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New CMU wall on existing 4" slab on grade

New CMU wall on existing 4" slab on grade

(OP)
I am a structural engineer on a project where we are putting 8" cmu walls (approximately 12' high) on top of an existing 4" slab. I'm not overly concerned with the bearing of the cmu on the slab (if the slab cracks it's acceptable), however even if i tie the wall at the roof, does anyone have any recommendations for the interface between the cmu and existing slab? Would you dowel into the existing 4" slab with 3" of embedment? I'm not sure exactly what this would achieve.

Suggestions anyone?

RE: New CMU wall on existing 4" slab on grade

ACI limits any footings to minimum 6" thick.(see section 15.7)

How are you going to defend using a 4" thick "footing" for a load-bearing wall?

 

RE: New CMU wall on existing 4" slab on grade

OK - I apologize in that technically you didn't say it was load-bearing in terms of supporting floors, etc.  It does take its own weight but I would normally still try to follow section 15.7 anyway.

 

RE: New CMU wall on existing 4" slab on grade

(OP)
I am sorry for not including that it's a non-load bearing wall.

I am thinking about putting #4 dowels @ 48" and attach to the bottom of deck using some sort of a slip connection. Do you think getting 2-3" embed is going to add a lot of resistance to sliding?

RE: New CMU wall on existing 4" slab on grade

What the short dowels would achieve, short of cutting in a new footing, which is what should be done here to achieve the proper connection, is to weakly limit the lateral movement of the wall in a seismic event.  That's about it.

I agree that the effectiveness would be very minimal.

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

RE: New CMU wall on existing 4" slab on grade

(OP)
In addition, i've seen a link for DoD website that includes some slab resistance in which i'm very close to getting. Depending on what k factor you use and what I can assume the modulus of rupture to be.

Here is the
link...http://www.wbdg.org/ccb/DOD/UFC/ufc_3_320_06a.pdf

If you go to page 14 (section/page 3-4) you will notice they include a 4" slab as having bearing resistance.

Also, since these walls are rather spread out and in isolated conditions, the statement in the commentary for chapter 15 which says that it is 'generally applicable to ... mats supporting several columns or walls...' doesn't appear to apply. I wouldn't consider this a 'mat' as it is not required to spread out a concentrated load.
 

RE: New CMU wall on existing 4" slab on grade

Thinking out of the box here, if you really do not want to cut in a new footing, what about grinding out a 1.5" deep X 3.5" wide slot periodically under the wall line in the slab.  

This would give more lateral resistance than the dowels, but would be more costly labor-wise, obviously.

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

RE: New CMU wall on existing 4" slab on grade

Your load is about 400 lb/lf.  Based on the tables in UFC, and considering that perhaps you have a 3000 psi slab with a modulus of rupture of about 500 psi, you are at about their limits, though I would consider their values to be conservative.

If the area has any forklift traffic in it, I would consider the doweling; otherwise, no (unless seismic of course...but then small dowels are not going to do much for seismic resistance anyway).

RE: New CMU wall on existing 4" slab on grade

With 3" embedment in a 4" slab, the dowels will only have have 1" of cover.  

RE: New CMU wall on existing 4" slab on grade

Can you tapcon a clip angle to provide lateral bracing for the wall?

Dik

RE: New CMU wall on existing 4" slab on grade

With a 4" slab - and not really knowing what the stiffness of the subgrade is - I'd be sure to warn the owner that settling and/or cracks in the floor would be very possible unless they cut out the slab and install a separate footing.

I know it's not load-bearing.  

Also consider closer spaced control joints in the wall to account for possible differential settling along the wall length.

 

RE: New CMU wall on existing 4" slab on grade

on a 4" slab the wall will almost definately crack due to differential settlement. Personally I would not take the risk.

RE: New CMU wall on existing 4" slab on grade

on a prison job i worked on, 2" embedment into the slabs was typical for partition masonry walls including cell walls. the walls were grouted solid and had #4@8" or #4@16" so they may have been running smaller numbers, but i wouldn't know on that. Seems a pretty good pin connection with rebar shear values at the least.

RE: New CMU wall on existing 4" slab on grade

(OP)
Darthsoilsguy2... I'm a bit concerned with grouted everything solid. This will only increase the weight of the wall which will cause more cracking/settlement issues. The code says to allow for 5psf interior pressure, so for a 14' tall wall your shear reaction would be 35 plf at the base. Without running the calc, i would assume that a #4 would be able to take quite a bit more than 12# (force at 8") or 47# (based on 16" spacing).  

RE: New CMU wall on existing 4" slab on grade

#4@8" is there so that a prisoner has to also cut through a bar to escape.  All walls with security risks are grouted solid. Partially grouted walls are cubby holes for contraband (knives, drugs, cigarettes, etc). i'd bet there is a more stringent code for interior pressure on prison walls.

point being i've seen these short dowels put in all the time on slabs-on-grade and slabs-on-deck.

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