When it comes to pre-engineered steel buildings, proper purlin bracing requires substantial linkage of building eave and ridge ends. Sag angles will not automatically stop breakdown and collapse, nor will strapping in simple parallel rows, a common building trend.

Every line of purlin bracing must be properly secured to the solid ridge angle or the channel at the ridge. This promotes opposition to the pressure generated by the energy of bracing from a double-sloped rooftop. It is not OK to simply install a sag angle along the ridge and call it “good.”

Parallel bracing needs to be regularly connected to the eave strut in one of two ways. This can be handled by crossing the purlin braces or through a direct attachment. Another method would be to use sag angles separating the first purlin along with the eave strut.

It will not be enough to affix the purlin brace to the eave strut’s bottommost flange purlin; this is due to the expansive difference in the torsional resistance of the eave strut. When a crossed brace can be introduced as a compression member this can help a lot with the dependability of the purlin.

Solid blocking between the primary “Z” purlin and the eave struts is usually a good design approach. Significant resistance to turning or twisting (torsion) as well as lateral buckling can be attained with blocking. The crossing technique described above may also have to be applied to the angle braces of some interior bays.

A concern in lateral purlin bracing is the expectation that the eave strut is immobile and as such a good location for attachment. However, the eave strut will have flux with the sheathing of the pre-engineered roof in addition to the purlins and won’t supply much sideways support for either. Eave struts can provide a lot of torsional support for individual purlins where the siding will be connected with closely spaced fasteners. In contrast, they can provide little support if purlin actions cause screws to slacken or the eave strut is not even joined to the structural wall.

Another efficient bracing system is to use crosswise schemed steel angles separating the top flange with one purlin to a bottom flange of the next purlin. This caps this bracing technique, for practical purposes, with configurations for through-fastened rooftops and negates standing-seam as an alternative. Using crosswise purlin braces allows every purlin to form a portion of a pyramid shape, which includes the pre-engineered steel roofing, the crosswise brace, as well as the purlin web. This procedure will only work right when the structure’s roof has the ability to endure compressive forces and is correctly attached to the purlins.

Just like using parallel purlin bracing, the feasibility of the diagonal brace scheme is very reliant on the capability of ridge channels or angles to counteract the substantial bracing forces of roof slopes. Applied correctly it can assist in the building cohesion of any pre-engineered steel structure.