Thicken slab for cmu wall

[dcceecy]

the cmu walls are interior partition walls about 10 ft tall, for some locations is 8" CMU , some is 6" CMU.

the cum wall will sit on slab on grade at bottom. the typical slab on grade is 4" reinforced with WWF. the wall will be braced at top to roof joists.

so obviously I need thicken the slab for those CMU walls.

I am thinking the thickened slab will be 8"x14"(wide), with (2)#4 continuous, with dowel #4 @ 32" OC to CMU from slab.

Does that sound right?

I did not check the wall its self yet. I plan to
grouted @ each course and reinforced with #4@32"0C (vertical)

Any suggestions and comments?

 

[ToadJones]

In situations like this, I always pour an footing below the interior walls also, similar to the perimeter. Then, the contractor can lay one course of block, perfectly level, then pour the slab around/in between walls.
This way you are no laying the block on the slab, which can be hard to pour flat and the walls are keyed at the base.

 

[Lion06]

That sounds pretty close. Our typical detail calls for the thickened slab to be twice the thickness of the slab, but 2'-0" wide at the base. We call for 2-#5 continous, and #4 @48" o.c. transverse, so what you're providing sounds reasonable.

This is, of course, for non-load bearing walls. For load bearing walls, provide a real footing.

 

[DHKpeWI]

If you are in a low seismic zone and the walls are non load bearing, the thickened slab should be fine. We have done this often for light industrial buidlings in Wisconsin and not had any issues. You might want to consider the joints in the slab to prevent cracking.

 

[dik]

I try to put these on a separate rectangular footing with the top of the footing flush with the slab. It's possible for the portion of slab that projects downwards to act as a restraint for the shrinkage of the slab and force a crack at the edge of the thickened portion. Also more difficult to coordinate sawcut joints.

Dik

 

[ToadJones]

Hell, I built my house like this.
Rather than use Lally columns and a beam, I poured a footing right down the center of the basement along with the perimeter footing. I laid one course of 4" block (4" wide, 8" tall) and built a load bearing wall on the block. The advantage being that I knew the block would be level and slab would be wavy. When complete you see 4" of the block sticking up with the stud wall on it. The concrete finisher ran his saw right up to the edge of the block. Worked out great.

 

[asixth]

Sounds right, I typically provide a thickening 1.5-2 times the thickness of the slab.

 

[rowingengineer]

Can we get an estimate of predicted movement for the site? less than 12mm (0.5"), 25mm (1"), 50(2")?

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[dik]

also, some jurisdictions have a minimum thickness for foundation work.

Dik

 

[DaveAtkins]

If you design this per the PCA Slabs on Grade book, you will be surprised how wide the thickened slab must be--maybe 3' or 4'. This is to control bending stress in the slab away from the wall.

DaveAtkins

 

[msquared48]

For partition walls, I just thicken the slab to 8 to 16" and widen the footing to 18 to 24" depending on how high the wall is and whether or not it is a shear wall that needs a grade beam. (I sometimes designate certain CMU partition walls to be shear walls)

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ThePenguin]

This topic is covered in DOD UFC 3-320-06A "Concrete Floor Slabs on Grade Subjected to Heavy Loads." This is essentially a reprint of US Army Technical Manual 5-809-12. Either way it can be obtained for free at the following link:

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_320_06a.pdf

I believe that the information relevant to wall loads is in Chapter 3. Hope you find this helpful.

- The Penguin

 

[abusementpark]

If it just a non-load bearing, interior partition, then the slab only needs to take the self-weight of wall.

We've generally been letting these wall rest on typical slab (non-thickened). Do you guys think a 5" building slab can't handle the self weight of a 10 foot non-load bearing CMU wall filled at 48" on center?

I don't believe we've had any problems with not thickening the slab for these walls.

 

[ThePenguin]

To connect the dots... Table 3-2 of the aforementioned DOD manual gives an allowable wall load of 565PLF for a 5" slab (wall on interior of slab). Taking the weight of an 8" CMU wall grouted 48" OC as 47PSF (ASCE-7 Table C3-1) this would give 470PLF for a 10' wall. This would agree with the previous post by abusementpark if the wall is only subject to its own weight. Additionally I beleive even higher design values for a 5" slab are given in "Designing Floor Slabs on Grade" by Ringo & Anderson.

 

[KootK]

I've run into an related issue recently. We detail slab thickenings below the walls with a control joint sawed / formed at the transition from thick to thin slab. The project architect wanted to move the joints underneath the CMU walls for aesthetic reason. I objected because I was skeptical that any kind of CJ in the thickened slab would allow the joint to do its job properly.

Thoughts?

 

[rowingengineer]

To the edge of the wall would be better, and if he wants to hide it make him batten out the edge. I would also spec a footing under the wall.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it