Experiments on local–distortional interaction buckling of cold-formed steel three-limbed built-up open-section columns

Cold-formed steel built-up open-section members, which are widely used in doorways, window frames, and corners, exhibit low torsion stiffness and high susceptibility to the deformation of the cross-section. Extensive research has been conducted on the pure buckling behaviour of cold-formed built-up members. However, few studies have investigated the local–distortional buckling interaction behaviour of complex built-up sections. This study aims to obtain a thorough understanding of the mechanics of three-limbed built-up open-section columns undergoing local–distortional interaction buckling through experiments and numerical simulations. Six lipped channels and 18 built-up columns were axially compressed under fixed boundary conditions. Finite element models were established and verified. Detailed parametric studies were conducted on the validated finite element models by varying the web height–thickness ratio, screw spacing, and slenderness ratio. The experimental results indicate that the studied built-up section had an evident assembly effect, and the bearing capacity of the built-up open-section column was sensitive to the web height–thickness ratio. Additionally, the numerical results show that, under local–distortional interaction buckling, the screw spacing had a minor influence on the ultimate bearing capacity of the three-limbed built-up open-section columns. The columns buckled in local–distortional interactions, with slenderness ratios ranging from 12.73 to 36.30. Finally, the validity and accuracy of the local–distortional​ interaction buckling calculation methods considering three-limbed built-up open-section columns were evaluated. With the current calculation methods, the prediction of the local–distortional buckling capacity of three-limbed open-section built-up columns is conservative.