Direct stiffness-strength method design for sheathed cold-formed steel structural members - Recommendations for the AISI S100

This paper presents a new design procedure for considering the bracing effect of sheathing boards that are attached to the cold-formed steel (CFS) structural members. The previous investigations show that the current AISI design specification for sheathing bracing design of CFS wall panels is unconservative (i.e. design standard predict a larger failure load than the experimental test results) due to exaggerated sheathing stiffnesses calculated from ideal loading conditions rather than worst-case loading conditions. Therefore, a new design procedure is suggested based on the performance (strength and stiffness) of the individual sheathing fastener connections. A new and simplified test setup is also introduced to simulate the realistic failure modes of the sheathing fastener connections using a conventional universal testing machine. A total of 67 individual sheathing fastener connection tests were carried out, including parameters such as ten different dimensions of the CFS stud (to account for the slenderness) and seven various sheathing board types (to account for the performance of each sheathing type). Based on the individual sheathing fastener connection test results, new expressions are formulated to predict the stiffness and strength of the individual sheathing-fastener connections. A statistical and application-oriented merit-based assessment indicated that the proposed expressions are appropriate for the use of sheathing braced design. In particular, the individual sheathing fastener-connection strength and stiffness arrived by the proposed expressions are conservative in predicting the failure mode and design strength of the sheathed CFS panel. Finally, this paper demonstrates the effectiveness of the proposed simplified design method by illustrating with the help of a design example, which quantifies the benefit of adopting this method in AISI design guidelines.