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Existing Purlin Bracing

Existing Purlin Bracing

Existing Purlin Bracing

Have the situation of an existing metal building with no documentation and unknown manufacturer. The owner's property insurer wants to verify that the existing 8" Zee roof purlins are adequately braced. The purlins are 24' span @ 5' o.c. with (1) 4" channel attaching the purlin midpoints in alternating purlin bays. They are supporting a standing seam roof and 20 psf snow. For some reason the insurer only thinks this is a concern at the bays along the outer wall and not in the interior area, although they the same roof framing exists throughout.

With limited experience in cold-form steel, I am having trouble determining what direction to take. Even after going through the AISI spec and commentary.

RE: Existing Purlin Bracing

If the brace is connected such that it prevents rotation of the purlins, then the unbraced length of purlin can be taken as 12'.


RE: Existing Purlin Bracing

Prior to the 1996 AISI Specification, many manufacturers assumed that their standing seam roof braced the top flange of the purlin without the use of supplemental braces. The 1996 Spec prohibited that practice and required one of two solutions for purlin design, 1) unbraced length based only on the use of discrete intermediate braces, or 2) running of a "base test" which is a single span test with 2 purlins and the roof panel. That is loaded in a vacuum box in either gravity or uplift loading to determine a capacity. That capacity is then divided by the full braced capacity to determine an R-factor. That R-factor is then used directly in the design as a capacity reduction. This reduction in particular is not a big issue in interior continuous bays because the typical controlling condition is bending/shear interaction at the end of the purlin lap near the support. Depending upon the age of the building it is certainly a possibility that this is functioning under the base test rules and they are needing extra reinforcement in the end bays vs. the interior bays, hence the channels only in the end bays.

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