The Study Of Out-of-plane Effective Length Factors Of Compressive Chords In Tubular Space Trusses With Triangular Sections

Effective length factor method is prevalent in stability design of truss structure. With a given effective length factor of truss structure, the critical buckling load is obtained by means of the Euler's Formula. Nowadays, the tubular space truss with the triangular section has been more and more popular in practice. There is, however, no provision of the effective length factor about the structure in the Code for Design of Steel Structures "GB 50017-2003" in our country. In the practical applications, the designers usually take the geometrical panel length for its effective length. If the chords do not have the lateral supports or if the lateral supports are insufficient, the truss chords would only be laterally supported by the braces which only provide elastic restraints for the chords. It follows that the method which regards geometrical panel length as effective length is unreasonable and unsafe. Therefore, based on the analysis of factors that influence buckling of chords, the suggestive formula of out-of-plane effective length factor for chords is proposed in this paper.In this paper, the following researches are conducted:â‘ Based on buckling influence factors of chords, 45 finite element models are generated with the familiar dimensions, restraints, slenderness ratios of chords, linear stiffness ratios between chords and braces, and the angles between braces.â‘¡In view of the initial imperfection and in accordance with the external fiber yielded theory, this research analyzes the finite element models that have been generated. This article analyzes the buckling modes of truss structures, the curves of load-displacement at pivotal position in truss structures and the influence of parameters in the modes on the buckling load of truss structures, in cases of different slenderness ratios of chords, different linear stiffness ratios between chords and braces and varying angles between braces.â‘¢Grounded on the analysis of the influence factors of the out-of-plane effective length factor, a suggestive formula is proposed about the out-of-plane effective length factor.Conclusions are drawn as follows:Among the influence factors discussed in this thesis, the slenderness ratios of the chords have the biggest influence on out-of-plane effective length factor. The linear stiffness ratios between the chords and braces have a comparatively smaller influence than do the slenderness ratios of the chords, and the influence of the angles between braces is the smallest. The out-of-plane effective length factors increase with the decrease of slenderness ratios of truss chords when the slenderness ratios are greater than 100; while the slenderness ratios are less than 100, the factors decrease with the decrease of slenderness ratios. The factors increase with the increase of the linear stiffness ratios between the chords and braces, and reduce along with the increase of the angles between braces. The impact of the angles between braces on the out-of-plane effective length factors can be basically neglected if the angles are within normal scope. Through the surface fitting based on the results of FEM analysis, this article puts forward the relational function between out-of-plane effective length factors, slenderness ratios of the chords and linear stiffness ratios between the chords and braces. In other words, this function is the suggestive formula about out-of-plane effective length factors.