Research On Seismic Performance And Design Method Of Reinforced Concrete Circular Columns Locally Strengthened By Using Near-surface-mounted Reinforcement

Reinforced concrete(RC)columns,which are the most vulnerable elements in a structure,are the main components of structures to resist lateral force and their failure will result in catastrophic consequences.The higher request of the seismic capacity of the existing structures in our country is put forward,and the seismic strengthening is required to meet the new seismic requirements.The existing research mainly focused on the enough ductility of the structures when an earthquake occurred to the strengthened structures,namely the energy dissipation of the strengthen structures.However,the residual performance of the strengthened structures after suffering from earthquakes or the recoverability of the strengthened structures with damage is neglected.This thesis adopts the near-surface-mounted(NSM)basalt fiber reinforced polymer(BFRP)bars and external BFRP sheets jacketing composite strengthening technique(hereinafter referred to as composite strengthening technique)to strengthened RC circular columns.The purpose of the strengthening technique is attempt to arrive in good matching of the stiffness and ductility of the strengthened RC columns and also is attempt to reduce the residual displacement of the strengthened columns under earthquakes to enhance their reparability.Thus,this thesis carried out quasi-static test on ten strengthened columns,in which four damaged columns are strengthened by the composite strengthening technique and the other six existing columns are strengthened by the composite strengthening technique.The effect of the various ratios of NSM reinforcement,different NSM materials,various styles of the wrapped sheets and various fiber volumetric ratios on the flexural capacity,seismic behavior and the residual displacement of the strengthened columns is systematically studied.The main research contents and corresponding results are summarized as follows:(1)Due to the deficiency of the existing damage model,this thesis proposed a modified damage-accumulation model of material based on displacement by taking into account the effect of damage accumulation under seismic loading.The proposed damage-accumulation model was used to modify material constitutive models of concrete and steel rebars to simulate the existing damage in the earthquake-damaged columns repaired by NSM reinforcement technique.The numerical values were compared with the experimental results.The results show that the modified damage-accumulation model can effectively calculate the residual strength and stiffness of the concrete and steel rebars in the RC columns after earthquake.Then,a computational model,in which the developed damage-accumulation models were integrated into the OpenSees finite element program to modify material constitutive models of concrete and steel rebars,was developed for simulating the earthquake-damaged columns repaired by NSM reinforcement technique.(2)Based on the modified damage-accumulation model,a quantitative formula of the NSM reinforcement technique is proposed for repairing the damaged columns.The numerical results show that the flexural strength of the damaged columns can be recovered by using the amount of NSM reinforcement calculated by the proposed formula.Furthermore,the efficiency of proposed quantitative formula is verified by the experimental data.(3)Ten strengthened circular columns were tested under cyclic load to verify the applicability and reliability of the composite strengthening techniques,which is NSM reinforcement and external BFRP sheets jacketing composite strengthening technique.The effect of various styles of NSM materials and the wrapped sheets,the various ratios of NSM reinforcement and various fiber volumetric ratios on the seismic behavior of the strengthened columns was systematically studied.In addition,the residual displacement of the existing columns strengthened by the composite strengthening technique was studied.The results show that the flexural capacity of the repaired columns was restored and enhanced;especially the NSM BFRP bars and external unidirectional BFRP sheet jacketing composite strengthening technique is the most effective in reducing the residual displacement of the strengthened RC columns,and enhancing the ductility and deformation capacity of the columns,but do not change the stiffness of the strengthened columns.In this way,the good matching of the stiffness and ductility of the strengthened RC columns can be realized.(4)In order to facilitate the application of NSM BFRP bars and external unidirectional BFRP sheet jacketing to practical engineerings,this thesis studied the calculation method of bearing capacity of strengthened RC circular columns.A formula of normal section bearing capacity considering the hoop confinement of BFRP sheet was developed for the existing RC circular columns strengthened by NSM BFRP bars and external unidirectional BFRP sheet jacketing,and the whole dimensionless m-n curve is obtained at the range from pure bending to compression for designers in structural design and checking.The developed formula is applicable to calculating the normal section bearing capacity for the strengthened columns and RC columns without strengthening.(5)In terms of residual behavior of the existing RC circular columns strengthened by NSM BFRP bars and external BFRP sheets jacketing under seismic loading,the quantitative relation between the amount of NSM BFRP bars and residual displacement of strengthened columns after earthquake was established and the corresponding calculation formula of decreasing proportion for residual displacement of strengthened columns was proposed.These were based on the analysis of the effect of NSM reinforcement ration on the decreasing proportion of residual displacement.The proposed formula can easily and effectively calculate the decreasing proportion for the residual displacement of strengthened columns,and also calculate the increasing proportion of the recoverability of strengthened columns after earthquake.Finally,design suggestions of NSM BFRP bars and external BFRP sheets jacketing are proposed for the application of practical engineering.