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There are three essential aspects to think about when planning the ideal purlin support plan for a precisely designed and anchored pre-engineered steel structure:
- Prevent lateral movement of the purlins and steel roofing assembly
- Impede rotation while decreasing torsion
- Involve lateral flange support
The two member flanges depend upon horizontal stabilization to enable adequate bracing capability. That is to say, they must be attached so as to halt lateral deflection of the two flanges at specified brace locations and at the ends. This effectively corrects a common standing-seam steel structure roof practice of introducing a single line of sag angles along the top of the purlin flange with sliding connections. To prevent purlin rotation under load, a single line of bracing in this manner is not high enough in the structure. The flange that must be constrained requires purlin bracing as close as possible to it. Building specs in which the bracing is away from the top flange are questionable; they may not provide both flanges with sufficient lateral deflection protection and may allow harmful rotation of the components.
Excellent purlin stability can be achieved by properly applied crosswise braces even if they are farther away from the flanges. Be aware that this bracing technique should only be acquired if a through-fastened steel structure roof will be installed. Fortunately, because of the well-deserved acceptance of standing seam roofing in steel buildings, sliding connections eliminate a lot of bracing issues. This roof configuration allows the benefits of diagonal bracing to be easily achieved by incorporating lines of bracing angles running alongside each other near the uppermost flange.
The employment of a through-fastened steel building roof does not rule out the prerequisite of proper purlin bracing, however. The steel rooftop itself can produce lateral, if uncertain torsional, support of the purlin. But the pre-engineered roofing diaphragm may not be substantial enough to counter lateral translation under loading from the whole system of purlins and roofing.
A better purlin support system is comprised of compact intervals of bolted channel blocking. Since bolts have a larger connection capacity than tabs or screws, this is a great alternative for reinforcement of the two purlin flanges as it negates translation and rotation. For smaller structures, a set of rows of angle braces can be affixed to the top and bottom flanges.
If purlin reinforcement is needed, it is crucial to correctly engineer the purlin intervals. The guideline for optimal purlin lateral support length as well as a workable configuration standard is a minimum non-reinforced purlin length of either between five and six feet, or a quarter of the length of the purlin span. A particular purlin section may distort and fail because of a deficiency in these critical calculations.
To build a stable, durable structure, think about these considerations when designing the proper purlin bracing method for your steel building project.
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