Reinforcement Design And Experimental Study Of Variable Section Joints Based On STM Model

Under the action of earthquake,the beam-column joint needs to bear a large shear force,which is the part of the structure that is easy to be damaged and destroy early.In the isolation structure,the part from the bottom column of the frame to the isolation bearing above is generally called the upper support pier.Due to the requirements of isolation bearing installation,the cross-section area of the upper pier is usually larger than that of the frame column,When the height of the upper support pier is low,it is approximately equivalent to the beam-column joint with variable cross section.Therefore,in the isolation structure,the connections between the support pier and the bottom column of the frame are mostly variable section nodes.At present,the design of engineering in China is usually considered according to the eccentric compression components,However,the section size of the upper support pier is large and the height is low.Under the joint action of the column end load and beam end load,the compressive stress of the upper support pier forms the force transmission mechanism of "oblique compression bar" inside,which should not be considered as the eccentric compression member.The stress of such variable section joints is relatively complex,and the internal strain of the member does not conform to the assumption of flat section,However,China generally according to the eccentric compression member(section internal force method)design.,So there is neither a reasonable calculation model nor a clear theoretical guidance for the design of such variable section beam-column joints.In view of this problem,the main research contents of this paper were as follows:(1)The finite element simulation was carried out for the variable section beam-column joints to analyze the force transfer mechanism of the variable section joints under the changes of the axial compression ratio of the column end,the stirrup ratio in the core area,the longitudinal reinforcement ratio and the concrete strength of the member.It was found that the compressive stress transmitted by the joint column end acts together with the compressive stress transmitted by the beam end,and the compressive stress would be transmitted diagonally down the column inside the joint,forming a "baroclinic bar".The increase of axial compression ratio would make the phenomenon of baroclinic bar more obvious,while the increase of concrete strength would weaken the phenomenon of baroclinic bar.(2)Based on the analysis of the force transfer mechanism and the stress trace of the variable section joints under the action of load,the calculation model for the design of the variable section joints was established based on the STM model theory.According to the calculation results of the model,three different design ideas were given.The first design idea of reinforcement "summation method",In this method,all the forces on the tie bar in the model were regarded as the forces that should be borne by the horizontal reinforcement in the joint area,and the forces on each bar were superimposed to carry out reinforcement design;The Second Design Idea "Maximum Method",The reinforcement was arranged according to the internal force of the bar at the maximum axial force in the joint area.;The third design idea "division design method",the reinforcement was arranged according to the axial force at the position of each bar in the tension-compression bar model in the joint area.(3)The traditional design method and three design methods based on STM model were reinforced and the components are designed,Quasi-static loading tests were carried out on these four components to study the seismic performance of ductility,stiffness degradation,skeleton curve and energy dissipation capacity of each component.The experimental results showed that the components designed by the "division design method" proposed in this paper had high energy dissipation capacity,strong deformation capacity,gentle stiffness degradation,and better aseismic performance than other components.Therefore,the "division design method" based on STM model proposed in this paper was more in line with the engineering practice.