The Effects Of Redistribution Of Internal Forces And Buckling Of Steel Bars On Mechanical Performance Of RC Columns

With the rapid economic development and improvement of people's living standards,coupled with the frequent occurrence of earthquake disasters in recent years which have bring about huge economic losses and heavy mental injury to people,people's requirements on the seismic performance of the structures are also getting higher and higher.RC frame and frame-shear structures are the major structural forms,where the columns are crucial vertical load-bearing components and main horizontal lateral-force-resisting components and its performance directly affects the seismic performance of the whole structure.Once the RC structure is loaded,its internal force will be redistributed.And its damage is usually caused by the buckling of the longitudinal reinforcement,which leads to the degradation of the structural load-bearing capacity.The bearing capacity and ductility deformation capacity of the RC structures will be overestimated without consideration of the redistribution of internal forces and the buckling behavior of longitudinal reinforcement,which can bring potential harm to the structure safety.So researchs on redistribution of internal force and buckling of longitudinal reinforcement are imperative.The main contents of this paper are as follows:(1)For the RC columns under axial compressive forces,its failure path and internal force redistribution process were analyzed.Based on the three basic equations,the process of axial internal force transfer was deduced.According to the relative magnitude of the yield strain of the longitudinal reinforcement and the peak strain of the concrete,the internal force redistribution process can be divided into two cases.When the yield strain of longitudinal reinforcement was less than the peak strain of concrete,the corresponding failure would be caused by the yield of longitudinal reinforcement,and the ultimate state was controlled by the ultimate strain of concrete.On the contrary,the failure state is caused by the ultimate strain of concrete strain and the crushing of concrete,and its final failure is controlled by the reinforcement yielding or buckling.(2)The basic mechanical properties of the longitudinal bars in RC columns were analyzed based on the energy method,including buckling length,buckling load and critical stress.Through the comparison of the modulus of the rectangular and circular sections,it was found that the explicit expression of the rectangular section modulus can be used to approximate the implicit expression of the circular section modulus.Through the use of longitudinal reinforcement-spring model,the bucking length and critical bearing capacity were deduced based on distributed elastic support and conclusion was made that the minimum value of the critical buckling load and buckling length has nothing to do with the number of series solution,which is dependent on restraint stiffness implied on the longitudinal reinforcement by the stirrups.Based on steel material model raised by Mander et al,the critical stress curve of buckling of steel bars was deduced,and the critical stress points of buckling were obtained.(3)Based on the deformation assumption of stirrups,the variable restraint stiffness imposed by stirrups on RC columns of circular and rectangular sections under different loads was analyzed.The constraint stiffness can be obtained in terms of the shape of the deformation by solving the geometric equation,the physical equation and the equilibrium equation,which is related to the mechanical parameters and dimensions of the stirrup.The restraint mechanism of longitudinal bars in different positions of rectangular RC column were analyzed and it could be concluded that the corner bars is mainly constrained by the axial deformation rigidity of the stirrup,and the bars in the middle section was constrained by the deformation mechanism of the membrance deflection.Based on the analysis of different types of section,it was deduced that the constraint stiffness of the composite stirrups was obtained by the supposition of the axial stiffness and the membrance deformation mechanism.(4)Numerical simulation was conducted on RC columns considering the influence of longitudinal bars buckling by OpenSEES programs.Based on the model of Zong et al,Pushover simulation of RC columns was conducted.The influence of each parameter was analyzed and compared with those in the paper to verify the simulation.Through the simulation of the seismic action of RC frames,it was concluded that the longitudinal reinforcement buckling accelerates the collapse of the RC frames.(5)In the chloride erosion environment,based on the bearing capacity model of axial compression RC column and the redistribution process of internal force,influence of the mechanical properties of the main reinforcement and stirrups of RC column as well as concrete cross section damage and steel bar buckling behavior were considered,based on the three stages of corrosion,the design models of different corrosion residual design capacity were established.Through numerical examples,the change of residual load of corroded RC column with the corrosion rate was calculated.Reached a conclusion that with the increase of column corrosion rate,the design ultimate bearing capacity of RC column decreased.The attenuation of the bearing capacity of corroded RC columns mainly occurs in the third stages of corrosion,the second stage is less attenuated than the first stage,and the attenuation of the third stage relative to the second stage is nearly half of that,mainly due to rust expansion cracking of protective layer concrete and decrease of RC column cross section,the reinforcing steel loses part of the protective layer concrete,and in the accelerated corrosion stage,so the axial stiffness of the RC column attenuates rapidly.