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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

RE: Compression Dowels

Your description confuses me. Can you post a detail? Are these wall verticals, and if so, at the bottom or top?

RE: Compression Dowels

Well, I'd have to say that your 2 ft x 2 ft. #5 bars are really in the wrong location. You never want to wrap a bar around a corner on the inside face. If there is ever any spreading of the walls, where the bars would go into tension, the "L" would try to straighten out and that would result in the surface cover concrete spalling away.

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

(OP)
Granted...IF the bar ever went into a tension condition, then I would agree with you. However, the condition, during the life of the vault, is that it will be under a constant soil pressure. Even should a condition occur where only one side of the vault is fully exposed, the side walls would take the lateral pressure in shear before the "exposed" wall is subjected to a reversal of stresses.
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

In a sense, you really just have a hooked bar going into the lower mat. If so, then extending the "hook" only 1 1/2" isn't really good practice.

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

As JAE said, it is poor detailing practice. But I have detailed plenty of walls without this starter bar on the inside face. It does nothing. The wall is keyed in for shear, and the bars would not be required in compression.

RE: Compression Dowels

(OP)
Hokie...That is argument I am making for this condition...Like I said, it was a condition that was placed by the contractor and was unfortunately not brought to light until after the concrete was poured.

RE: Compression Dowels

I agree with JAE.
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

TX?
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

I don't believe the detail was built as drawn... In fact I'll state that it wasn't. You cannot build what was drawn.

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

CEL,
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.

RE: Compression Dowels

I mean that TX was correct about a rebar with a standard hook not being physically able to make the turn on a 40mm cover inside to inside condition. So it didn't happen, something else did.

RE: Compression Dowels

Well, there was 3" of cover specified to the top of the footing. So other than the drafting error of showing the bar turned at a right angle, I don't doubt they built it that way.

RE: Compression Dowels

No, but even with 3" on the top of the horizontal portion, you can't fith the bat with a correct bend...

RE: Compression Dowels

I didn't say it had the correct cover around the bend or in the wall, I just said it could have been built that way. Anyway, we all agree it is an unacceptable practice, bending a bar around an internal corner like that.

RE: Compression Dowels

The inside bar should have been extended to the bottom. If there's a moment at the base, then the inside bars would be in compression at the joint and the bar being placed at the top would be ineffectual. It's good to get into the practice of a correct detail.

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

(OP)
I agree with you all that the condition is a very poor condition. Like I said earlier, the bars were tied by the contractor to the top mat of steel, and I was not given the opportunity to review the shop drawings or inspect the rebar prior to the pour. It was not detailed to tie into the top mat.

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?

RE: Compression Dowels

Don't know your code, but off the top of my head it cannot meet the minimum cover requirements...

RE: Compression Dowels

Do you need 3" cover on the inside?

Dik

RE: Compression Dowels

(OP)
No...this is an electrical vault. There is no exposure to the elements.

RE: Compression Dowels

Do you require cover to provide a 2 hour FRR? If so, that would be your governing condition, else, it appears the construction works as 'built'... no exceptions?

Dik

RE: Compression Dowels

(OP)
No...There no extraneous conditions for the concrete that would require a deeper embedment. The top mat of the footing/slab was detailed to have a 2" cover. The detail sketch I had provided was an as-built of the field conditions as relayed by the client and photographs of the rebar and formwork.

RE: Compression Dowels

My reference to shear friction and development of the bars refers to the tension created when the aggregate along a cracked plane tries to slide. The bars will go into tension to restrain the widening of the crack and prevent perching of aggregates (that is, assuring aggregate interlock restrains the movement at the crack.) ACI 318 requires that bars used for this purpose be fully developed on each side of the expected shear surface.

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

(OP)
I understand what you are saying about the shear friction. Without researching the exact particulars of the code reference for shear friction (Deadlines this week..lol), it would seem that the shear friction issue would need to be considered where shear reinforcing is required for the shear loads. Fortunately, in this case, the concrete shear capacity exceeds the shear load.

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

The bars on the inside face of that wall would not go into tension until actual failure had initiated. Therefore, I do not see how they could be used as shear friction reinforcement even if developed. But in the interest of full disclosure, I don't use shear friction at all, as I consider the concept to be "black magic".

RE: Compression Dowels

Shear friction if the mechanism by which a concrete section carries shear after it cracks (or across a cold joint.) This is also called interface shear. The reinforcement clamps the two surfaces together sufficiently to resist movement across the interface.

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