Study On Mechanical Properties And Mechanism Of Steel Frame Outer Shell Joints

As the important hinge of force transferring in the steel frame system,the beam-to-column joints should meet the requirements of seismic performance such as strength,stiffness,so that the components can work together and ensure the effective transmission of the load.Square steel tube column and H-steel beam outer shell joints have good seismic performance,it is very important to study the mechanical properties and the design method of the new joints.In this paper,the low-cycle reciprocating load tests of the outer-shell joints were carried out,and the numerical analysis and theoretical calculation were combined to do a systematic research.The main research contents and results are summarized as follows:1.The low-cycle reciprocating loading tests of eight new-type square steel column-H steel beam outer shell joints were carried out,the joints are connected by T-stub and full bolts.The test parameters include the section size of beam-column joint and whether pouring concrete in the column.The mechanical properties of moment-angle hysteresis curves,skeleton curves,energy dissipation capacity etc under the action of reciprocating load are analyzed.The results show that the bearing capacity of the outer shell is high,the hysteresis curves are plentiful,the stiffness and strength of the joints are stable and the ductility is good.The cross section size of beam-column joints and whether pouring concrete in square steel column have great influences on the bending capacity and the initial stiffness of the joints.2.Based on the hysteresis curves and the skeleton curves obtained from the tests,the hysteresis rules and the restoring force characteristics of the outer-shell joints were analyzed.The triangular line skeleton curves of the outer shell joints were obtained by the test fitting method and dimensionless processing,and the data of the specimen under the reciprocating load was studied by regression analysis,then the stiffness degeneration laws of different stages were obtained.On this basis,a three-fold restoring force model suitable for the outer shell joints was proposed.By comparing with the experimental curves,the rationality of the proposed restoring force model is verified,which provides theoretical references for the nonlinear seismic response time history analysis.3.The finite element analysis model of the outer shell joints under low cyclic reciprocating load was established.The model considered the non-linearity of the material and geometry,the interaction between the bolt and the plate,as well as the steel and the concrete,the method of unit selection,boundary condition and meshing was given.Compared with the experimental results,the failure modes,hysteresis curves and skeleton curves of the joints confirm the validity of the finite element model.On this basis,the stress distribution of the components in the nodal domain is analyzed.4.Based on the experimental results and the numerical model,the failure modes of the outer shell joints and the characteristic points of the moment-angle hysteresis curves of the whole stress process were analyzed,and the shear transfer path of the nodal area was discussed.The mechanical properties of the joints under the materials,geometrical and load parameters were studied systematically.The effects of the parameters such as the yield strength of the steel,the stiffness ratio,the bending capacity ratio and the parameters of nodal zone on the impact of failure modes and moment-angle relationships were analyzed,which laid the foundation for the reasonable values of outer shell joints in the design.5.Based on the component method in the EC3 specification,the outer shell joints were split into multiple components,and the T-stub tensile stiffness,the column flange tensile stiffness and the shear stiffness of the column web were deduced respectively.The effective components were transformed into a simplified mechanical model composed of a series of springs and rods.Then,the formula for calculating the initial rotational stiffness of the joint was derived.It is proved that the mechanical model of the outer shell joint is simple and effective.From the principle of component method,it can be seen that the bending bearing capacity of joints is determined by the constituent elements,and is controlled by the component with the least carrying capacity in the process of damage.On this basis,the calculation method of the bending capacity of the joints is put forward,which is in good agreement with the experimental results.Therefore,under the premise of the basic design information,the initial bearing capacity and the failure mode of the outer shell joints can be preliminary estimated.6.In order to achieve the performance-based seismic design of the steel frame structure,the performance-based design idea was referred to the quantitative research of the performance index of outer shell joints.Referring to the division of structural performance of seismic code in China and the FEMA356 specification,the performance levels of the joints were divided into three stages:Immediate Occupancy,Life Safety,Collapse Prevention.The inter-layer displacement angle was used as the index parameter of the quantified structure performance level.Combined with the current domestic and international codes for the deformation limit and related research results,the recommended values of performance indexes for steel frames with outer shells under various performance levels were given,and through the deformation capacity calculation model of the joint,the expression of the inter-layer displacement angles of the steel frame under different performance levels were deduced,the relationships between different performance states and the joint performance indexes are established.