Design of Cold-Formed Steel Back-To-Back Connected Built-up Beams

Built-up sections are commonly used in cold-formed steel (CFS) structures. The built-up sections are either welded or mechanically fastened based on the need and necessity. However, the current design standards lack specific design methods for CFS built-up members as the development is in progress. The American Iron and Steel Institute (AISI) specifies that only the beams fabricated from back-to-back connected C-channel built-up section as a prequalified geometry for design. This paper presents experimental and numerical investigation on the CFS built-up beams fabricated from back-to-back sigma sections. A total of 15 tests were carried out, containing three different lengths and five different intermediate connection spacing. The test specimens were laterally braced, webs and flanges were stiffened to avoid lateral torsional and local buckling so as to exhibit failure due to distortional buckling. Further, parametric studies were carried out using numerically validated model with five different lengths, two different sheet thicknesses and varying distortional slendernesses totaling to 160 specimens. The numerical results were summarized and compared with the design strength predictions by AISI's direct strength method (DSM). The comparison indicates that the design predictions in general are conservative, however, over conservative for low slenderness range (λ_d ≤ 0.673). Therefore, a new limit is suggested for increasing the inelastic reserve buckling strength based on the experimental and numerical results.