Bracing for Uplift Conditions

Whenever purlins are stabilized by roofing or top-flange bracing, they are considered laterally supported only for downward loads, which produce mostly compressive stresses in the purlin s top flange. (Owing to continuity effects, some areas of the top flange located near supports will be in tension.) But what about the situations when wind produces upward forces and the bottom flange acts mostly in compression?

According to one model,15 the maximum compressive stress during uplift occurs at the intersection of the purlin s bottom flange and its web. There, the purlin is unbraced. As Tondelli16 demonstrates, the bottom flange can also be in compression under downward loading in short interior spans of uniformly loaded continuous purlin runs. A similar situation can occur under partial roof loading (see Fig. 5.12).

Behavior of roof purlins braced only on the tension side is extremely complex and poorly understood. The AISI Specification has undergone many changes in this regard; the latest major modification appeared in the 1989 Addendum. The design approach of the 1989 Addendum described here was retained in the 1996 AISI Specification1 and was further fine-tuned in the 2002 North American Specification for the Design of Cold-Formed Steel Structural Members.4

In order to calculate the nominal moment strength M _n of the member with one flange attached to through-fastened roofing, a reduction factor method is presented in Equation C3.1.3-1:

where<i class="emphasis">M</i><sub<i class="emphasis">n</i></sub> = nominal moment strength<i class="emphasis">S</i><sub<i class="emphasis">e</i></sub> =<i class="emphasis"> effective</i> elastic section...