Study On Hysteretic Behavior And Strain In Panel Zone Of Composite Concrete Filled Steel Tubular Joint With External Stiffening Ring

Composite concrete filled steel tube(CFST)is a new type of structure developed on the ordinary concrete filled steel tube.It has the advantages of high bearing capacity,good plastic toughness,convenient construction,remarkable economic benefits and broad application prospects.The research of composite CFST joint mostly refers to the concrete filled steel tube joints and improves them.CFST joint.with external stiffening ring have the advantages of uniform stress distribution,stiffness,plasticity,bearing capacity,and good seismic performance.It is also the main joint form recommended by the Technical Specification for Rectangular Steel Tube Structure and Technical Specification for Concrete-filled Steel Tube Structure.Therefore,this paper intends to study on the external stiffening ring between composite CFST column and steel beam joint.This paper conducts an experimental study on composite CFST joint specimens based on digital speckle correlation method(DSCM),obtain the bending moment-beam column relative angle(M-?)hysteresis curve,the panel zone shear force-shear angle(V-?)hysteresis curve and the evolution process of deformation field in the panel zone of the joint,analyzes the bending,shear resistance and seismic performance of the joints such as ductility,stiffness and energy consumption.After that,the yield mechanism of the panel zone of the joint was studied by ABAQUS finite element software with the beam-column line stiffness ratio.Research indicates that:(1)Bending moment-beam column relative angle(M-?)hysteresis curve and the panel zone shear force-shear angle(V-?)hysteresis curve based on DSCM shows good energy consumption and late deformation.According to the initial rotational stiffness,the CFST joint with external stiffening ring belongs to a semi-rigid joint,but still belongs to the strong joint category.Shear deflection in panel zone of CFST joint contributes larger to relative column deformation of beams and columns,the influence of the shear deformation of the joint domain on the structural deformation cannot be ignored,and shear strain distribution is inclined at 45°.The shear strain at the joint of the specimen with beam bending failure develops slowly with the increase of load,and the shear strain in the joint of the specimen with column-end bending failure develops rapidly.Ultimate shear strains of joint with column-end bending failure are 2 to 3 times of ultimate shear strains of joint with beam-end bending failure.(2)Bending moment-beam column relative angle(M-?)hysteresis curve and the skeleton curve of ABAQUS finite element simulation agrees well with the test results,the difference in ultimate bending capacity is within 15%.The failure mode of the joint is greatly affected by the beam-column line stiffness ratio.The axial compression ratio has a greater influence on the ductility,has a relatively small effect on the failure mode.The joint with smaller beam-column line stiffness ratio,the failure mode is dominated by the bending of the beam end,resulting in the destruction of the plastic hinge of the beam end;the joint with larger beam-column line stiffness ratio,the failure mode is dominated by the bending of the column end,resulting in the destruction of the plastic hinge of the column end.The strains in panel zone are concentrated in the diagonal area and there are the weak areas in panel zone.(3)As the line stiffness ratio of the beam-column increases,the initial stiffness of the joint increases,Flexural bearing capacity also increases with increasing stiffness ratio and remains constant after reaching a certain value.The results of tests and finite element analysis show that the composite CFST joint with external stiffening ring have good strength,rigidity,deformation ability and energy consumption capability.Meet the seismic requirements of strong j oint-weak component.