Compression Dowels

[rwpe]

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

 

[hokie66]

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

 

[rwpe]

Yes, these are wall verticals and at the bottom. Attached is a sketch I issued on the project.

 

[JAE]

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.

 

[rwpe]

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.

 

[JAE]

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.

 

[hokie66]

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.

 

[rwpe]

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.

 

[TXStructural]

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.)

 

[hokie66]

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.

 

[CELinOttawa]

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).

 

[hokie66]

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.

 

[CELinOttawa]

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.

 

[hokie66]

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.

 

[CELinOttawa]

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

 

[hokie66]

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.

 

[dik]

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

 

[rwpe]

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?

 

[CELinOttawa]

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

 

[dik]

Do you need 3" cover on the inside?

Dik