EFFECTIVE LENGTH FOR WEB MEMBERS IN TUBULAR TRUSSES

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  • 鋼管トラスウェブ材の座屈長さに関する研究
  • コウカン トラスウェブザイ ノ ザクツ ナガサ ニ カンスル ケンキュウ

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In tubular trusses web members usually buckle in an out-of-plane mode because restraint at the member ends supplied by the chords is greater on in-plane rotation than on out-of-plane rotation. In this paper the effective length for out-of-plane buckling of the web members is evaluated. The web member is assumed to be a bar with two rotational springs at each end. One of the springs represents the stiffness of the chord to out-of-plane rotation at the panel point and the other spring denotes the local stiffness of the chord wall. The effective length is obtained from the buckling analysis of the model. The end rotation has a component that produces out-of-plane bending deformations of the chord. Since the stiffness to this rotation component is neglected, and since the stiffness of the web member under tension is also neglected, effective lengths obtained from this analysis give an upper bound solution. Buckling strengths of web members observed in truss tests are plotted against slenderness ratios according to the above analysis. The results are compared with various column curves. It is shown that effective lengths obtained from the above analysis lead to buckling strengths somewhat greater than SSRC Curve 2 or the allowable stress (for short-term loading) according to the AIJ Design Standard. These column curves have been recognized to be applicable to cold-formed hollow sections, like the tubes used for these specimens, which verifies that the analysis gives lower-bound solutions. In order to study how the effective length varies with each of restraining factors at the web member ends in actual trusses, effective lengths are computed for a large number of tubular trusses with various dimensions by using the present analytical procedure. From these numerical results, effective lengths for web members are found to be represented by two simple formulas, which are useful for more rational design of tubular trusses.

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