Global buckling strength of discretely fastened back-to-back built-up cold-formed steel columns

This study proposes a design equation for built-up cold-formed steel (CFS) columns to eliminate the inconsistencies present in Section I1.2 of AISI S100 specification. The proposed equation was developed by using the results from the authors' compound spline finite strip-based numerical framework and 228 experimental and numerical results from literature on built-up CFS columns, failing by minor axis buckling. Investigations were conducted using numerical models with mode-specific deformations incorporated in compound spline finite strip framework. The shear slip behaviour between the individual columns, characterised by (i) fastener spacing and (ii) slenderness ratio of fully-composite cross-section, was investigated using parametric variations. The study shows that the modified slenderness ratio (MSR) in Section I1.2 of AISI S100 yields conservative strength predictions with increased fastener spacing. This is possibly because the MSR considers the effect of fastener spacing and overall slenderness ratio as disjoint and treat them as additive components. In reality, they interact, and hence a compound slenderness ratio (CSR) is proposed with a term that captures the interaction of these two entities. The prediction of strength by the proposed CSR and direct strength method (DSM) is in good agreement with the results of experimental and numerical studies, with the mean close to one. When incorporated in the DSM design equations, the proposed CSR yields matching reliability with AISI S100.