14 - Direct stiffness–strength method design for cold-formed steel structural members with sheathing bracing

Abstract

This chapter presents a new design method for considering the bracing effect of sheathing boards that are attached to CFS (cold-formed steel) structural members. It also presents a detailed summary of history, development, and flaws of the available sheathing braced design guidelines of the American Iron and Steel Institute. Previous investigations have shown that the current American Iron and Steel Institute (AISI) design specification for sheathing bracing design of CFS wall panels is unconservative (design predictions>experimental strength) due to exaggerated sheathing stiffnesses calculated from ideal loading conditions rather than worst-case loading conditions. Therefore a new design method is suggested based on the performance (strength and stiffness) of the individual sheathing-fastener connections. The current test setup of the AISI is improved to simulate realistic failure modes of the sheathing-fastener connections. The test results revealed: (1) the prominent influence of tensile modulus of the sheathing board and the geometric dimensions of the CFS stud (lever arm), (2) the sudden and catastrophic failure modes and they should be considered in the form of coefficients in the design expressions to prevent an unsafe design, and (3) the sheathing thickness did not influence the performance of the sheathing board (strength, stiffness, and failure mode). Based on test results, new expressions are formulated to predict the stiffness and strength of the individual sheathing-fastener connections. Finally, the effectiveness of the proposed simplified design method is illustrated by a design example, which quantifies the benefit of adopting this method in AISI design guidelines.