Accumulation of snow or ice on the roof may cause the purlins to rotate from their original orientation, compromising their structural integrity. This phenomenon called purlin “rollover” can cause purlins to buckle and eventually, collapse.
Prior to 1996, there were reported instances of actual roof collapse incidents because of purlin “rollover”. The American Iron and Steel Institute (AISI) recognized the design flaw and established a standardized test procedure and clarified how adequate lateral bracings for purlins can be achieved. The revisions are contained in the 1996 release of the Specification for the Design of Cold-Formed Steel Structural Members.
The new standard requires that a “base test”, done according to AISI standards, be performed first. The test returns the amount of lateral support that is provided for the purlin, by the deck and clip. The design of standing seam roofing systems for steel buildings should either include purlin braces or the capacity used in the design of the purlins be reduced, based on the results of the base test.
Some of you may ask why should this happen when building codes were supposed to be factored in, in the design of steel buildings. Building codes were indeed incorporated in the design, but the designers may have used incorrect assumptions about the ability of the deck and clips to provide adequate lateral bracing for the purlins.
The AISI recognized these incorrect assumptions, prompting them to update their standards.
Make sure that your steel building supplier/manufacturer has been given a AISI-MB certification to ensure that your steel building is of high quality and designed based on the existing standards.