Buckling and Strength of Cold-Formed Thin-Walled Built-Up Columns under Axial Compression: Tests and Code Evaluation

This study examines the axial-compression behaviour of cold-formed thin-walled steel built-up columns. Sixty concentric axial tests are carried out on six cross-section configurations (single C- and U-sections and four screw-fastened built-up sections) with lengths from 500 to 3000 mm. The measured capacities are compared with predictions from the Chinese code GB 50018-2002 using the Effective Width Method (EWM) and from AISI S100-2007 using both EWM and the Direct Strength Method (DSM). For closed multi-limb built-up sections, members with slenderness λ < 70 failed mainly by local buckling, whereas λ > 70 led to global buckling. For open built-up sections, λ < 80 is governed by distortional buckling and λ > 80 by global buckling. Overall, GB EWM provided the closest strength predictions for the built-up members, while AISI DSM is generally conservative. The results offer practical guidance for the design of cold-formed steel built-up compression members.