Experimental investigation and design considerations on cold-formed steel built-up​ I-section columns subjected to interactive buckling modes

An experimental investigation into the behavior of cold-formed steel (CFS) built-up column assembly compressed between fixed ends is presented in this paper. The built-up column assembly is designed to be doubly symmetric and locally slender to study the interactive buckling mode of failures. A total of forty-one columns were tested including different cross-section dimensions, lengths, intermediate connection spacing, and slendernesses. It is experimentally shown that the back-to-back connected doubly symmetric built-up cross-section assembly columns fail predominantly in interactive local and flexural–torsional buckling due to the shift in the line of action of the internal force caused by local buckling deformations. The influence of intermediate fastener spacing was also prevalent in the failure modes of the doubly symmetric CFS built-up columns. The appropriateness of the AISI's maximum intermediate connection spacing limitation is verified to prevent global instability failures. The test and design results comparison indicated that the current AISI's DSM design curve for interactive buckling is unconservative for the CFS built-up columns with predominant interactive local–global failure mode vulnerability. Therefore, a modified design curve for interactive buckling is suggested based on the test results with reliability analysis.