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There are three essential factors you must consider when planning a purlin strengthening method that will ensure an accurately engineered and secured pre-engineered steel building. Firstly, lateral motion of the steel roofing and purlins as a whole must be prevented; secondly, rotation, twisting, or turning of the complete structure must be avoided; and finally, a sideways flange support should be put in place.
For this design to have an optimal outcome, the two member flanges should have sideways support. Ideally, bracing should be used to secure the flanges—therefore counteracting lateral movement at designated brace spots, and at the ends of the flanges.
The bracing corrects a usual standing-seam pre-engineered roof routine, by creating a single line of sag angles along the top of the purlin flange with sliding connections. To avoid purlins rotating under load, the single line of bracing in this particular manner is too low. It is essential you place purlin bracing as closely as possible to the flange that needs to be restrained. If the bracing is further away from the top flange it is unlikely to provide the two flanges with lateral movement protection, and rotation may damage the members.
However, such a bracing method should only be applied if you have chosen a through-fastened pre-engineered steel roof. Superior purlin reliability can be provided by properly-applied diagonal braces, even though the purlins are spaced beyond the flanges. By removing such bracing concerns, there is a well-deserved popularity of standing-seam rooftops for steel structures using sliding connections. This type of roof configuration allows the benefits of diagonal bracing to be easily achieved by including parallel lines of bracing angles beside the uppermost flange. The need for proper purlin bracing, however, is emphasized by your selection of a through-fastened pre-engineered roof. Horizontal (but not necessarily torsional) reinforcing of the steel purlin can be accomplished by the steel rooftop. Furthermore, the pre-engineered roofing diaphragm may not be engineered to stop lateral movement from occurring in the roofing and purlins.
The preferred system for purlin reinforcement is to have close intervals of bolted channel blocking. This is an excellent way of bracing so that the two purlin flanges work to prevent rotation and movement, using bolts that have a greater connection capacity than screws or tabs. Two rows of angle braces attached to the top and bottom flanges can be used for smaller structures. For any chosen purlin reinforcement technique it is critical that you have the purlin intervals accurately planned. Twisting and disintegration of the purlin area may be seen where a miscalculation of intervals has been made.
A good way to figure purlin spacing is to specify the purlins’ lateral buttressing measurement as a number representing the unbraced purlin length of either a range of between 60 to 72 inches or 25% of the purlin span–whichever is least. You may find it useful to refer to the information given in this article when selecting the right purlin buttressing scheme for your next steel structure project.
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