In this study, a distortional buckling behavior of cold-formed steel (CFS) built-up closed section columns was revealed and investigated. A total of 12 C-section, 12 U-section, and 24 built-up closed section columns assembled using self-drilling screws at their flanges were tested under concentric axial compression. The distortional buckling behavior and mechanical properties of the experimental members were investigated, and their buckling modes and failure states were studied. Finite element models were established and verified. The effect of the screw spacing on the distortional buckling behavior and mechanical properties of the CFS built-up columns was explored using three hypothetical models. The results showed the following: I. The number and half-wavelength of the distortional buckling half-waves differ with the variation in the screw spacing. II. When the screw spacing is less than 0.9${\lambda }_{\mathrm{c}}$, the ultimate capacities of the CFS built-up columns are greater than the sum of those of the C- and U-section single columns, i.e., the former exhibit the built-up effect (1+1>2). III. When the screw spacing is greater than 0.9${\lambda }_{\mathrm{c}}$, the ultimate capacities of the specimens gradually approach the sum of those of two single columns (1+1$\approx$2). Additionally, five different models were proposed to predict the critical load of the distortional buckling. Finally, the ultimate capacities of the built-up columns were calculated using the direct strength method, and it was found that it is conservative to calculate the ultimate capacities of the built-up columns using this method.

在这项研究中，揭示并研究了冷弯型钢 (CFS) 组合封闭截面柱的扭曲屈曲行为。在同心轴压下测试了总共 12 个 C 形截面、12 个 U 形截面和 24 个组合封闭截面柱，这些柱在其法兰处使用自钻螺钉组装。研究了实验构件的扭曲屈曲行为和力学性能，研究了其屈曲模式和失效状态。建立并验证了有限元模型。使用三个假设模型探索了螺钉间距对 CFS 组合柱的扭曲屈曲行为和力学性能的影响。结果显示如下： I. 扭曲屈曲半波的数量和半波长随着螺钉间距的变化而不同。二、螺杆间距小于0.9时${\lambda }_{\mathrm{C}}$，CFS组合柱的极限承载力大于C型和U型截面单柱的极限承载力之和，即前者表现出组合效应（1+1>2）。三、当螺杆间距大于0.9${\lambda }_{\mathrm{C}}$, 试件的极限承载力逐渐接近两根单柱 (1+1$\approx$2）。此外，提出了五种不同的模型来预测扭曲屈曲的临界载荷。最后，用直接强度法计算了组合柱的极限承载力，发现用这种方法计算组合柱的极限承载力是保守的。