Numerical investigation of the strength and design of cold-formed steel built-up columns

The paper concerns built-up cold-formed steel (CFS) structural members composed of three or four component lipped channel sections. Finite element (FE) models are calibrated against recently conducted experimental results and used to perform a parametric study of the strength and behaviour of CFS built-up section columns, considering column the length, section thickness and screw spacing as independent variables. The strengths thus obtained combined with experimental ultimate strengths are compared with strength predictions obtained using the North American Specification for the Design of Cold-formed Steel Structural Members, which specifies the use of a modified slenderness ratio for built-up members composed of two sections connected back-to-back. The same data is used to assess a recently proposed design method for predicting the strengths of the studied 3C and 4C built-up sections, and potentially multi-cell built-up sections more generally, based on effective rigidities coupled with the Direct Strength Method. The proposal includes a design procedure for built-up sections experiencing flexural-torsional buckling, which is neither explicitly stipulated in current design standards nor discussed in-depth in previous studies, and generally provides accurate strength predictions. A reliability analysis is included in the paper to calculate resistance factors to accompany the proposed design provisions.