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

 

[rwpe]

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

 

[dik]

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

 

[rwpe]

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.

 

[TXStructural]

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.

 

[rwpe]

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.

 

[hokie66]

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".

 

[TXStructural]

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.