Design of cold-formed steel built-up columns subjected to local-global interactive buckling using direct strength method

摘要

This paper presents the structural behavior of doubly-symmetric built-up cold-formed steel (CFS) columns that fail in local-global (flexural-torsional) interactive buckling modes. The built-up column investigated in this study is formed (back-to-back) by connecting two identical unlipped channel sections with the use of self-drilling screws. A total of 45 column tests were carried out, with various parameters such as slenderness of the section, length and intermediate spacing between self-drilling screws. The test results indicate that the axial ultimate strengths of the built-up columns are governed by buckling of the individual channel section and an unstiffened flange element due to higher local buckling stress (lambda(1) 1.5) resulting in an interactive buckling. The test results also indicate that the effect of intermediate spacing depends on the failure mode of the CFS built-up cross-sections. The various options available to predict the elastic critical buckling stress for global flexural buckling and design strength is presented. The design strengths predicted by the current direct strength method (DSM) is unconservative when compared with the axial compressive strength of the CFS built-up column with local-global interactive buckling. Therefore, suggestions to prevent the incorrect application of the DSM method are elucidated.