TABLE OF CONTENTS As structural engineers have long known, an unbraced compression flange of any single-web flexural member, even of a perfectly symmetrical one loaded through its web, has a tendency to buckle laterally under vertical loading. A singly symmetrical (C section) or a point-symmetrical (Z section) cold-formed purlin is even more susceptible to buckling because it has its shear center in a location quite different from the point of loading application, which is typically the middle of the top flange. Plus, the principal axes of a Z section are inclined to the web, and any downward load produces a lateral component. Because of these factors, the unbraced C and Z sections tend to twist and to become unstable even under gravity loading on a perfectly horizontal roof.
In sloped roofs, the purlin web is tilted from the vertical position, a fact that further complicates the problem of twisting. Gravity loading acting on a sloped C or Z purlin can be resolved into the components parallel and perpendicular to the roof, both of which tend to overturn the purlin, although in the different directions if the purlins are properly oriented as shown in Fig. 5.14. A computation based on the member geometry quickly finds that the two components equalize each other when the slope equals the ratio of the dimensions of the purlin s flange to its depth. For example, for an 8.5-in-deep Z purlin with a 2.5-in-wide flange, this slope is...
