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RE: Basic Roof Diaph. Question

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Tension only rod bracing does not work without compression struts.  Analyzing
cold formed z members for combined compression and bending is nasty  by hand.

Also for z purlins you need to lay them out depending on the slope of the
roof: have adjacent purlin top flanges facing each other for a low slope roof
and purlin top flanges facing up slope for a high slope roof.


-----Original Message-----
From: Kevin Below [mailto:kbofoz(--nospam--at)gmail.com] 
Sent: Monday, December 17, 2007 4:57 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Basic Roof Diaph. Question

Andrew, the objective is to transfer the transverse loads from the plane of
the roof to the wall bracing.  You can do it with a roof diaphragm or by
horizontal bracing.  I think the main issue is whether you want to accept a
standing-seam roof.  It's a more important decision than the type of lateral
restraint. 

Kevin


On Dec 17, 2007 5:25 PM, Andrew Kester, P.E. <akester(--nospam--at)cfl.rr.com> wrote:


	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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