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RE: Thickened Floor Slabs

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I tend to avoid thickened slabs because they will serve as a restraint as the slab on grade shrinks.  However, if the wall occurs about half way between contraction joints in plan, I may be inclined to use a thickened slab.

 

The cracking in any concrete slab on grad will primarily be due to shrinkage, but the nature of the applied load from the masonry wall must be considered.  If the grade is sand there will be no time dependent settlement.  If it is clay, any differential settlement will be time dependent. 

 

If an independent footing is out of the question, and you are inclined to use a thickened slab, place rebar in the upper portion of the slab, perpendicular to the wall, and stagger the ends of the bar in plan, and lap your slab reinforcing onto the thickened slab rebar.

                

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The rebar will not prevent cracking but the rebar will help control the size if the crack.

 

If you do in fact use WWR in your slab on grade, specify flat stock (not rolled), specify something greater than 10 gauge, and make them support it on dobies.  This can be the last operation before the ready mix truck shows up.  Flat work can be done right. 

 

Also make sure that your masonry wall is adequately braced at the top.

Regards,
Harold O. Sprague

-----Original Message-----
From: Al Grathwol [mailto:AGrathwol(--nospam--at)BoyleEngineering.com]
Sent:
Tuesday, October 07, 2003 12:48 PM
To:
seaint(--nospam--at)seaint.org
Subject: RE: Thickened Floor Slabs

 

I would thicken the slab at the walls - 4" seems too thin.

-----Original Message-----
From: Lester Unger [mailto:lesterunger(--nospam--at)yahoo.com]
Sent: Tuesday, October 07, 2003 8:39 AM
To: seaint(--nospam--at)seaint.org
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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