Buckling and Collapse Behavior of Screw-Fastened, Built-Up Cold-Formed Steel Columns of Varying Cross-Section Size Experimental Investigation

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An ongoing experimental effort using built-up cold-formed steel (CFS) columns is discussed in this paper. The quantification of partially-composite action, determination of member end fixity, and observation of buckling and post-buckling behavior is presented. Specifically, the section studied herein is a common back-to-back, lipped channel section with two self-drilling screw fasteners connecting the webs of the individual channel sections along their length. The specimens are 6 ft (1.83 m) in height and are bound by 1 ft (305 mm) of track. Previous experimental work by the authors has focused on studying the effect of sheathing, end conditions, and fastener spacing on composite action, and a separate series of tests examined the effect of fastener layout on local and distortional buckling. The tests presented herein conclude with a study of 16 different cross-sections, where each section is tested with two web fastener layout types as determined via AISI S100 (2016) section I1.2. A total of 32 monotonic, concentric compression tests are completed with 15 position transducers monitoring displacements at key locations. Material properties and initial imperfections are quantified for each specimen before testing. Results show a vast range of deformation behavior, with local-global interaction and flexural-torsional modes common in many of the sections. End fastener groups boost capacity only when minor-axis flexural buckling is observed. Also, the column end conditions are determined to be semi-rigid, but almost fixed-fixed for all sections. Future work includes nonlinear FEA modeling validated by test data and a new design approach using finite strip-based modeling and the Direct Strength Method.

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