Interactive failure mode and Design of Cold-formed Steel Closed Cross-section Built-up Columns

The structural behavior of Cold-formed Steel (CFS) built-up closed cross-section columns is investigated. The CFS built-up Closed Cross-Section columns are designed in various shapes with various slenderness ratios to verify the influence of intermediate fastener spacing and check the appropriateness of the Direct Strength Method (DSM) of AISI. A total of 595 axial compression test results were incorporated in this study with various end conditions, including the results from the literature. The design parameters such as local slenderness, global slenderness, intermediate longitudinal fastener spacing, and length of the column are varied. The failure modes of the column are summarized and the reason for them is explained. The influence of intermediate longitudinal connection spacing was observed in the axial loading capacity and failure modes. The investigation results indicated that the lesser value of intermediate fastener spacing to local buckling half-wavelength ratio increased the strength of the column while the higher ratio increased the vulnerability of local-global interactive buckling. The test results including ultimate load and failure modes were compared with the direct strength method design predictions of AISI and existing literature. A modified DSM approach is proposed with a new local slenderness expression to consider the influence of intermediate fastener spacing in the design strength of cold-formed steel built-up closed cross-section columns.