4" CMU on 8" CMU 4

[XR250]

I have an Arch. who want to use one course of 4" CMU on top of an 8" CMU foundation wall. She wants to do this as it is a slab on grade with 2x4 framed walls and does not want any CMU visible from the interior. (see attached - drawn as 2x6, but wants 2x4). I am pushing for a 2x6 wall so we can use 6" CMU and get some rebar in it, but she is hurting for interior space.
I was thinking about installing 1/2" threaded rod in the top one or two courses of the 8" CMU and trying to slide the 4" CMU over it and grout the 4" CMU with a sand mix (the rod would be used as the anchor bolts and reinforcing for the 4" CMU). Does this sound reasonable or are the face shell thicknesses different enough that this would not be possible?

 

[manstrom]

It is damn near impossible to get any reinforcing or anchor bolts in a 4" CMU block. 6" yes, 4" no.

You can also use an 8" shoe block (an L shaped block). That would be much more practical and would allow for reinforcing.

Can you get away with a 2x4 wall and still meet insulation requirements? Not sure where you are, but R19 insulation won't fit in a 2x4 wall.



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[SWEG]

If it's a commercial building then you should probably go with 2x6. Just about anywhere in the country you'll need 2x6 walls to meet energy code these days. If 2x4 walls are allowable and you are having trouble with 4" CMU, I'd do an 8" thick turn-down slab edge on top of the 8" CMU. You'll need some L-shaped dowel bars from slab on grade hooked down into the 8" CMU to resist settlement shear loading at the face of 8" CMU. Additionally, the contractor usually complains about this method because they have to form the exterior face of the slab on grade to pour the slab. However, the end product is really nice and the architects typically like not having a joint between slab on grade and CMU, especially after some time when the slab has shrunk and some slab settlement has occurred. The long-term byproduct of that method can be pretty poor, aesthetically, in some cases.

 

[XR250]

It is residential. 2x4's are fine in our area.
8" Header blocks or similar restrain the slab leading to cracking and are no good for perimeter insulation.
All I really need is for the threaded rods to get thru the cores of the 4" CMU and it will essentially post-tension it once they are tightened.

 

[jfmann]

Consider concrete (with vertical bars) to replace upper block.

John F Mann, PE

http://www.structural101.com

 

[XR250]

@jfmann;

That is thought, but would probably just switch the whole wall to concrete at that point to keep it under one trade. Might end up doing that as the house value could justify the extra expense.

 

[XR250]

If anyone is interested, we solved the issue. We are going to build the 2x4 stud wall on top of the 8" CMU and use the face of the stud wall as the slab edge form.
Better structurally and better insulation. It will require some additional coordination with the framing, but the contractor seems to feel it is the best solution.

 

[hokie66]

Termites will love that.

 

[jfmann]

Ouch......good luck!.....appears to be high risk for long-term decay since water will find way behind edge of slab (and there is exposure to inside from under slab also)......at very least, use treated-wood. However, I find hard to believe competent masonry contractor could not simply install what amounts to simple curb on top of block wall.

John F Mann, PE

http://www.structural101.com

 

[XR250]

@Hokie & JFmann:

Does not seem to be any worse for termites than the original detail. They will find ANY wood that is near grade. The wood will be at least 8" above grade and it is flat site. The framing will be flashed and insulated with blue board at the interface. The slab will bear on 2" of foam on 4" of washed sotne. It is unlikely that there can be a moisture problem. They will also likely use an in-wall insect treatment system such as this

http://www.pestdefense.com/taexx.

That is what the local tract builders use when they do a monolithic slab.
FWIW, Installing a concrete curb requires a significant amount of formwork.

 

[msquared48]

Tell the space conscious Architect that if she does it here way, there will not be any space left when the building fails. The space will be maintained with your way.

Mike McCann, PE, SE (WA)

 

[XR250]

Nice Mike
I'll see what I can do.

 

[CELinOttawa]

Love the comment Mike! On a related note: I once had a PhD insist that drawings go out of the office showing 5/8" (M16, or 15M) bars [let's just conceal the nation this happened, to protect the guilty, shall we...] every 16".

I tried to explain, but I was "just" a B.Eng. Following this, and the gong-show that I had to get the company out of, I was placed in charge of monthly practical training sessions for all of the company's engineers.

I tell you what, I learnt more from the training sessions than I imparted on others. But never did we find a way to reliably reinforce a 4" CMU! lol.

 

[concretemasonry]

Despite 47 years experience in engineering/masonry construction, I have hard hard time even seeing a 4" block (4x8x16) in the detail shown.

According to ASTM C90, a a "4" (4x8x16) unit is 3-5/8" thick x 7-5/8" high x 15-5/8" length. Apparently, there is some misconception of the specifications or just wrong terminology or usage.

The detail shown could be revised to show a 8" bond beam at the top of the 8" wall and then a 6" block (filled with grout) and appropriate reinforcement/bolts used for the 6" wall of the above structure.

I have no idea what the 3 units making up and 8" height are. They look like 2" or so high solid units that could be drilled. There are often 6x4x16 units that are used domestically and especially internationally, that have cores that will allow for reinforcement before grouting/filling.

According to most codes, 4" thick CMUs cannot be legally grouted because of the required size for a grouts space and coverage, but the they have been used for decades after approvals from nit-pickers.

That sure looks like a very neat and pretty detail drawing, but even more expensive to use.


Engineer and international traveler interested in construction techniques, problems and proper design.

 

[CELinOttawa]

You might be able to use some form of epoxy in this way, but how successfully and how durable a result? Who knows?!

I think the idea should just be put to bed, and the issue explained yo the Architect. Best not to try something like this.

 

[XR250]

Agreed, although I do have one more idea (just for grins). What about bolting a PT 4x4 to the top of the 8" CMU? Use that as the screed for the slab and then could easily fasten the wall to that with structural screws.

 

[CELinOttawa]

Don't use 4x4 in that way... Much too likely to have widespread longitudinal cracking and not be able to perform...

 

[CELinOttawa]

Mind you your idea works for a standard wall to be built atop... Just need to address stability.

 

[XR250]

@CEL

"Much too likely to have widespread longitudinal cracking and not be able to perform…"

Can you elaborate on this?

Thanks

 

[CELinOttawa]

Sure: It is a matter of proportions. When you have a 2x4, you get the occasional shake and the occasional crack here and there. Rarely do you get a long, deep, crack running the length of the member.

Such cracking tends to be related to the residual strain caused through the stack drying, kiln drying, or really any old type of drying in wood.

If you go to any lumber yard you'll quickly note that many 4x4 and larger, particularly square, members will have long cracks separating them into two or more "sub-elements". This is not particularly critical in their traditional uses (ie: low-load columns), but would start causing trouble if they were part of a shear transfer, collector, anchorage, etc.

Does that make my concern more clear?

If you want to pursue this, just use a double bottom plate where a single doesn't suffice.