Structural Behaviour of Cold-Formed Steel Built-Up Closed Cross-section Columns - Assessing the Influence of Parameters and Design Methods

Structural behaviour of cold-formed steel (CFS) built-up closed cross-section columns subjected to local, global, and local–global interactive failure is investigated. The objective is to assess the appropriateness of six different Direct Strength Method (DSM) based designs proposed by various researchers including the current AISI codification. A total of 595 built-up column test results from the existing literature were used for assessment. The comparison of test results and design predictions indicated that the primary influential parameters that inhibit or induce the local–global interaction failure in closed cross-section built-up columns were not well incorporated into the designs. Owing to that, the DSM design predictions are largely scattered; conservative by a maximum of 258% and unconservative by 60%. The reliability analysis was carried out to statistically assess the design methods. It is found that five out of six designs are reliable but do not represent the actual structural behavioral trend. Following this, representative sample results and failure modes were analyzed for identifying the influential parameter that causes design methods to result in inaccurate failure load. The analysis shows that the strength of the column increases with a decrease in intermediate fastener spacing (less than the local buckling halfwave length) due to the flange overlapping effect while the column strength decreases due to local–global interactive buckling caused by a larger intermediate fastener spacing (higher than the local buckling halfwave length). Finally, the paper presents future directions on how the DSM should evolve for the accurate design of built-up closed cross–section columns.