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RE: Basic Roof Diaph. Question

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For a typical PEMB, each transverse frame resists the tributary lateral load of its respective bay as distributed by the girts and spandrel purlin. For longitudinal loads it is common to use X-braced bays or portal frames constructed 90 degrees to and in between the transverse frames. At these same longitudinal resisting bays (since your typical standing seam, roof has no shear capacity) the loads dumped into the roof at the end "gables" and or wind columns are transmitted to the braced bays or portal frames via horizontal diagonal bracing (HDB) within (typically just beneath) the standing seam roof such that the HDB and the adjacent transverse frame members within the roof plane act like a truss to distribute the load to the braced/portal frames.


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From: Andrew Kester, P.E. [mailto:akester(--nospam--at)cfl.rr.com]
Sent: Mon 12/17/2007 5:25 PM
To: seaint(--nospam--at)seaint.org
Subject: Basic Roof Diaph. Question


Maybe I am not keeping up on state of the art roof systems, but....
 
We are designing a 2 story office building with a flat roof, 2nd floor is steel composite floor, walls are CMU. Rectangular shaped, nothing unusual.
 
The contractor/GC is an old PEMB guy, and for some reason wants to use steel light gauge Z-purlins and a 24GA standing seam type roof instead of steel joists and 1.5B steel deck, which is what we are highly reccomending (standard on any project like this). We asked if we can get tech info on the deck for gravity and diaphragm of this type of roofing and they said it does not have an allowable diaph. shear value. We insist that we need a 1.5B or sim. steel roof deck for our diaphragm, whether we use purlins or not. They are asking is there a way to use diagonal rods or X-bracing in the horizontal plane to substitute for the diaphragm. This is without adding any steel columns or moment frames, still all CMU bearing walls.

We insist this will not work and we need a proper roof diaphgram. Are we missing anything??? (I mean without getting into a fancy, complex theoretical discussion where you can get diagonal flat straps at some very small spacing to work for small loads with huge deflections....)
 
Thanks,
Andrew
 
Andrew Kester, P.E.
Principal/Project Manager
ADK Structural Engineering, PLLC
1510 E. Colonial Ave., Suite 301
Orlando, FL 32803

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