Study On Seismic Behavior Of RC Columns In Flexural-shear Failure

Reinforced concrete column is the most fundamental lateral force-resisting component in RC structures.In performance-based design,predicting the seismic performance of columns is the first step to achieve a specific performance-based design level.RC column may fail in flexural-shear failure under seismic loadings,and its seismic behavior is significantly different from the columns in flexural failure mode.In this study,seismic behaviors of reinforced concrete columns in flexural-shear failure are investigated by experiments,finite element simulation and theoretical analysis.The main contents of this study are summarized as follows:(1)Eight seismic tests were performed for large scaled RC rectangular columns that focused on flexural-shear failure,the test parameters were the shear span ratio,axial-compression ratio,hoop spacing and loading history.When the second-order effect due to axial compression is neglected,the backbone curve of a flexural-shear-governed column exhibits strength deterioration after the peak strength,which is obviously different from the typical backbone curve of a flexural-governed column.The improved measurement method using statically determinate truss composed of LVDT transducers is able to measure the nodal-displacement vectors accurately based on the deformation compatibility condition,and the overall deformation diagrams of the column body were drawn.The shape of overall deformation of the flexural-shear critical columns,which are significantly affected by shear force and shear deformation,were transformed from the shape of flexural deformation to shear.(2)Deformation contribution of flexural,shear,and bond-slip behaviors were separated from the total deformation based on the nodal-displacement vectors,and the influence of each deformation component on the seismic behaviors of the column was systematically studied.It was found that the shape of hysteresis curve,energy consumption capacity,and the rate of stiffness degradation were discriminative in each deformation component.Energy dissipation capacities contributed by flexural,bond-slip,and shear behaviors were evaluated according to test parameters,and the effect of specific test parameter on the energy dissipation was not significant.Energy dissipation capacity contributed by bond-slip behavior can be ignored.The bond-slip deformation of columns reduces the ductility and have a negative impact on the seismic performance of columns on account of the changes in the contribution of bond-slip deformation.According to the change laws of stiffness degradation of each deformation component,a Method based on the proposed damage index related to stiffness degradation of each deformation component was developed to identify column failure mode.The prediction results are basically tallies with the experimental phenomena.(3)The calculation results from the plastic hinge formula,Uniaxial Shear-Flexure Model(USFM),and Axial-Shear-Flexure Interaction(ASFI)method for reinforced concrete columns under different loadin g histories are compared.The plastic hinge formula based on section analysis failed in predicting the degradation process of the flexural capacity of RC columns affected by shear force and deformation.The correction of the compression softening of concre te in Modified Compression Field Theory(MCFT)can lead to flexural capacity of the simulated columns degradation with the increase of the loading displacement,but the ASFI and USFM methods overestimate the degradation rate of the flexural capacity.Numerical simulation to all the tests of RC columns is conducted using nonlinear FE analysis software VecTor2 based on MCFT and Disturbed Stress Field Model(DSFM),and the prediction curves are tallies with the experimental curve well.According to the test data of displacement components,a further verification of simulated curves is conducted.Except for the bond-slip displacement component,the amount and trend of stiffness variation in each displacement component simulated by VeTor2 is very close to the test.While,the shape of shear displacement hysteretic loops is basically symmetrical in simulation,which is different from the test results.(4)During the cyclic loading,the cover concrete in the plastic hinge zone was divided by the shear cracks,losing the bonding to the hoops in progress,and the accumulated residual strain of hoops was significant.The transverse deformation of the plastic hinge zone was not affected by the change of the loading direction,and increased with the increase of the accumulated lateral drift defined as the sum of the absolute values of the lateral drift in the both loading direc tions.According to the tendency of the lateral deformation increment,the lateral strength degradation is related to the lateral drift for columns subjected to monotonic loading,and the accumulated positive or negative lateral drift for columns subjected to cyclic loading.A damage index based on lateral deformation is proposed and verified in predicting capacity degradation for RC columns.(5)Shear demand of identical columns under different test setups are not the same due to the variety of the degrees of P-Δ effect,and the tendency of the influence of P-Δ effect on the ultimate deformation capacity of RC columns is opposite on columns failed in flexural failure mode and flexural-shear failure mode.Based on the energy balance approach,one reasonable explanation on the mechanism in the relationship between the confinement and shear resistance of lateral reinforcement was put forward.The relationship between the strength degradation of column and the energy dissipation of shear deformation and the second order effect is established and proved by test data.A formula based on the energy dissipation of shear deformation and the second order effect to estimate the ultimate displacement of RC columns is derived,and the predictions gave a good agreement with the test results.