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Re: Thickened Floor Slabs

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Even though the numbers for a 4" slab may work theoretically, you have to
look at actual construction practices:

 - The granular layer directly under the slab isn't always perfectly
uniformly compacted.  For typical slab loads this is fine.  For a
concentrated line load it may not be.  The heavier loads may cause isolated
settleing, which will cause cracking in the slab or wall.  A thickened area
under the wall with nominal reinforcing (we use 2 cont. #4 bars) will add
significant stiffness to the slab directly under the load and will help
prevent cracking.

 - Almost every slab core I've ever seen featured the WWF within 1/2" of the
bottom of the slab due to the highly effective "pull-up" method of WWF
placement during the pour.  While this may actually help the bending caused
by the wall, it will do little or nothing to prevent shrinkage cracking.  If
a shrinkage crack is adjacent and parallel to your wall, your punching shear
is greatly reduced.

Jason Kilgore
Leigh & O'Kane, L.L.C.
816-444-9655 (FAX)
----- Original Message ----- 
From: Lester Unger
To: seaint(--nospam--at)
Sent: Tuesday, October 07, 2003 10:39 AM
Subject: Thickened Floor Slabs

What factors do I need to look at for 6" interior non-bearing masonry walls
on 4" floor slabs on grade? My company had some typical details of thickened
floor slabs under all 8" and 6" interior masonry walls. We no longer use
these details. Typically we put all 8" walls on wall footings and 6" non
bearing walls on 4" slab on grade. I checked the floor slab as if it were a
footing and found it to be OK with a 6" cmu wall 14' high on it. The
construction manager is insisting on the thickened floor slabs at the 6"
walls. I don't feel these are necessary. Comment would be appreciated.

Thank you.

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