Experimental Research On Extremely Low-cycle Fatigue Behavior Of Partially Concrete-filled Stiffened Steel Box-section Bridge Piers

Partially concrete-filled steel bridge pier is widely used due to its light weight,high strength and good ductility.However,according to the investigations of structural damage after several earthquakes,it was found that another new failure mode may appear in addition to local buckling failure of steel bridge piers under severe earthquakes.The steel bridge piers will fail after tens of cycles or even several cycles when they are subjected to the action of great plastic strain amplitude.This fracture has obvious ductile characteristic,which is different from the traditional high-cycle fatigue failure and is called as extremely low-cycle fatigue.In order to ensure that the seismic performance of this type of piers are fully exerted,it is of great significance to carry out the extremely low-cycle fatigue behavior test of partially concrete-filled stiffened steel box-section bridge piers,and to study this behavior in depth.In this paper,the extremely low-cycle fatigue test of partially concrete-filled stiffened steel box-section bridge piers is carried out.The effects of normalized flange’s width-to-thickness ratio,normalized slenderness ratio,axial load ratio and concrete grade on the extremely low-cycle fatigue of the specimens are investigated.Based on the experimental and numerical simulation methods,the extremely low-cycle fatigue behavior is studied.The principal research contents and achievements are as follows.(1)A total of 12 specimens are designed to investigate the effects of normalized flange’s width-to-thickness ratio,normalized slenderness ratio,axial load ratio,and concrete grade on the extremely low-cycle fatigue behavior of the specimens.The size of specimens,loading device,loading pattern and measurement scheme are introduced into details.The results of finite element pre-analysis show that the designed experimental scheme can achieve the expected research purpose.(2)The results of extremely low-cycle fatigue test of the partially concrete-filled steel bridge piers are analyzed.By correcting the initial stiffness of lateral load-displacement hysteretic curves,the actual lateral load-displacement hysteretic curve of the specimens are obtained.The failure mode of specimens is characterized by ductile failure.Although extremely low-cycle fatigue failure and local buckling occur for all specimens,the specimens are finally dominated by extremely low-cycle fatigue failure.The fatigue crack propagation of the specimens can be divided into the following three stages,which are pre-crack initiation,stable crack propagation and rapid crack propagation,respectively.It was not until the rapid crack propagation stage that the bearing capacity of the specimens began to decrease significantly.After the initiation of extremely low-cycle fatigue crack,compressive strain of uncracked region in the middle of flange increases rapidly.The initiation and propagation of extremely low-cycle fatigue crack will delay the occurrence of local buckling.With the increase of the ultimate displacement amplitude,the damage of the concrete adjacent to the flanges becomes more serious.(3)The test results show that the larger the normalized flange’s width-to-thickness ratio is,the more obvious the local buckling becomes,and the more severely the concrete damages.Both the extremely low-cycle fatigue crack initiation life and the local buckling occurrence cycle of the specimens tend to decrease with the increase in the normalized flange’s width-to-thickness ratio and the normalized slenderness ratio.The larger the normalized flange’s width-to-thickness ratio is,the smaller the maximum bearing capacity becomes,and the worse the ductility and energy absorption capacity of specimen are.The normalized slenderness ratio has little effect on the maximum bearing capacity,but the specimen with larger normalized slenderness ratio has worse ductility and energy absorption capacity,slower stiffness degradation.With the increase in axial load ratio,the extremely low-cycle fatigue crack initiation life of the specimen increases,and the local buckling occurs earlier.The maximum bearing capacity and stiffness degradation of the specimens are kept almost the same with different axial load ratio.When the concrete grade of the specimen is reduced,the maximum bearing capacity decrease,and the ductility is improved.The specimen in-filled by C20 concrete has slower stiffness degradation,more serious concrete damage and worse energy absorption capacity.(4)By comparing the lateral load-displacement hysteretic curves,strain and failure mode of the test with those obtained from numerical simulation,the validity of the proposed numerical simulation method is verified.It is concluded that the non-local damage method of Ge model with modified coefficient proposed in the thesis can accurately predict the extremely low-cycle fatigue crack initiation life of partially concrete-filled stiffened steel box-section bridge piers.