ROTATIONAL RESTRAINT AND DISTORTIONAL BUCKLING IN COLD-FORMED STEEL FRAMING SYSTEMS
ResumoA series of cantilever tests on joist/stud-sheathing assemblies were conducted to determine the rotational restraint that sheathing provides to the flange of a cold-formed steel floor joist or stud. This rotational restraint, characterized by the stiffness, k, can partially or fully retard the distortional buckling mode. Distortional buckling, common in conventional cold-formed steel members such as the lipped channel, is characterized by significant rotation of the flange at the flange/web juncture. Cantilever tests were conducted for different thicknesses, depths, and flange widths, of the cold-formed steel member, and for two fastener types, and three sheathing types: plywood, oriented strand board (OSB), and gypsum board. The testing demonstrated that the rotational stiffness may be decomposed into two parts: connector, and sheathing. The connector stiffness is due to the rotation of the fastener in the flange of the cold-formed steel member, and is most significantly influenced by the thickness of the cold-formed steel member itself. The sheathing stiffness is due to bending of the sheathing itself, and may be highly variable. The results formed the basis for a new design method adopted in American standards (AISI-S210-10) for incorporating restraint into design strength predictions for the distortional buckling mode.
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