Distortional buckling behavior of cold-formed steel built-up closed section columns

In this study, a distortional buckling behavior of cold-formed steel (CFS) built-up closed section columns was revealed and investigated. A total of 12 C-section, 12 U-section, and 24 built-up closed section columns assembled using self-drilling screws at their flanges were tested under concentric axial compression. The distortional buckling behavior and mechanical properties of the experimental members were investigated, and their buckling modes and failure states were studied. Finite element models were established and verified. The effect of the screw spacing on the distortional buckling behavior and mechanical properties of the CFS built-up columns was explored using three hypothetical models. The results showed the following: I. The number and half-wavelength of the distortional buckling half-waves differ with the variation in the screw spacing. II. When the screw spacing is less than 0.9 lambda(c), the ultimate capacities of the CFS built-up columns are greater than the sum of those of the C- and U-section single columns, i.e., the former exhibit the built-up effect (1+1>2). III. When the screw spacing is greater than 0.9 lambda(c), the ultimate capacities of the specimens gradually approach the sum of those of two single columns (1+1 approximate to 2). Additionally, five different models were proposed to predict the critical load of the distortional buckling. Finally, the ultimate capacities of the built-up columns were calculated using the direct strength method, and it was found that it is conservative to calculate the ultimate capacities of the built-up columns using this method.