Study On Longitudinal Bar Buckling And Shear Behavior Of Reinforced Concrete Columns

Longitudinal bar buckling and shear failure are common failure modes in reinforced concrete columns under earthquake,both of which can lead to the reduction of seismic performance and post-earthquake repairability of reinforced concrete(RC)columns,and need to be be emphasized in the structural design,analysis and assessment.The current research has paid insufficient effort on the longitudinal bar buckling of column,there is no unified conlusion on the influence factors of bar buckling,and the simulation studies on the seismic performance of columns considering the longitudinal bar buckling are incomplete.Moreover,most of the current codes adopt simple construction measures to prevent bar buckling,but lacking of the relevant material constitutive model and force analysis model for calculation.In addition,although many studies have been conducted on the shear behavior of columns,there still exist some problems which require further improvement,such as the analysis of axial-shear-flexure interaction and the calculation of the shear capacity contribution of longitudinal reinforcement.In this dissertation,the buckling analysis and constitutive model of reinforcing bar,the seismic performance analysis of reinforced concrete column considering longitudinal bar buckling and shear effects,and the calculation of shear capacity of reinforced concrete column are studied.The main contents are as follows:(1)Based on the mechanical characteristics of monotonic compressive buckling of reinforcing bars,a buckling analysis method for reinforcing bar under compression is established through theoretical derivation,then it's verified against the experimental data of reinfinforcing bar buckling carried out in this dissertation and the previous studies.The influence of the bar length-to-diameter ratio,the yield strength,and the ultimate strength-to-yield strength ratio on the buckling behavior of reinforcing bars is studied by using the proposed method,based on which a constitutive model of monotonic compressive buckling of reinforcing bar is established.The results show that with the increase of the bar length-to-diameter ratio,the will obvious stress softening effect can be produced in reinforcing bars due to compressive buckling,and the decrease of yield strength and the increase of the ultimate strength-to-yield strength ratio will slow down the decrease of stress bearing capacity of buckling bars.The influence rule of relevant factors can be reasonably described by the proposed constitutive model.(2)By introducing a strain-dependent post-yield stiffness ratio formula into the traditional Bouc-Wen model,a hysteretic constitutive model for reinforcing bar under under the cyclic load of tension and compression is developed.The hysteresis characteristics of the reinforcing bars,such as the stiffness degradation,strength degradation,and compressive stress softening effect,can be described effectively using the proposed model.Moreover,a calculation flowchart based on the Newton-Raphson method is introduced to solve the proposed model and the genetic algorithm is used to analyze the collected experimental hysteresis loops of reinforcing bars to identify the model control parameters.Then the corresponding prediction equations of control parameters in terms of physical parameters of the reinforcing bar are established based on the results of identification and theoretical analysis.Accuracy of the proposed hysteretic constitutive model and parameter prediction equations is verified through the comparison of results between calculation and experiment.(3)Based on the hysteretic constitutive model of reinforcing bar proposed in this dissertation,the formula of tangent modulus of material is derived.Then the uniaxial material model of reinforcing bar is implemented in OpenSees computing platform by using C++,and the validity of the model is verified against the experimental data.Nonlinear fiber beam-column element models are established,through which the influences of longitudinal bar buckling on the seismic performance of reinforced concrete columns and frame structures are studied.It's found that the longitudinal bar buckling will reduce the lateral bearing capacity of columns to a certain extent,which is related to the axial compression ratio,shear span ratio,volume stirrup ratio,longitudinal reinforcement ratio and other parameters.The buckling of longitudinal bars in columns will aggravate the interstory drift response of frame structures under strong earthquake.(4)Based on the traditional fiber section analysis method,modified compression field theory is introduced to describe the shear behavior of reinforced concrete columns.The whole loading process of the column is divided into two stages,i.e.,bending-control stage and shear-control stage,and the tension and compression zones of the control section are analyzed separately,considering the effect of compressive buckling of longitudinal reinforcement and the P-? effect,then a load-deformation analysis model of reinforced concrete columns under axial-shear-flexure interaction is proposed,and it's effectiveness is verified against the collected experimental data.It's found that the force mechanisms of RC columns under different failure modes are quite different,and the proposed model can be used to reasonably analyze the load-deformation performance of RC columns.(5)Based on the cross-section force equilibrium and deformation compatibility analysis,the shear capacity of reinforced concrete columns under different displacement ductility conditions is calculated and the effects of axial compression ratio,shear span ratio,stirrup ratio and longitudinal reinforcement ratio are investigated.Using the collected experimental data,a prediction equation of the shear capacity of flexural-shear critical reinforced concrete columns is proposed,which makes up for the shortcoming of inadequate consideration of the shear contribution of longitudinal reinforcement in previous studies.It's found that the shear bearing capacity of RC columns is influenced greatly by the above parameters.Compared with the prediction equations of the existing codes and researches,the proposed equation further considers the influence of the longitudinal reinforcement,and its calculation results are better consistent with the experimental data.