Seismic Performance Of Damaged Reinforced Concrete Column Retrofitted By Steel Jacket

With the in-depth study of performance-based seismic design theory,the damage assessment method and retrofit technique of reinforced concrete(RC)structures after earthquake have been further developed.Tests and field investigation show that the damage of RC frame structure is mainly concentrated in the plastic hinge area at the bottom of the column on the first floor,especially the column members controlled by flexure and flexure-shear.In order to rapidly retrofit earthquake-damaged RC columns and restore the safety of RC structures,a method of using steel jacket(circular steel jacket and square steel jacket)to locally repair the plastic hinge failure area at the bottom of the earthquake-damaged RC column with flexural failure or flexural-shear failure is proposed in this paper,which is characterized by using the excellent performance of steel jacket confined concrete to restore the degraded confining stress and shear strength of the damaged column.The steel jacket extends to the bottom of the column,which is different from the construction detail of steel jacket confined concrete with a gap at the bottom,and the construction activity is simple and rapid.The finite element analysis and seismic test of the damaged RC column retrofitted by steel jacket are studied in this paper.The main research work and conclusions are as follows:(1)Based on the regression analysis of the experimental results of reinforcing steel,the relationship between unloading stiffness and plastic strain,and that between the parameter controlling Bauschinger effect and plastic strain are established.The buckling model and the buckling length of longitudinal reinforcement in the RC column are considered.The relationships between the low-cycle fatigue damage,strength degradation and plastic strain under cyclic loading are established,and the influence of the buckling effect is taken into account.A modified uniaxial constitutive model of reinforcing steel considering stiffness degradation,strength degradation,fatigue damage and buckling effect is proposed,which is suitable for finite element analysis of damage and retrofit of RC columns.(2)Based on the regression analysis of the experimental results of concrete,the equations for the elastic modulus and the corresponding strain of the peak stress in the backbone curve of plain concrete are established.Based on the test data of confined concrete,the equations of peak stress and corresponding strain of confined concrete are established.Based on the test data of concrete under monotonic loading and hysteretic loading,the equations for the control parameter of the backbone curve,unloading plastic strain and tangent stiffness of zero point after unloading in the hysteretic model are established.A modified uniaxial constitutive model for plain concrete and confined concrete considering damage progress is proposed,which is suitable for finite element analysis of damage and retrofit of RC columns.The equation of peak stress for confined concrete is suitable for the concrete confined by stirrup and steel jacket,which is verified by the experimental data.(3)The proposed modified constitutive models of reinforcing steel and concrete are implemented into Open Sees,an open source finite element software framework.By comparing the computational results with the test results,it shows that the proposed modified reinforcing steel model can accurately simulate the unloading stiffness and the influence of Bauschinger effect in the cyclic loading test.The computational results considering buckling effect and lowcycle fatigue damage effect are more consistent with the test results,which verifies the rationality of the proposed modified reinforcing steel model.By comparing the computational results of concrete with the experimental results,it shows that the proposed modified concrete model can accurately simulate the backbone curve of plain concrete and confined concrete with different compressive strengths,and the hysteretic curve of confined concrete under different confining stresses,which verifies the rationality of the proposed modified concrete model.Based on the proposed modified models of reinforcing steel and concrete,the finite element analysis results of RC columns are in good agreement with the test results.(4)Low cyclic loading tests of 2 original RC columns and 12 retrofitted specimens of damaged RC columns are carried out.The effects of pre-damage,retrofit methods(circular steel jacket,square steel jacket or enlarged section)and construction detail at the bottom of steel jacket on the seismic performance of damaged RC columns after retrofit are investigated.The failure mode,hysteretic curve,bearing capacity,ductility,energy dissipation capacity,stiffness degradation of the original and retrofitted specimens of RC columns are analyzed.The results show that the seismic performance of damaged RC columns retrofitted with circular steel jacket is the best,followed by square steel jacket,and the strengthening effect of enlarged section is the lowest.The steel jacket without gap at the bottom shows a better convenience for construction and a better seismic performance.(5)The finite element analysis of damaged RC columns retrofitted with steel jacket is carried out by using the modified reinforcing steel and concrete models proposed in this paper,and the results are compared with the seismic test results.The results show that the proposed modified reinforcing steel and concrete models can better evaluate the material damage state of RC columns under different pre-damage degrees,and obtain the initial damage properties of the damaged materials.Compared with the finite element analysis results of existing simplified or complicated reinforcing steel and concrete models,the computational results based on the material models proposed in this paper are in better agreement with the seismic test results.The proposed modified constitutive models of reinforcing steel and concrete considering the damage process and the repair method of earthquake-damaged RC columns retrofitted with steel jacket can provide theoretical and experimental basis for the development of performancebased post-earthquake evaluation and retrofit for buildings.