Sectional Buckling Tests of Built-up Cold-Formed Steel Columns

The paper presents tests investigating the local and/or distortional (sectional) buckling of built-up cold-formed steel sections subjected to pure compression. The experiments were designed to study the influence of key parameters, including buckling mode, cross-section geometry and fasteners spacing, on the ultimate sectional capacity of built-up columns. Three singly- or doubly-symmetric built-up sections were tested, comprising two distinct component lipped channel sections; one buckling predominantly in the distortional mode and one buckling predominantly in the local mode. The component sections were connected using self-drilling screws at three different spacings. Material properties, geometric imperfections and residual stresses of the specimens were measured prior to testing. A total of 36 specimens with a length of 1 m were tested under concentric compression between fixed ends. The specimens failed in either distortional or interactive localdistortional buckling modes. Compared to the sum of the capacities of the component sections, the ultimate capacities of the built-up columns failing predominantly in the distortional buckling mode increased significantly with the reduction of screw spacing, whereas the increase was minor for specimens that failed in the interactive local-distortional buckling mode. The ultimate strengths of the specimens are predicted using the current Direct Strength Method, which indicates either conservative or overly unconservative predictions depending on the level of composite action considered in the elastic buckling analysis.

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