nirias
Civil/Environmental
- May 16, 2011
- 9
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?
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?
