the seismic performance of the in-service steel bridges was discussed.The elastoplastic hysteretic properties and fracture mechanism of the Q345qD structural steel after being fatigue damaged were analyzed through experiments,such as the high-cycle fatigue,low-cycle fatigue,and high-low cycle fatigue tests.According to the test results,the kinematic hardening model,the combined hardening model,and the two-surface constitutive model considering the initial fatigue damage was proposed.The relationship between the constitutive model parameters and the initial damage level of the material was established.The results show that the elastic modulus,the yield strength and the length of yield plateau gradually decrease along with the increase of the initial damage level.When the initial fatigue damage reaches a critical level,the yield plateau even disappears.The fracture surface caused by high-cycle fatigue is relatively flat,and the fatigue microcrack originates from the surface,and the fracture is a single-fatigue source type.The fracture surface caused by low-cycle fatigue is relatively rugged,and it is formed by multiple fatigue sources in different height.The characteristic of the fracture surface caused by high-low cycle fatigue lies between the characteristic of the high-cycle fatigue and low-cycle fatigue failure sufaces.The fracture surface is partly rugged and flat,the grain near the surface is very smooth,while it is very sharp around the internal defects.The constitutive parameter prediction formulae are proposed.The improved two-surface model reaches a perfect agreement with the test results,the improved combined hardening model agrees well with the test results,while the accuracy of the improved kinematic hardening model is relatively poor.In order to study the elastoplastic hysteretic performance of the in-service steel piers,the bearing capacity and seismic performance of the single-column pipe and rectangular steel piers were analyzed,and the bearing capacity prediction formula with different initial fatigue damage level were proposed.The results show that,for the pipe-section steel piers,the bearing capacity and ductility are affected by the axial compression,the diameter-thickness ratio and the slenderness ratio.For the rectangular steel piers,the spacing ratio of the diaphragms has a significant impact on the seismic performance of the structure.After considering the initial fatigue damage level,the load-bearing capacity of the steel bridge pier reaches the peak point earlier and then drops rapidly,and the ductility failure occurs earlier locally at the bottom of the piers.Finally,the elasto-plastic seismic performance of an in-service steel arch bridge was analyzed.The influence of the fatigue damage accumulated during its service life on the seismic performance was discussed.The results show that structural instability was not occurred under the excitation of the strong earthquake.As the initial damage level increase,the peak stress of the arch-column joint gradually decreases,while the dynamic plastic strain range increases,indicating the ultra-low cycle fatigue damage level increases rapidly.The ultra-low cycle fatigue damage index DULCF has a linear relationship with the initial fatigue damage level.