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 (λ_l > 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.

本文介绍了在局部-整体（挠曲-扭转）交互式屈曲模式下失效的双对称组合冷弯型钢（CFS）柱的结构性能。通过使用自钻螺钉连接两个相同的无唇槽部分（背对背）形成了本研究中研究的组合柱。总共进行了45个柱测试，并使用各种参数，例如截面的细长度，长度和自钻螺钉之间的中间间距。试验结果表明，建成列的轴向极限强度是由专用信道部分和非加强凸缘元件的屈曲，由于较高的局部屈曲应力支配（λ_升 > 1.5）导致互动屈曲。测试结果还表明，中间间距的影响取决于CFS组合横截面的破坏模式。介绍了可用于预测整体弯曲屈曲的弹性临界屈曲应力和设计强度的各种选项。与采用局部-整体交互式屈曲的CFS组合柱的轴向抗压强度相比，当前的直接强度法（DSM）预测的设计强度并不保守。因此，阐明了防止DSM方法错误应用的建议。