urgent CMU needs thickened slab?

[boffintech]

Picture for a moment: a non-load bearing interior CMU wall (8” x 12’ tall with 1 #5 vert. bar at 48” oc) on a 5” concrete slab-on-grade with WWF on 4” of #57 w/vapor barrier.

Is it usual or unusual to have a depressed area (a thickened slab) for this wall? Does the IBC require a thickened slab for this condition?

The #5 dowels have a standard ACI hook. Without a thickened slab area under the wall the hooks of #5 dowels only have a couple on inches of cover as they just hook into the slab. Will this bar be fully developed? Should I be worried or are these some of those so-called “fixing hooks”?

Some on these walls have roll up garage doors (all interior storage rooms) and the EOR has called for 2 #6 bars at each jam. No thickened slab area is called for so these big #6 hooks are just hooked into the slab.

 

[LowLax]

I'd say it depends on the strength of your soil. The soil would have to be pretty bad for the wall to cause noticable settlement, but the jamb loads might be big enough to cause concern.

 

[haynewp]

I usually put in a thickened slab if the wall is more than about 8 feet and so do other engineers I know. The slab might calculate out as being alright with a 12 foot wall, depending on the soil properties and slab strength. The #6 bar thing may not have been so well thought out. Is there anything bracing the top of the walls? If so, then the embedment of the dowels may not be that important.

I don't know of the IBC requiring a thickened slab, but I do recall it requiring the tops of partition walls of a certain height to be braced in seismic areas.

 

[boffintech]

All of walls are braced at the top by angle iron. Brackets are bolted at 4' oc to the underside of the elevated concrete slab above (on the sides of beams or joists or directly to the slab if needed) and angle iron is welded to brackets.

 

[JAE]

We usually use thickened slabs for walls such as yours. Technically, ACI requires that all footings be at least 8" thick (or is it 6" - can't remember right now) so you probably ought to have at least that (8").

This is where engineering overlaps into art - you could do a calculation to see if the slab itself would be capable of resisting the shear caused by the self-weight of the wall - but this would be based on a thin floor slab and the position of the WWF is always a big unknown - so your d is varying all over the place.

Most engineers don't sweat this - a thickened slab isn't that expensive to do - just a minor headache for the contractor to dig out prior to placing the VB, WWF and concrete.

 

[boffintech]

This slab was placed today so the EOR got the 5" of slab he spec'd out. It will probably be OK.

 

[jjeng2]

If the wall is braced at the top I wouldnt be too concerned. In general it is good practice to thicken the slab under interior masonry walls.