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RE: 2-in conc slab = rigid diaph?

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Thanks, Will, for the response.  In the situation I have, the span between shearwalls in either direction is not too bad.  I can reasonably assume the waffle grid is stiff enough to be considered “rigid.”

David Topete, SE


From: Will Haynes [mailto:gtg740p(--nospam--at)]
Sent: Thursday, December 14, 2006 4:53 PM
To: seaint(--nospam--at)
Subject: Re: 2-in conc slab = rigid diaph?


ACI 2002 section 21.9.4 gives 2" as the minimum required for a diaphragm. I am not sure if it can automatically be considered rigid.



Will Haynes


On 12/14/06, David Topete <dtopete(--nospam--at)> wrote:

I am doing a peer review.  The structures are masonry, single story.  Some have pitched roofs.  Others are flat roofs.  The flat roofs are made of a c.i.p. concrete waffle grid, 8" joists, 2 feet c.c. each way in a 12" deep system.  The slab itself is only 2-in thick with 6x6 wire mesh reinforcement.  Can I assume the slab itself as essentially zero, and depend on the grid for diaphragm action to transfer loads to the LFRS?  If anyone has a quick answer, it'd be most appreciated.


David A. Topete, SE

Structural Engineer


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