No. R965, December, 2019

Rasmussen, KJR, Khezri, M, Schafer, BW and Zhang, H
The Mechanics of Built-up Cold-Formed Steel Members
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The paper presents methods of analysis of built-up sections in which the discrete locations of fasteners is accounted for explicitly, rather than by smearing their effect using continuous shear flexibility as in current approaches. By considering fasteners at discrete points, it is possible to analyse the effects of placing additional fasteners at the ends (end fastener groups), to account directly for actual support conditions and to determine the optimum locations of fasteners. The paper first outlines the linear analysis of beams in flexure and introduces the notion of the effective flexural rigidity to account for partial composite actions. Closed form solutions are provided for five load and end support cases to demonstrate the application of the analysis. Next, the paper describes the linear analysis of built-up sections in torsion, considering first uniform torsion followed by non-uniform torsion. Closed form solutions are obtained for the effective torsion rigidity (GJeff) of built-up sections featuring closed loops. A framework is also presented for determining the effective torsion rigidity (EIw,eff) of open built-up sections in non-uniform torsion. The paper concludes with the analysis of built-up sections subject to flexural buckling. A general variational buckling equation is derived followed by an energy type method for calculating buckling loads for common end support conditions, including columns supported on flexible end tracks. Closed form solutions are presented for up to seven rows of fasteners longitudinally. While by nature approximate, the solutions are shown to be highly accurate. Comparisons are made between the presented closed form solutions and buckling load predictions obtained using current design provisions.

Built-up member; Discrete fastener; Effective rigidity; Flexure; Torsion; Buckling; Cold-formed steel

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