A Purlin Supporting Technique for Pre-Engineered Steel Buildings

There are a few important factors to consider when planning the best purlin reinforcing configuration for your pre-fabricated, pre-engineered steel structure. These aspects involve the need to avoid lateral translation of the whole compilation of roofing and purlins, to minimize rotation and any turning or twisting (torsion), and to establish sideways flange bracing.

There should be horizontal stabilization of the two member flanges if this plan is to work. In other words, by applying bracing, the horizontal deflection of the two flanges should be impeded at chosen brace spots and at the ends. Establishing only a single line of sag angles alongside the uppermost of the purlin flange with sliding connections, a familiar standing-seam pre-engineered steel roof system, is remedied with this approach. In this application the single line of bracing is too low to prevent purlin rotation under load. A manufacturer’s design specification where the bracing is further away from the top flange is unlikely to provide the two flanges with sideways deflection protection and harmful rotation of members. The flange that needs to be constrained should have purlin bracing as near as possible.

A high degree of purlin reliability can be produced by correctly installed diagonal braces despite being at an interval from the flanges. This type of bracing procedure should only be used once a through-fastened building roof is selected. This is ordinarily not a complication, as the popularity for standing-seam roofing for pre-engineered and pre-fabricated steel structures that have sliding connections removes a lot of bracing concerns. By incorporating lines of bracing angles running alongside each other close to the highest flange, this roofing system allows the benefits of diagonal bracing to be accomplished.

The need for appropriate purlin bracing, however, is emphasized by the choice of a through-fastened steel roof. On its own the steel rooftop can provide sideways, but not essentially torsional, reinforcement of the steel purlin. The steel roofing diaphragm may not be sound enough to avert lateral translation under loading from being introduced to the configuration of roofing and purlins.

Compact patterns of bolted channel blocking are the better arrangement for buttressing of purlins. With the insertion of bolts that have a greater connection capacity than that of screws or tabs, this becomes a great approach to reinforcement of the two flanges of purlins opposing rotation and translation. Twin rows of angle braces possibly secured to the top and bottom flanges can be chosen for smaller structures.

It is essential to have the correct purlin spacings generated for any recommended purlin support process. A good guide for spacing is to choose between designating the purlin sideways support span at the smallest number of either the maximum unbraced purlin length of between 60 and 72 inches, or a quarter of the purlin distance—whichever is less. Miscalculations can result in buckling and possible loss of a purlin sector, so it is imperative that all figures are correct. When choosing a suitable purlin bracing approach for your steel structure project, refer to the points considered in this article to aid your decision.