This paper presents a numerical investigation of the effects of discrete fasteners on the buckling behaviour of cold-formed steel (CFS) built-up columns and concludes with the assessment of the applicability of the current direct strength method (DSM) to the design of these members. To this end, the compound strip method is first utilised to accurately model the buckling behaviour of CFS built-up columns under axial compression via introducing connecting elements to represent the contribution of discrete fasteners. A series of elastic buckling analyses with different levels of composite action are carried out to obtain the "signature curves" corresponding to the considered CFS built-up sections. The performed studies comprise of variations in cross-sections, end boundary conditions and fastener spacing. Utilising the developed numerical techniques, the ultimate strength of the sections are estimated and an adjustment procedure is suggested to extend the current DSM equations to the design of CFS built-up columns. The proposed adjustment is verified against the existing experimental data and shown to be capable and accurate in capturing the expected behaviour of CFS built-up members with or without stiffeners.
In this paper, the compound strip method is utilised and extended for the stability analysis of cold-formed steel built-up sections. The discrete fasteners are incorporated in the analysis such that they can be placed anywhere within the length of the member. A beam element with geometrical and material properties of the fastener is adopted to model the connection between the individual sections. The stiffness matrix terms for the connection element are derived and added directly to the global stiffness matrix of the considered built-up sections. The proposed numerical technique is verified through various examples and the results are compared with the finite element solutions. The results obtained demonstrate that the developed technique can accurately and efficiently model the behaviour of cold-formed steel built-up sections. Furthermore, the influence of the fasteners properties, their longitudinal spacing and also the boundary conditions on the overall buckling behaviour of built-up sections is examined through numerical examples. The simplicity of the proposed technique facilitates an extensive parametric study of cold-formed built-up sections including the search for optimal placement of fasteners and combinations of component sections in built-up profiles.
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