Here are some things to remember when you calculate properly anchored and designed purlin reinforcements for your pre-fabricated, pre-engineered steel structure: Avoid lateral translation of the whole aggregation of roofing and purlins; allay rotation and alleviate any turning or twisting; and put in horizontal flange bracing.

The two-member flanges need lateral stabilization for this design to work right. Very simply, they should be secured with braces to prevent sideways deflection at specific places and at the ends. Installing a solitary line of sag angles alongside to the top of the purlin flange with sliding connections - a common standing-seam pre-engineered steel roof process - is corrected this way. To hinder purlin rotation under load, the singular line of bracing in this method is too low. It is vital to situate purlin bracing as close as feasible to the flange that needs to be constrained. Be suspicious of a manufacturer’s design when bracing is away from the top flange, it probably won’t provide the two flanges with lateral deflection protection and may cause destructive rotation of members.

This type of bracing should only be used on a through-fastened steel structure roof. Properly applied diagonal braces can produce higher purlin capability even if they are placed at an interval away from the flanges. This is normally not a problem since standing-seam roofs for pre-engineered and pre-fabricated steel buildings with sliding connections have become popular because they remove a number of bracing issues. This roofing system allows diagonal bracing by including lines of bracing angles running alongside each other by the topmost flange.

The need for proper purlin bracing is highlighted by the selection of a through-fastened pre-engineered steel roof. By itself the steel rooftop can provide sideways, but indefinite, torsional buttressing for the steel purlin. The pre-engineered steel roofing diaphragm, unfortunately, may not be enough to prevent lateral translation from loading applied to the array of purlins or roofing.

For bracing of purlins the proper arrangement contains compact patterns of bolted channel blocking. This application uses bracing of both flanges of purlins - repelling translation and rotation with the inclusion of bolts that possess a larger connection capacity than the use of screws or tabs. For smaller buildings, a pair of lines of angle braces attached to the bottom and top flanges can be used.

It is vital to have suitable purlin spacing established for any selected purlin bracing scheme. The options for the purlin sideways support intervals, and a good guide for spacing, are a minimum figure of between the largest non-reinforced purlin length of either five feet or 72-inches or 25% of the purlin distance. Bad calculations can lead to purlin sections warping or falling apart. The facts examined in this article need to be mulled over when picking the proper purlin bracing plan with any building project.