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- To: <seaint(--nospam--at)seaint.org>
- Subject: horizontal roof bracing 2
- From: "Andrew Kester, PE" <akester(--nospam--at)cfl.rr.com>
- Date: Tue, 18 Dec 2007 13:27:35 -0500
Thanks Mathew, Gerard, et al for your input.
It is in Florida which is ZERO seismic, V=110mph...
I also am a big guy and do not like doing inspections on PEMB roofs!! I feel like the deck acts more like a tensile fabric then it does a rigid gravity member :)
Also just to clarify, it is not a PEMB. It is a 2 story office building, CMU shear and bearing walls. The contractor/client has done PEMB his whole career and wants to put a typical PEMB roof system on this building. We do not see how to accomplish this with diagonal rods or flat straps with no steel frames, beams, or anything else but light gage purlins and CMU walls. We understand how this sytem works on a PEMB steel frame system as per Mathew's excellent explanation, just not with CMU shear walls.
We have no other steel other then the roof girders and the purlins and that is for gravity loads. We have no horizontal roof girts to take the lateral out-of-plane loading from the top of the CMU walls to the end shear walls (and no moment frames either). Horizontal X-braces would result in a large concentrated diagonal tension force into the bond beam with nothing in compression to resist it as in a moment frame system, except for the purlins... I can see how perhaps flat straps attached diagonally in both directions at a small spacing may work in theory, but a lot would have to be overcome in this situation and a lot to detail. I also think this would create excessive deflection that would be incompatible with our CMU walls.
The GC I believe just sold the client on a "maintenance-free" standing seam metal roof and now realizes it will be redundant over top of a steel roof deck. I don't think they have ever done a non-PEMB structure so steel joists, deck, rigid insulation and membrane system is foreign.
Please CC me directly as I get the digest mode, thanks!
Andrew Kester, PE
ADK Structural Engineering, PLLC
1510 E Colonial Ave., Suite 301
Orlando, FL 32803
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