Buckling behavior of closed built-up cold-formed steel columns under compression

The versatility of cold-formed steel sections ensures the possibility of combining multiple individual sections forming built-up members with significantly higher load-bearing capacity. This paper reports detailed experimental, numerical and analytical research on closed built-up cold-formed steel columns with four different cross-section shapes under compression. Twelve experimental specimens were fabricated using individual C, U, and Σ-shaped sections and connected using fasteners. The material characterization and imperfection measurements were also performed. The interaction between the different components and the influence of spacing between connectors was investigated for a relevant range of slendernesses using the finite element method, providing recommendations concerning the effective use of fasteners to attain composite action between the individual shapes. The overlapping plates tend to buckle simultaneously when the spacing is smaller than the local buckle half-wavelength. The suitability of the design codes, namely, Eurocode and North American design predictions, according to Effective Width Method (EWM) and Direct Strength Method (DSM), respectively, were assessed for the built-up members. Reliability analysis was undertaken, and it was observed that generally, the DSM with the modified slenderness ratio is the most reliable methodology.