There are three important factors to consider in formulating the ideal purlin bracing system for a steel building that is correctly designed and anchored. These, then, are to prevent lateral translation of the entire assembly of roofing and purlins, to prevent rotation and to relieve any torsion, and to introduce lateral flange bracing.
            If this scheme is to work there must be lateral stabilization of the two member flanges. In other words, with the implementation of bracing they need to be adhered as to prevent lateral deflection of the two flanges at certain brace areas and at the ends. This effectively remedies a common standing -seam roofing practice of introducing a mere single line of sag angles in parallel to the top of the purlin flange with sliding connections. The single line of bracing in this application is too low to prevent purlin rotation under load. It is crucial to put purlin bracing as close as possible to The flange that needs restraining. A manufacturer’s design specification where the bracing is further away from the top flange is suspect for providing both flanges with lateral deflection protection and harmful rotation of members.
            Superior purlin stability can be provided by properly installed diagonal braces even if they are placed at some length away from the flanges. As a reminder, this type of bracing system should only be utilized when a through-fastened steel roof is selected. This is not generally a problem as the well-deserved popularity of standing-seam roofing for steel buildings with sliding connections removes a lot of bracing concerns. This roofing system allows the characteristics of diagonal bracing to be achieved easily by the addition of lines of bracing angles running in parallel near the top flange.
            The use of a through-fastened roof does not preclude the necessity of proper purlin bracing, however. The roofing, on its own can provide lateral, but not necessarily torsional, bracing of the steel purlin. Additionally, the roofing diaphragm, unfortunately, may not be substantial enough to stop lateral translation under loading from being introduced to the entire array of purlins and roofing.
            The better system for bracing of purlins consists of tight intervals of bolted channel blocking. This is an excellent approach to bracing of the two flanges of purlins against rotation and translation with the addition of bolts that have a greater connection capacity than the use of tabs or screws. Optionally, a couple of rows of angle braces attached to the bottom and top flanges can be used for smaller structures.
            It is critical to have the proper purlin spacings developed for any chosen purlin bracing system. Lack of critical calculations can cause buckling and failure of the purlin section. A good template for spacing is to choose between specifying the purlin lateral support measurement at the lowest number of either the maximum unbraced purlin length of between sixty and seventy two inches or 25% of the purlin span.
            Consider most of the points brought up for consideration in this article when choosing the proper purlin bracing system for your next steel building project.