Experimental Studies on the Composite Action in Wood-Sheathed and Screw-Fastened Built-Up Cold-Formed Steel Columns

This paper addresses an ongoing computational and experimental effort on the quantification of composite action in built-up cold-formed steel (CFS) columns used frequently in CFS framing. 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 studs. Previous elastic buckling studies concluded that for industry-standard built-up columns designed according to AISI S100-12 Section D1.2, up to 85% of fully composite action is achieved in global buckling. In recent testing detailed herein, the components of (and attached to) a built-up column as installed in a CFS frame such as tracks and sheathing were studied as they contribute to the increase in composite action and consequently, axial capacity of the columns. Sixteen monotonic, concentric compression tests of 2 standard sections at 6 ft [1.83 m] in length and with varying fastener layouts and sheathing conditions were performed. Column deformation was monitored using 17 strategically placed position transducers. Results indicate a large increase in composite action with the addition of OSB sheathing. In addition, built-up column-to-track connections as well as prescriptive end-fastener groupings designated by the specification provide a column end condition which more closely approximates a fixed end condition rather than the more commonly and conservatively assumed pin end condition. Future work includes two successive phases of testing which will compare a wide range of built-up section types to current design provisions and elucidate the effect of fastener spacing and layout on local and distortional buckling modes, as well as numerical modeling of fastener layouts in a finite strip modeling domain for use in design. 

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