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?

 

[XR250]

Got it. I am familiar with that cracking. Not a whole lot of uplift or shear transfer in this structure and I typically go 32" o.c. with anchor bolts which would help.
Anyhow, at this point I think the Arch. is just gonna have to accept a 2x6 wall.
It would be nice if someone would manufacture a thin-walled 4" CMU that could be used in this application.
FWIW, there are hundreds of houses built in our area that just use one or two courses of 4" brick instead of the proposed CMU. No rebar and the anchors just grouted into the holes of the brick. The Arch's think I am crazy not to sign off on things like this.

 

[CELinOttawa]

I've seen a lot of crazy first world construction, not to mention what gets done in the name of shelter and roads in developing and third world.

Your concerns are rational, and your refusal to sign of commendable.

 

[RFreund]

I'm confused, why such push back against the 4" cmu. It essentially acts as blocking and a pour stop? It is only one course tall, right? It only needs to transfer shear and compressive loads. The anchor bolts can be embedded into the 8" block below. It's like an 8" grout pad, no? You are concerned with the "hinge" at the bottom of the wall I suppose? Can't you use #3 bar by code? I mean don't get me wrong I wouldn't do a 4" cmu full height wall and they probably would have a very difficult time getting #3's into a full height wall, but this is one course. Sure 6" is probably better, but I'm not understanding the push back against this (maybe I'm miss reading the tone of the thread).
I suppose the detail has about zero tolerance when you figure trying to line up anchor bolts and dowels. Maybe that in itself would talk me out of it. Yeah probably, use the 6".

EIT

http://www.HowToEngineer.com

 

[XR250]

Here is what the Arch. and contractor have agreed on. I actually do not think it is too bad. The wood sill and bottom plate are PT.

@RFreund - There is uplift as well and the prying force from the hinge to deal with. In reality, the 4" would probably work, just not meet code for grout space.

 

[CELinOttawa]

why such push back against the 4" cmu

Because between construction tolerances, insufficient space for grouting and the impracticality of wetting the interior surfaces and achieving a surface saturated dry condition in such a weird place without clear outs...

Just not a good idea. I doubt it would behave as you are thinking, but I will be the first to say I simply don't know. With that in mind it is once again not a good idea since we are meant TO KNOW our structures are safe.

Cue someone coming on to tell me I'll being overly conservative, and or dramatic.

 

[KootK]

I'm with CEL in that my preferred way to do this detail would be not at all. But then market pressure is what it is.

I think that your post-tensioning idea is really pretty clever. Suppose:

1) You shift the block wall towards the exterior such that it is centred under the 4" masonry. That way, you can sink your rods into the block below without being tight against the face shells.

2) You use whatever masonry product and anchor rod combination that allows you to pass the anchor rod through the masonry. Constructibility may be an issue. It would be ideal if it could be drilled in adhesive anchors.

3) Use a generous washer plate under your post-tensioned anchor rod nuts.

I don't love this, but I could live with it. Heck, given the scale of this, it could probably survive as unreinforced masonry if it could survive construction in the first place.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I like that idea of moving the 8" CMU out. It may give aesthetic problems if the grade drops off, however.
Tons of these have been built un-reinfoced without any failures that I know about (yet)

 

[Ingenuity]

The wood sill and bottom plate are PT.

I think that your post-tensioning idea is...

Crossed-wires with the acronyms, I think.

KootK, I think XR250 was meaning for "PT" to be pressure treated, not post-tensioning...

Eng-Tips need a FAQ of engineering acronyms...so we all "speak" the same "language".

 

[KootK]

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.

@Ingenuity: both versions of "PT" have been discussed here, surprisingly. Although, given the particular situation, maybe "pretensioned" would have been a more apt term.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.