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Steel Buildings and the Requirements of Purlin and Girt Bracing

• October 21st, 2006

            The proper implementation of girt and purlin bracing in modern steel buildings is critical. Incorrect bracing can cause serious lateral buckling when there are vertical loading forces introduced. Cold-formed “C” or “Z” purlins are susceptible because they have shear centers in an area separate from the point of loading application atop the flange. These factors can overwhelm any non-braced “C” or “Z” section and can make them twist and become unstable even under the most forgiving of loading and roofing applications.
            Any pitched roof will show a tilting of the purlin web from a vertical position which can exacerbate the problems of twist. The gravity load at the top flange of a “Z” purlin can be remedied by the assistance of components that are perpendicular and/or in parallel in either a steep or gently pitched roof. Torsion loads can actually increase in roofs as they become closer to a horizontal plane rather than severe slope configurations.
            Any pitch above the horizontal plane should allow for purlin settings to be facing upslope. An alternating purlin positioning should be specified for that particular roof application. This, however, may not be true of standing-seam roofs because of lateral bracing requirement deficiencies provided for purlins. Single-slope structures often utilize opposing purlins in a like orientation to create greater bracing forces and better stability.
            There are three main factors that need to be considered and achieved by adequate girt and purlin bracing. It is crucial that the braces be designed and spaced to negate any crippling at attachment points. It is necessary to point out that the purlin bracing systems recommended for steel buildings today do not all meet the minimums of these three factors. The design phase of the project is crucial to proper bracing selection.
            One factor is the prevention of the negative impact of the entire assembly of roofing and purlins due to lateral forces. Even with the best construction processes and adequate lateral and torsion bracing, a single-slope roof unit with purlins situated in one direction will augment some upslope movement. The solution for this is reinforced anchoring at the ends of the bracing system that is strong enough to negate the aggregate bracing forces. Double-sloped roofs utilize angles or ridge channels for the same effect. Another solution is the introduction of a roof diaphragm that can transmit these bracing forces to the sturdy primary frames. The primary frames absorb the bracing force actions.
            Another factor in correct purlin and girt bracing is the restraint of the purlin or girt from any rotation and the easing of torsion. Any loading encourages rotation and this includes the pipes and ductwork that are attached to roof purlins. Purlin bracing will relieve this problem along with attaching hanging building accessories from the web and not the flanges.
            The third factor is to introduce flange bracing that is lateral in application. Exterior or interior member flange can be in compression and thus dependant on load direction. Both flanges may require lateral bracing. Smaller non-braced lengths of section in the weakest direction can be achieved by making the brace spacing of tighter design.
            Proper girt and purlin bracing design can extend the serviceable life of any steel building.