Study On Seismic Performance Of Flexural-shear Failure RC Circular Column Considering Number Of Loading Cycles

Reinforced concrete column components(including frame columns,bridge piers,piles,etc.),are the main load-bearing members of the structure.Under the strong earthquake action,low-ductility flexural-shear failure(or shear failure)occurs easily because of the improper shear and seismic resistance measures.In order to avoid the unfavorable failure mode of reinforced concrete columns in earthquakes,the current Code for Seismic Design of Buildings(GB 50011-2010)adjusts the coefficient of enlargement of column end moment and the limit value of axial compression ratio on the basis of the original standard,but there are still some imperfections,especially for the calculation of shear capacity of circular columns and analysis of seismic performance,such as the lack of the model directly used to calculate the shear capacity of the cylinder,the insufficient understanding of the seismic failure mechanism of the flexural-shear failure reinforcement columns,the influence of different loading cycles on the failure mode and the seismic performance of the columns.In order to solve the above problems,the pseudo-static test of reinforced concrete columns subjected to flexural-shear failure under different loading modes(mainly considering the number of cyclic loading)is conducted to study the influence of loading mode and cycle times on the seismic performance and shear capacity of the columns.The main contents and results are as follows:(1)8 specimens of reinforced concrete columns with different shear span ratio are subjected to monotonic loading and repeated loading of different cycles,and the effects of loading mode and cycle times on the shear capacity,ductility,hysteresis curve,hysteretic energy dissipation,equivalent viscous damping,stiffness degradation and various deformation components of cylindrical specimens are analyzed.The results show that different loading cycles have significant influence on the shear capacity and seismic performance of reinforced concrete circular columns.As the number of cyclic loading increases,the cylindrical specimens are more prone to flexural-shear failure,the proportion of shear deformation after yielding is obviously increased.Compared with monotonic loading,the shear capacity and the deformation capacity of the column under cyclic loading degrade significantly with the increase of the number of cyclic loadings;and energy dissipation capacity of the column under cyclic loading is cyclically degraded due to damage accumulation,and it tends to be obvious with the increase of the number of cycles and the displacement amplitude,especially after the peak load.(2)Based on the pseudo-static results of reinforced concrete columns in this paper and the PEER test data,the seismic failure mode and shear capacity of reinforced concrete columns are analyzed.Then,on the basis of the design specifications of concrete structures in China,the calculation model of the shear capacity of reinforced concrete column with the influence of plastic deformation(displacement ductility)is put forward.On this basis,the influence of the number of loading cycles on its shear capacity is taken into account.The results show that under horizontal loads,the shear capacity of RC columns decreases with the development of plastic deformation.The shear capacity of reinforced concrete columns subjected to flexural-shear failure depends on the load corresponding to the shear failure,rather than the peak load of the skeleton curve.The proposed model fully considers the influence of plastic deformation and cyclic loading on the shear strength,and the calculated results agree well with the experimental data.It can be used to determine the seismic damage mode and calculate the shear capacity of reinforced concrete circular columns.(3)Based on the test results of 8 flexural-shear failure reinforced concrete columns under monotonic loading and cyclic loading of different loading processes,the influence of loading cycle times and displacement amplitude on flexural-shear failure of reinforced concrete cylindrical skeleton curve is analyzed,and the calculation formula of strength degradation rate is obtained.The method for calculating the skeleton curve of a flexural-shear failure column considering the number of loading cycles and displacement amplitudes is proposed.According to the characteristics of the hysteretic curve of the flexural-shear failure column,a hysteretic model of the bending shear failure column considering the pinch effect and different loading history is established.