Design of Slender Built-Up Back-to-Back CFS Channel Columns with Pinned Warping-Free Ends

Built-up back-to-back cold-formed steel (CFS) channel members (BBC members) are utilized as CFS compression members with enhanced load-bearing capacity in building structures. In this study, finite-element (FE) models of BBC compression members with pinned warping-fixed and pinned warping-free ends were established and verified using test results available in the literature. Using the developed FE models, detailed parametric studies were conducted to assess the behavior and capacity of slender BBC compression members with pinned warping-free ends, varying cross sections, and screw arrangements. The results were first compared with those of their individual (channel) members and BBC members with pinned warping-fixed ends. They were then used to evaluate the direct strength method (DSM) design rules in the CFS design standards used in North America and Australia/New Zealand. Finally, new reliable design rules based on the DSM equations and some analytical equations in the literature were proposed for the BBC compression members with pinned warping-free ends, together with suitable recommendations. This paper includes the main details of this study with a summary of its important findings and proposals.