Design of locally buckling cold-formed steel built-up columns formed by unlipped channel sections

This study proposes a design procedure for the local buckling strength prediction of cold-formed steel (CFS) built-up I and box section columns based on the Effective Width Method (EWM) and the Direct Strength Method (DSM). For this, the ultimate strength of the built-up sections made of two plain unlipped channels is obtained using the finite element (FE) analysis. First, some compression tests on built-up sections are performed to validate the FE models. Then, a detailed parametric study using the validated FE models is conducted for sections with a wide range of slenderness (1≤λl≤3) where λl is the square root ratio of yield stress to the local buckling stress of the cross-section. The results show that the local buckling strength of built-up I sections is equal to the sum of its section’ strength, whereas for box sections, the strength can be 20%–25% higher as the nesting of sections will alter the local buckling characteristics. The EWM (with k factor approach) and authors’ DSM based design procedure can predict the strengths of the built-up I sections well but predict conservative strengths for the built-up box sections. This issue is resolved in this paper by proposing a simple cross-sectional model based on the constrained local buckling deformations of the built-up box section. When the buckling stress results of the proposed model were used with the EWM and authors’ DSM design procedures, the predicted local buckling strength results matched well with those of the present numerical study. In addition, the proposed design procedure provided more accurate strength predictions than the existing DSM and those presented in the literature for the built-up section columns