| Steel tubular structures were widely used in industrial buildings, long span public buildings, offshore platforms and bridges because of good performances and beautiful shape. Unstiffened joints were usually used in those types of structures, which were the weakness for the complex load condition. It is well-known that if concrete filled chord is adopted in the steel tubular K-joint, the composite joint can get excellent resistance against the plastic deformation of the chord wall near the weld of the compressive bracing, because the chord wall is now supported by the core concrete. Compared with tubular joints with hollow section chord, joints connecting CFST chord can be different in stiffness, stability, resistance capacity and failure model. Fatigue resistance can be improved as well.This paper mainly concerned with the mechanical behavior of gap circular steel tubular (CFST) K-joints with concrete filled in the chord which are subjected to the axial load predominantly. Finite element analysis modeling was used in this paper.The work performed can be summarized in the following:(1) Finite element modeling (FEM) of concrete filled steel tubular K-joints was carried out with the ABAQUS software platform. Selection of appropriate material properties and consideration of the interaction between concrete and steel tube wall were described in the paper. Decisions on element type, meshing procedure and solution algorithm were also presented. The FEM method was verified against experiments performed by other researchers, and the analysis result showed a good agreement with the experimental data.(2) Parametric studies were performed on CFST K-joints using the FEM method proposed earlier. Typical loading-displacement conditions for the far ends of the K-joints were investigated, and one of them was then selected as the boundary condition of the studied K-joints in the following parametric analysis. The loading path was also defined. The parametric analysis was designed to investigate the influence of the following parameters on the performance of circular CFST K-joints: diameter to thickness ratio of the chord, ratio of the bracing diameter to the chord diameter, ratio of the bracing thickness to the chord thickness, chord's axial load ratio, concrete strength, gap, and joint eccentricity. Six typical failure modes of the composite joint were analyzed in this paper. They are: (1) plastic deformation of the tensile bracing near the welded joint, (2) local buckling of the compressive bracing, (3) punching failure of the chord wall near the weld of the tensile bracing, (4) strength failure of weld toe of the tensile bracing, (5) concrete local bearing failure near the weld of the compressive bracing, and (6) strength failure of the chord.(3) By analysis to the characteristics of typical load-deformation curves of the CFST K-joints, the developments of the forces and deformations in the joint region were investigated. The stress and strain distribution over the joint region and the interaction between the steel tubular wall and the core concrete were also studied.(4) A simple method was proposed to calculate the resistance of circular CFST K-joints. Coefficients were determined by curve-fitting to the results of the parametric analysis to consider the influences of the chord's diameter to thickness ratio and the chord member's axial load ratio. The ultimate capacity of the circular CFST K-joints was obtained by carefully introducing these coefficients into the formula for local buckling of cylindrical shell, which was used to estimate the ultimate resistance capacity of circular composite K-joints. It was shown to agree well with the FEM results. |
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