Research On Seismic Displacement Performance Of Flexural And Flexural-shear Damaged RC Columns Strengthened With FRP Strips

A large number of seismic damage investigations show that not all reinforced concrete columns have flexural failure with good deformation capacity.Some columns have shear failure with poor deformation capacity and flexural-shear failure with certain deformation capacity.Among them,flexural-shear failure is considered as an acceptable failure mode by many scholars.Therefore,it is necessary to study the failure mechanism and deformation capacity of this failure mode.Firstly,five RC cantilever columns with different stirrup ratios were designed for low-cyclic loading test.Under the condition of constant longitudinal reinforcement ratio,by controlling the amount of stirrups,the component presented one shear failure,three bending shear failure and one bending failure.The development,failure process and final failure mode of bending cracks and shear cracks of RC columns with different failure modes are analyzed.In order to further explore the change of displacement performance of RC columns with different failure modes before and after reinforcement,five reinforced columns with basalt fiber reinforced polymer strips were designed by selecting typical shear failure,bending-shear failure and bending failure columns,and low-cycle cyclic loading tests were carried out respectively.It is found that when the reinforcement amount was the same,the strip reinforcement had the best effect on the ductility energy dissipation index of shear failure columns.For shear failure columns,it was found that the improvement of ductility energy dissipation index of shear failure columns by three-layer and two-layer strip reinforcement was not significantly different.ABAQUS finite element software was used to analyze the influence of axial load on the displacement performance of flexural-shear failure columns before and after reinforcement.It was found that with the increase of axial compression ratio,the deformation capacity of columns before and after reinforcement was significantly reduced,but the peak bearing capacity was slightly increased.The improvement ability of strip reinforcement on the failure mode of columns was gradually weakened.When the axial compression ratio is between 0 to 0.5,the improvement effect of strip reinforcement on column ductility and energy dissipation capacity was obvious.When the axial compression ratio exceeds 0.5,the improvement effect of strip reinforcement on column ductility and energy dissipation capacity was poor.The relationship between the failure mode and ductility coefficient of the test column and the simulated column before and after reinforcement was comprehensively analyzed.It was found that the ductility coefficient of the bending shear failure column before reinforcement was between 2 to 4,and the ductility coefficient of the bending failure column was greater than 4.After reinforcement,the ductility coefficient of flexural-shear failure column was between 2 to 7,and the ductility coefficient of flexural failure column was greater than 7.Combined with the failure mode and test data of the column,the plastic hinge length of the column was calculated by using the ’ longitudinal reinforcement yield zone method ’,’ concrete crushing zone method ’ and ’ sectional curvature method ’.It was found that the plastic hinge length of the column before reinforcement was between 0.7h to 1.5h,and the plastic hinge length increased gradually with the increase of stirrup ratio.The plastic hinge length of bending failure column was about 1.7h.After reinforcement,the plastic hinge length of bending shear failure column is between 0.6h to 1.7h.The plastic hinge length of columns before and after reinforcement decreased with the increase of axial compression ratio.In order to explore the relationship between the ductility of reinforced columns and the amount of strips,considering the influence of column section shape and strip spacing on the strength of fiber cloth,the calculation method of effective constraint coefficient of BFRP strips was given,and the calculation formula of column displacement ductility coefficient with stirrups and strip reinforcement as the main parameters was deduced.