Direct strength method for cold-formed steel built-up columns: Experimental database evaluation

Codified Direct Strength Method (DSM) equations and modified ones proposed in the literature have not been tested collectively using experimental data on a wide range of built-up cold-formed steel (CFS) columns that failed in local, local-global, distortional, global and interactive buckling modes. To fill this gap, an open-access database was compiled encompassing compression axial test data for 1194 CFS built-up columns, consisting of 599 closed, 485 open, and 110 half-closed cross-sections, along with their material and section properties, fastener spacing, boundary conditions, and failure modes. Then, codified and modified DSM local-global, distortional, and global buckling curves were compared with the corresponding experimental data. Next, the effect of critical elastic buckling stress estimation models, namely single-section and built-up models, on DSM design predictions has been evaluated. Finally, the load and resistance factors of the codified and modified DSM equations were determined according to AISI S100. Based on the reliability analysis, original DSM equations for local-global and global buckling were unconservative, while the distortional buckling equation ensured the safe design of built-up columns. Targeted modifications proposed in the literature for the design of specific types of CFS built-up columns have enhanced prediction accuracy while maintaining the simplicity of the DSM framework and supporting safer, more efficient design practices. Single-section-based critical sectional elastic buckling stress estimation models provided more accurate DSM predictions than built-up models. The AISI specification for fastener spacing influences the capacity of built-up columns. Increased spacing reduced strength, especially in columns prone to flexural and interactive buckling.