Direct Stiffness-Strength Method: An Alternative Design Approach to AISI for Sheathed Cold-Formed Steel Z-Section Structural Members Subjected to Bending

This paper presents a design approach, the direct stiffness-strength method (DSSM), for the strength prediction of sheathed cold-formed steel (CFS) structural members. The DSSM considers each sheathing-fastener connection as a bracing point and incorporates its stiffness and strength in the CFS wall panel design. The DSSM is formulated and validated based on large experimental test results. The motivation for this work is to address research gaps in the latest American Iron and Steel Institute (AISI) standards for the sheathing braced design of CFS structural members. A total of 41 individual sheathing-fastener connection tests were carried out, including parameters such as four different sheathing board types (to account for the performance of different sheathing) and seven different dimensions of the CFS Z-sections (to account for the slenderness). A new set of expressions was formulated to predict the stiffness and strength of the individual sheathing-fastener connections (bracing points) based on the observations from tests. A design-oriented assessment (comparing the design results with full-scale wall panel test results) indicated that the expressions formulated for DSSM are appropriate for the use of sheathed CFS Z-section member design. Further, this study used a design example to demonstrate the appropriateness of the DSSM approach. Since the introduction of the sheathing braced design concept in conjunction with the direct strength method (DSM) established by the American Iron and Steel Institute in RP13-1, this approach has gained traction among researchers to further enhance it for the various failure modes of the CFS structural members. However, there is an inconsistency in the sheathing bracing design concepts of the AISI design standards. The proposed DSSM is a simple one; it considers each sheathing fastener connection as a bracing point and incorporates its stiffness and strength in the CFS wall panel design. This proposed DSSM can be completed in two steps by checking the bracing ability of the provided sheathing. The DSSM method can be used with any design specification for determining the moment capacity of the CFS member with the contribution of the sheathing board. The design steps involved in DSSM are demonstrated with flowcharts and a design example. The DSSM proved that use of sheathing boards can eliminate the additional steel bracings at the intermediate length and breadth of the CFS wall panels, but steel bracings are required at the building corners for stability. This design method can realize up to a 15% reduction in steel consumption in the CFS building construction.