Analysis and Design Assumptions for Continuous Cold-Formed Purlins

Abstract

In the design of continuous roof purlins or wind girts made of cold-formed steel members, several assumptions in analysis and design are commonly made. Typically, these continuous members are Z- or C-shaped. Continuous through fastening between cold-formed decks or panels and one of the flanges of the purlin or girt has been the usual method of providing lateral bracing. With the increasing popularity of standing-seam roofs, discrete point bracing of the underlying roof purlins is becoming fairly common. In either case, because the bending moment changes sign in these continuous elements, there are regions where the compression flange is not continuously braced. The lateral-torsional buckling mode of failure frequently governs design strength. This paper addresses the assumptions made in the analysis and design of these continuous elements, shows the effects and questions the appropriateness of many of the assumptions, and presents safety concerns. The examples presented are for continuous Z-purlins subjected to gravity loading. The conclusions reached, however, are entirely applicable to other shapes, other loading, and other applications, such as continuous girts.