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Purlin bracing in metal building

Purlin bracing in metal building

Purlin bracing in metal building

(OP)
I'm reviewing a proposal for a pre-engineered metal building (PEMB) with standing seam roof and don't know what I should be looking for in terms of purlin bracing. The snow load is 200 psf (315 ground snow) with a pitch of 6:12, so purlins experience significant off axis loads that make lateral bracing necessary and there are serious in-plane forces with no apparent roof diaphragm that might meet them.

The standard purlin bracing is just three lines of "knock in" sheet metal braces that extend from the top of each purlin to the bottom of the adjacent purlin. A more robust version of that bracing involves x-braces between purlins except at the ridge where there is only a single tie to the first purlin on the opposite side of the ridge. Typically there are 3 lines of purlin braces per bay. The only in-plane roof bracing I have seen on PEMBs is tie rods in an "X" between bays. That keeps the portal frames stable but does not address purlin stability.

With a 6:12 pitch, 200 psf snowmeans 100 psf in-plane force apparently resisted only by sheet metal purlins loaded about the weak axis. The attached sketch shows a potential failure mode for purlins that might result that would not be resisted by the x-braces or by the standing seam roofing (attached with a few sheet metal screws per foot at the eave line). The sketch is not to scale and for clarity I only illustrate one line of purlin x-bracing. The structure is 50 ft x 70ft overall and bay spacing is 23 ft 8 in.

Is there a proven detail I should be looking for that would keep all of the purlins from moving laterally?

RE: Purlin bracing in metal building

Can you not specify a roof diaphragm for the building?
Seems not only do you have a purlin translation problem but also a torsional one as gravity act downwards, not a a 26 deg. angle.
I would like to see two back to back "Cee" shapes or an I-beam as a purlin oriented vertically with some sort of beveled plate on top. Of course, the economy of a PEMB goes out the window in this scenario.

RE: Purlin bracing in metal building

Wouldn't this be on the PEMB supplier to figure out? It is their component, they should figure out how to best brace it.

RE: Purlin bracing in metal building

Aren't the sag rods connected all the way to the other side of a dual slope roof preventing the purlins from minor axis bending?
i can understand this as an issue if it's a monoslope roof.

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