Compression Dowels
Compression Dowels
(OP)
I have a project in which I have designed a concrete vault, completely underground. The walls were designed using a "fixed-end" beam analogy. Therefore, at the support ends of the wall, we have a negative moment where the inside face of the concrete is in the compression region of the beam, with a point of inflection approximately 2'-0" from the support.
An issue has come to light on the project with regards to the dowels ("L" bars) tying the compression steel into the top mat reinforcing steel of the footing/slab of the vault. The question has been raised that the dowels have not embedded deep enough into the footing. I have searched throughout the ACI and have not found any requirements for the compression steel, yet the Owner's representative insists that there is (without providing an ACI section) and has gone as far as to reject the work in place. The rep's reason for the rejection is that he feels that the wall should be treated as a two way slab and should conform to Chapter 13 of the ACI, which requires that the bottom steel extend 6 inches over the support. I believe, in this case, that the comparison is incorrect to treat the wall as such.
Have I missed something? Common sense tells me this is ok, but I thought I should ask...
Help??
RWPE
An issue has come to light on the project with regards to the dowels ("L" bars) tying the compression steel into the top mat reinforcing steel of the footing/slab of the vault. The question has been raised that the dowels have not embedded deep enough into the footing. I have searched throughout the ACI and have not found any requirements for the compression steel, yet the Owner's representative insists that there is (without providing an ACI section) and has gone as far as to reject the work in place. The rep's reason for the rejection is that he feels that the wall should be treated as a two way slab and should conform to Chapter 13 of the ACI, which requires that the bottom steel extend 6 inches over the support. I believe, in this case, that the comparison is incorrect to treat the wall as such.
Have I missed something? Common sense tells me this is ok, but I thought I should ask...
Help??
RWPE





RE: Compression Dowels
RE: Compression Dowels
RE: Compression Dowels
Now if you know for certain that the corner will never spread open, then that effect won't occur.
I don't believe that the 6" ACI requirement really applies here but your detail isn't really good detailing practice. The lower, horizontal leg of the bar should have been dropped to the bottom of the slab.
RE: Compression Dowels
It should be noted that the original detail was shown with the dowel located in the middle of the slab, but the shop drawings (reviewed by the Owner's rep) showed the bars as I had indicated on the sketch. The contractor had placed the bars as indicated in the shop drawings. My detail was more of an "as-built" sketch to keep the record drawings up to date. In an effort to help out the contractor (our client), I reviewed the conditions in conjunction with the ACI and could find nothing within the code that dictated the depth of embedment for this condition. I realize it is not the best of conditions.
RE: Compression Dowels
The bar, as positioned, really doesn't do anything. It can't keep the theoretical horizontal joint between bottom mat and wall together because it isn't developed past the joint.
It also probably isn't needed for compression reinforcement (I'd be surprised if you had used it as such)
It also can't serve as a shear friction dowel across the joint because of lack of development.
RE: Compression Dowels
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And to pick a nit, never draw a right angle in a bar, they have a bend radius which will clip the corner, and if it is not accounted for properly, there will be thin cover or an exposed bar (another problem which goes away if the horizontal leg of the bent bar were dropped to the bottom layer of reinforcement.
Unless the concrete has sufficient shear capacity on its own, there needs to be a fully developed bar crossing the cold joint to engage shear friction. (As JAE also points out.)
RE: Compression Dowels
Shear friction? There is a bar in tension on the outside. The bar on the inside would be in compression, so would have no clamping effect.
RE: Compression Dowels
And I agree with all the "that's a crap detail" as well as Hokie about the "it doesn't matter to the function", but I'll add that it DOES matter to know what they really did build... Because that isn't it.
My money is on the embedment depth being deeper, or the bend radius being violated (ie: way less than 6d).
RE: Compression Dowels
Why do you say you cannot build it that way? I have seen it set up exactly that way...until I made them change it.
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Shear friction is not an issue, the wall will be in bearing at the 'notch' in the footing.
The only thing to check is if the hook development length is OK... it appears to be, but haven't done any sums on it.
Dik
RE: Compression Dowels
We all realize from an "engineering judgment" standpoint, this is NOT the way to do it. But from a technical standpoint, does the condition violate any part of the ACI?
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Dik
RE: Compression Dowels
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Dik
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RE: Compression Dowels
Also, just because there is a formed joint does not meant there will not be a crack that forms at the base of the key, allowing movement. This is why I made the observation about shear resistance along a cracked surface.
As far as ACI requirements:
The bars on the exterior face provide continuity across the joint, and will act as flexural reinforcement and as reinforcement to resist shear by shear friction. You do not indicate the embedment depth of theses bars into the footing/base slab, so we cannot know if they are actually developed, as would be required to meet your design intent of a fixed-end condition for the wall. I would suggest that the wall should be designed as a simple beam, or with a sufficiently robust detail used for the corners (which it may have been.)
As for the bars on the interior face ACI 318-11 says, "12.5.5 — Hooks shall not be considered effective in developing bars in compression." The use of a hooked bar can be construction aid, but it doesn't shorten Ld. As pondered by others in this thread, I wonder if compression reinforcement is needed.
I have seen quite a few electrical vaults with perpetual standing water or excessively humid interiors, so an assumption of dry conditions may not be realistic.
RE: Compression Dowels
The tension bars extend to the bottom mat of the slab/footing and extend an additional 4'-0" into the wall (approx. 4'-9" total Ld length).
The interior bars, where they occur in the compression region, were not considered in the flexural design of the wall. The only reason they are there is that ACI requires reinforcing on each face of the wall for walls 10" thick and greater. So in essence, the bars are really not doing anything and the dowels provided were more of a continuity thing than anything else.
Point taken on the water in the vault. As a side note, the joints were designed with a rubber, dumbbell waterstop across the joint.
RE: Compression Dowels
RE: Compression Dowels