桁架受压腹杆的面外稳定和支撑体系

桁架受压腹杆在平面外的计算长度通常取其几何长度。然而在工程设计中，腹杆两端并不总是设有防止出平面位移的支撑构件。在未设支撑的情况下，两端节点实际上是弹性支承点，其弹簧刚度取决于在支点上相连的其它杆件。仅当弹性支承的刚度满足一定要求时腹杆平面外计算长度才等于其几何长度。本文分析弦杆对腹杆提供的约束刚度，并给出刚度不满足要求时的平面外计算长度。

THE OUT-OF-PLANE STABILITY OF TRUSS WEB MEMBERS AND BRACING REQUIREMENT

The out-of-Plane stability of compressive truss diagonals is a spatial problem which has not been well studied. The Chinese GBJ17-88 Specification of Steel Structure Design Prescribes that the effective length of these members shall be equal to their geometric length between panel points. In case one of the ends (or both ends) is not fully supported laterally, other members connected to the end have to provide an elastic support of adequate stiffness in order that the diagonal could be designed as hinged-hinged compression member. Otherwise, the effective length of the diagonal should be larger than its geometric one.In this paper, the stiffness provided by either compression or tension chords in different cases is analysed, and the effect of initial crookedness of compression chord is taken into account.Formulas are Presented to compute the required stiffness of chord member and the effective length factor for inadequately constrained compression diagonals. The decrease in flexural rigidity of chord members of monosymmetric section due to torsion is also discussed.