Considerations for Proper Purlin Bracing
There are a number of factors to consider to insure that the proper purlin bracing engineering has been applied to a steel building system. We will address several of these topics in this article.
Purlin bracing needs to be correctly installed for wind uplift conditions. If purlins are held in place by top-flange bracing or roofing they are actually supported laterally for compressive stresses caused by downward loads in the top flange. The bottom flange in compression, however, needs to be considered with the upward force of wind.
Approved formulas applied to steel building purlin bracing systems have shown that it can be assumed that the entire load capacity of a roof framed with “Z” purlins encountering wind uplift is just over two-thirds of its ability to resist downward loads. The roof, then, by extrapolation should be able to support a wind uplift of over two-thirds of the live or design snow load without additional bracing introduced for the bottom flange. However, bottom flange bracing is essential to prevent purlin rotation under loading and to help with stabilization the purlin.
The question of correct purlin bracing for standing-seam roofs also arises. Standing-seam steel roofs are specifically engineered to contact and expand with ambient temperature fluctuations and have a certain amount of calculated movement allowed in coordination with the supporting structure. There is indirect attachment of purlins to standing -seam roofs by means of concealed clips. The sliding tolerances of the roof are determined by the specific clip design. A tolerance of up to one and a half inches in roof movement is safely allowed in most applications. This design is questioned as to its actual ability to provide lateral bracing to the purlins, although friction between the purlins and roof does have some bracing characteristics. Use of discrete purlin bracing has recently been introduced in order to help reduce the standing seam roof’s propensity for poor purlin bracing.
Purlin bracing stability must be addressed not only between supports but at the supports themselves. There are a number of methods used to accomplish this, most centering around the use of an anti-roll clip design. Stable rafter to purlin connections can be achieved with utilizing an angle clip with gusset, a welded clip with gusset, an angle clip, or by means of a welded purlin clip. The choice of the best attachment in through-fastened roofs is variable to the amount of roof pitch, forces on the clips, and other factors.
Improper purlin bracing can lead to problems. Major wind and snow events can defeat improper bracing and may lead to total structural failure of the steel building. If the forces of nature cause a purlin to “roll” more than a few degrees from its designated position it is critically weakened. When gravity load is then introduced to these misaligned purlins torsional stress enters in and the purlins can simply fail and add to the collapse of the entire steel support structure.
Take steps to ensure that the purlin bracing system planned for your steel building project is more than adequate.