Study On The Performance Of Joint Element Of Concrete Frame Strengthened By External Prestresing

Because of the historical conditions,the existing large number of safety reserves in China can not meet the requirements of the current seismic design code to varying degrees.To improve the seismic performance of the buildings with lower safety reserves,the only technical way is to reinforce them.In the existing strengthening methods of frame joints(such as enlarging section method,CFRP reinforcement method,FRP reinforcement method,etc.),the reinforcement effect is limited because they can not overcome the spatial obstacles at the joints,that is,they can not directly restrict the circumferential direction of the core area of the joints.External prestressing reinforcement method for transverse piercing of beam end is simple in construction,controllable in quality,able to overcome the space obstacle of reinforcement of frame joints and realize direct reinforcement,and there is no stress lag phenomenon after reinforcement.On the basis of the existing reinforcement technology of reinforced concrete frame joints,a new reinforcement device for reinforced concrete frame joints is proposed,which uses punching and piercing screw at the beam end,prestressing the screw and applying centripetal restraint to the joints(columns),so as to reinforce the joints,overcome the shortcomings of non-uniform restraint in the core area of the existing reinforcement joints and the traditional reinforcement frame.Spatial obstacles caused by the existence of orthogonal beams with frame joints.Aiming at the proposed new joint strengthening device,the bearing capacity,stiffness and seismic performance of the strengthened joints are analyzed and studied respectively.(1)Based on the relevant literature,the influencing factors of joint bearing capacity are analyzed;combined with the characteristics of joint reinforcement devices,the mechanical characteristics of joints before and after reinforcement are analyzed.According to the contribution of screw and arch rod to joint bearing capacity,the calculation formula of joint bearing capacity after reinforcement is put forward by superposition principle.(2)According to the three principles of yield criterion,flow rule and hardening rule in elastoplastic theory,combined with the strengthened joints of angle steel,screw and arch rod combined devices proposed by our research group,and the transmission mechanism and deformation characteristics of joints,the constitutive relationship of concrete and the constitutive relationship of spring and bow rod are established on the basis of the "super element" model proposed by Mitra and Lowes.The stiffness matrix of 8-node plane frame joints strengthened by external prestressing is established according to the deformation relationship,equilibrium relationship and deformation coordination relationship of the joint elements.The stiffness matrix includes plane eight-node stiffness matrix,bond-slip stiffness matrix,shear spring stiffness matrix of joint interface and external screw stiffness matrix.Then the total stiffness matrix of the joint is assembled.It lays a foundation for the secondary development of joints on ABAQUS platform.(3)On the platform of finite element software ABAQUS,the models of reinforced and unreinforced frame joints are established respectively.This paper introduces the modeling process of frame joints from five aspects: material attributes(concrete,steel),component assembly and interaction,element selection and meshing,prestress application,load and boundary conditions setting,and makes technical preparations for seismic performance analysis of reinforced frame joints.(4)On the basis of the above work,eight RC node models are established and the failure modes and seismic performance of the specimens are analyzed by means of reciprocating loading.The hysteretic curves,stiffness degradation curves,skeleton curves,characteristic load values and energy dissipation of the eight node models are obtained respectively.The results show that the plastic strain of the unreinforced joints is concentrated in the joint area,the concrete is buckling,the stirrups of the joints are all yielded and the shear failure of the joints occurs;the plastic strain of the strengthened joints is concentrated at both ends of the beam,the stirrups of the joints are not yielded,and the bending failure of the beam ends occurs in the reinforced models.The seismic performance of the strengthened model is improved significantly,the yield load is increased by 20%-25%,the maximum load value is increased by4%-9%,the ductility coefficient is increased by 22%-27%,and the energy dissipation capacity is increased by more than twice,and the stiffness degradation is gentle.The reinforcing effect of the reinforcing device is remarkable.(5)The influence of different reinforcement parameters(steel strip spacing,prestress and axial compression ratio)on the reinforcement effect is analyzed by monotonic loading method.The results show that with the decrease of the steel strip spacing,the more uniform the stress in the core area of the joint is,the force form of the baroclinic bar gradually disappears;with the increase of the prestress,the initial stiffness of the joint increases obviously;with the increase of the axial compression ratio,the yield load increases slightly,and the joint tends to brittle failure.That is to say,the axial compression ratio of members can not exceed the limit in reinforcement design.