Study On The Bearing Capacity And High And Low Cycle Fatigue Coupling Damage Of Steel Bridge After Strong Earthquake

After the 1994 Northridge earthquake in America and the 1995 Kobe earthquake in Japan,the ultralow cycle fatigue induced by strong ground motions and post-earthquake serviceability of steel bridges became the focus of many researchers.However,the pre-existing high-cycle fatigue and the degradation of the post-earthquake strength of steel bridges haven't drawn enough attention yet.At present,studies on the combined effect of high and low cycle fatigue concern only on engine blades or aerospace materials,which are completely different material with structural steels.Therefore,the obtained conclusions cannot be applied directly on the seismic design of steel bridges.Further studies are still needed regarding on the material strength deterioration induced by ultralow cycle fatigue loading and its effect on the structural post-earthquake strength.In this paper,based on the research about the combined effect of high and low cycle fatigue of steel bridges under strong ground motions in the National Natural Science Foundation of China(No.51 708485),the material strength deterioration caused by the repeated loads was studied.The effects of the mechanical properties degradation on the seismic performance and post-earthquake strength of steel members and bridges were studied,respectively.On the basis,high-cycle fatigue of a steel bridge was investigated using the real-record vehicle stream from the Beijing to Shanghai expressway.Meanwhile,the low-cycle fatigue of the bridge under strong ground motions was analyzed.Finally,the combined damage of high and low cycle fatigue of the arch-column joint on the bridge was analyzed.Firstly,the cyclic softening of the commonly used structural steel Q345 under repeated loading was studied.The relationship between the material strength deterioration and the plastic energy dissipation density was proposed.The existing constitutive model was modified to take the tensile strength deterioration into account.Seismic performance analysis and post-earthquake strength evaluation of steel bridge piers and a steel arch bridge were carried out using the modified constitutive model.The effect of material strength deterioration on the mechanical behaviors of steel members and bridges were investigated.The results show that,Q345 is a typical kind of steel material with an obvious cyclic softening behavior.The tensile strength deteriorates along with the increase of the maximum tensile effective plastic strain that the specimen has ever experienced.The predicted results by the modified constitutive model agree very well with the experimental results.The tensile strength of the material deteriorates faster in the first load cycles than that in the mid-late load cycles.The modified constitutive model can predict the cyclic softening accurately.When taking into account the material strength deterioration,the loading capacity of the steel member decreases.However,the deterioration is not so significant.When considering the pre-existing seismic damage,the residual strength of the bridge sustains 90%of the original value.However,the ductility the seismic damaged bridge decreases dramatically.The real-record vehicle stream from the Beijing to Shanghai expressway was adopted to conducted influence line analysis of a steel bridge,and the stress-history during the service time of the arch-column joints were obtained.The reservoir method was used to obtain the fatigue stress spectrum of these joints.Miner's rule was used to obtain the high-cycle fatigue damage evolution during the service time.Finally,the elastoplastic seismic analysis of the steel bridge was performed to study the low-cycle fatigue damage of these joints.The combined damage of high and low cycle fatigue was investigated then.The results show that,the fatigue damage accumulates of these joints during the service time under the impact of the vehicle stream.The high-cycle fatigue damage increases linearly during the service time when Miner's is utilized The high-low cycle fatigue failure can be observed when the bridge is hit by Chichi earthquake after being in service for 20-24 years.The high-low cycle fatigue failure can be observed when the bridge is hit by Niigate earthquake after being in service for 40?57 years.If the high cycle fatigue damage accumulated during the service time is neglected during seismic analysis,the evaluation results are in the unsafe side.Therefore,the service time should be taken into account when conducting fatigue evaluation of a steel bridge under strong ground motions.