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Re: Waffle Slab Shear Thickness

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I don't know of a published study, but it makes sense.  The joist grid alone without the slab would have rigidity and shear transfer capacity in a "space frame vierendeel" manner; with the slab a waffle is an incredibly stiff system.  I would consider a waffle system the truest form of rigid diaphragm, and would not consider shear transfer within the diaphragm to be a problem unless there were really irregular shapes and openings.
Interesting question.
----- Original Message -----
Sent: Tuesday, August 30, 2005 4:28 PM
Subject: Waffle Slab Shear Thickness

I am looking at the concrete floor/roof diaphragms of an existing multi-story building.  The floors are either 2 1/2" or 4 1/2" thick with integral concrete joists at 2'-6" on center that frame into integral concrete beams.  My question is what effective floor thickness would you use for resisting the seismic diaphragm forces?  The easy and quick response would be to just use either the 2 1/2" or 4 1/2" floor thickness but I was hoping to take advantage of the joists.  Is there such a thing as an "effective floor thickness" for resisting seismic diaphragm shears in a waffle slab system?


Thomas Hunt, S.E.
ABS Consulting