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Basic Roof Diaph. Question

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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