Analysis of Sheathed Cold-Formed Steel Wall Studs

The strength of sheathed wall systems is substantially greater than unsheathed walls with the same studs. Testing on studs sheathed on both sides with dry gypsum board demonstrate strength increases as large as 70% over unsheathed studs (Miller and Peköz 1993, Miller and Peköz 1994). The current AISI Specification for sheathed wall stud systems are complicated, only apply when the sheathing is identical on both sides, have several onerous prescriptive requirements, and the mechanical model employed has been questioned when compared to experimental results. This paper summarizes the deficiencies of, and arguments against, current models and suggests avenues of research for improvements. Analysis discussed herein, directed at answering the many open questions, leads to numerous interesting conclusions about the behavior of sheathed wall systems. Sheathing diaphragm stiffness should not yet be abandoned as the basic mechanical model. Diaphragm stiffness, per stud, is non-uniform and is not solely derived from stud spacing as assumed in AISI (1986) nor is it solely independent of stud spacing as assumed in AISI (1996, 2001). Shear stiffness, either locally of the material, or globally for the diaphragm, is the fundamental manner in which the sheathing resists weak axis buckling of the stud. For sheathing on one-side only, or highly dissimilar sheathing, cross-section distortion plays a role in the behavior and none of the existing (AISI) or proposed (Lee and Miller 2001) models account for this. Finally, freely available numerical tools provide a means to assess elastic buckling capacity of sheathed studs even with one-sided or dissimilar sheathing.