Stability Of Compressive Chord And Behavior Of Eccentric Joint In Steel Tubular Truss

In recent years, the steel tubular truss which utilizes welded tubular joints has found its wide application in the space and long span structures. Current design theories and methods in the tubular structures are mainly focused on the load-carrying capacity of tubular joints. Traditional truss theory has been used in the design of the truss for checking stability, where all joint was treated as hinged connection. This tends to exaggerate the effective length factor for the compressive members because of omitting joint rigidity contribution. In steel tubular trusses, when the compressive members under the worst loading condition will buckle, the other parts will provide a bending restrain to the buckled members. Therefore, the constraint effect resulting from the other members should be considered in the stability analysis of the members. To this objective, the overall buckling analysis of the structure should be carried out. In this paper, the plane rectangular steel tubular trusses are selected as analytical object and the following research are performed:First, the factors was founded which might determine the out-of-plane and in-plane buckling modes of the compressive chords of the plane steel tubular truss. That is the ratio of out-of-plane stiffness to in-plane stiffness of top chords and braces. Then a judging method for determine the buckling modes is obtained which is expected to be useful for engineers.Secondly, according to the global stability analysis for the whole structure, a group of members which include three segment chords was selected as our analytic objects. We treated the constraint effect from the adjacent members on the buckling member as rotation spring constraint. Then the eigen equation for calculating the critical load is generated. After simplification of the equation, the relationship between the effective length factor and the effective bending stiffness ratio was founded. Further the relationship is modified by comparing the results obtained from FEM, and a useful and practical stability calculation method for engineers is proposed. Due to the fact that the axial force of the member will affect its constraint stiffness, it has been considered in this method when calculating rotation stiffness, which was never done in current design code. For the fabrication convenience, usually a gap among the K-joints will be kept. But it might cause the joint eccentricity, and further results in additional moment at both ends of the chords and braces. For predicting the effect of joint eccentricity, a geometric and material nonlinear shell element was chosen to analyze the whole truss subjected to full loading process. The influence of joint eccentricity on the ultimate load-carrying capacity as well as the collapse process of the structure, is estimated, and a reasonable design was recommended. Finally, the comparison of the distribution of axial force and moment resulting form joint eccentricity is completed by employing beam elements.