Adequate purlin bracing in pre-engineered steel buildings calls for substantial connections between the eave and ridge ends. A common building technique known as sag angle, or strapping in basic aligned rows, does not necessarily prevent breakdown and failure of purlin braces.
A correctly installed row of purlin bracing is anchored to the strong ridge angle or a channel at the ridge. In the case of a double-sloped rooftop, this helps impede the compression introduced by the accumulated energy of bracing. A sag angle placed along the ridge is not strong enough to do the job.  
Parallel bracing is generally attached to the eave strut in one of two manners. It can be accomplished by means of a direct connection or by crossing the purlin braces. Or, it can be attained with the use of sag angles between the initial purlin and the eave strut.
Purlin stability is not easily achieved by an exchange of the purlin brace with the eave strut’s bottommost flange due to the effect of torsional opposition on the eave strut. Instead, the strength of the purlin can be considerably fortified when a crossed brace is introduced as a compression member.
Superior design in blocking placement results in the separation of the first “Z” purlin and then the eave struts, which can create great opposition to turning or twisting (torsion) and prevent lateral buckling.In certain circumstances, if an exceedingly wide all-steel building is being considered, the crossing method detailed earlier should also be affixed to the angle braces for given interior building bays. As long as they understand the design of the building, its use and local weather conditions, today’s steel building manufacturers can produce buildings with adequate support that last a long, long time.