Interface Shear Between Wall and Slab. Does Reinforcement Need to be Fully Developed on Compression

[Quade999]

Hi Everyone,

If I have a situation like the one below where I have a retaining wall connected to a footing slab with the reinforcement having enough embedment for tension development only, does the reinforcement on the compression bending side of the wall need to be fully developed for the compression yield strength at the interface? Basically, the hook extending into the slab has enough embedment to attain yielding in tension but not in compression (as hooks don't contribute to compression development). For interface shear, the codes say that the reinforcement has to be fully developed on either side, but I take it that they mean fully developed for the yield strength in tension (as the reinforcement provides a clamping force. i.e. resists the shear with tension). If both bars shown have full development for the tension yield strength, then do they both contribute to interface shear resistance? Or do I need to meet compression development length requirements?

Thanks

 

[r13]

The development length for bars in tension with hooked end is l_dh (see figure below), which is identical to the development length for compression (l_d = 0.02d_bF_y/√f_c' = 0.02(60000)d_b/√f_c' = 1200d_b/√f_c'. You need to use smaller bars to reduce the development length, or increase thickness of the slab.

 

[r13]

 

[Quade999]

I'm looking at this with respect to AASHTO code. See below. Because there is enough cover on the hook, the tension development length can be multiplied by 0.8. This makes the tension development length approx 10 inches, while the compression development length is approx 13 inches.

 

[r13]

Looks like you are reinforce the retaining wall, IMO, unless you are utilizing the compression steel in design for a specific reason, the compression steel, theoretically, is not required, thus, no development requirement. If you feel the need to develop it, you can locally thicken the slab to accommodate the development length. Or as suggest before, use smaller size bars at a closer spacing, to reduce the development length, which is the preferred solution.

 

[Quade999]

In design, we normally ignore any benefit that compression steel gives (which is done here) for conservatism. I am just wondering if the area of both layers of reinforcing can be used to calculate interface shear capacity, or if only one layer can? My thought is that the shear friction will pull on both layers in tension, therefore they can both be utilized for interface shear capacity.

 

[BAretired]

I agree with that. In fact, the compression bars may be better as they have less likelihood of bursting out of the wall than the tension bars.

BA

 

[r13]

If you want to utilize the compression steel to resist shear, then, that is now a shear friction reinforcement, and shall be developed per shear friction requirement. But that not the way it is done for shear. Usually, the retaining wall should be sized to avoid requiring shear reinforcement, otherwise, you need to provide shear reinforcement similar to that in the flat slab, not the shear friction reinforcement. Now allow me summarize my responses:

1) Compression steel usually is not a must have (structurally) for retaining wall front face, but for good practice and crack control.
2) Ideally, the concrete stem shall have adequate strength to resist shear alone. However, if necessary, proper shear reinforcing (J hooks/shear loops) shall be provided to resist the excessive beam shear as in a flat slab, shear friction alone does not solve the problem.
3) Reduce bar size with a closer bar spacing to fully develop the compression steel. Now, this is a conservative practice.

 

[Quade999]

The wall itself has adequate shear resistance with the concrete shear resistance on its own. It's just at the interface where the friction of the concrete alone would not be enough and requires the benefit of the dowels. My assumption was that for interface shear, both layers of steel would be pulled on in tension and therefore both effective at providing resistance.

 

[r13]

You can also provide shear key at the interface of the wall and slab to increase shear resistance.

 

[JoelTXCive]

Your rebar is hooking in the wrong direction. Your primary tension rebars need to hook forward into the toe of the footing so that a 'closing moment' is created. There is lots of research on this along with older threads on Eng-Tips.

I usually lap and extend the primary tension bars 1.3ld into the footing bars.

The compression bars should be hooked back into the heel with a standard hook extension. (or via the other methods people have mentioned)

Here is a snip from the CRSI detailing guide. (they are only using a standard hook for both tension and compression though)

 

[JoelTXCive]

Here is one of the older threads.......

https://www.eng-tips.com/viewthread.cfm?qid=401855

 

[r13]

I don't really want to get into the dispute on the effectiveness of orientation of hook tail in column/wall footings. Basically I think the most important thing is having sufficient development length, and sufficient spaces around the development length to allow for quality bonding of the bars been developed. I don't think ACI has made a big deal out of it, so far. Link

 

[JoelTXCive]

Retired, I'm with you on needing sufficient development length; but there is a moment at the base of the stem and that moment must be transmitted to the footing. Having the tension bar tail oriented so that a 'closing moment' is created is a much more efficient way of getting that moment into the footing. I don't worry about this for concrete walls with minimal out of plane loading, but for retaining walls, I do think it is important.

Wait until Koot sees this thread. He will school all of us on "wrapping a moment around a corner."

 

[r13]

Yeah, I see quite a few people are excited about this topic. I agree that the tension bar for retaining wall should pointing outboard though.

 

[BAretired]

I have to admit, I don't see how it makes any difference which way the hook faces, but I'm sure someone will educate me. It seems to me that the only purpose for the hook is to act like an anchor for the bar and it does that whichever direction it faces. A plate welded to the end of the bar would serve just as well (probably better).

BA

 

[r13]

BA,

I think you are inline with present standing of ACI, or even CSA, but not to a lot of folks in the old thread cited by Joel. For retaining wall, I do place the hook tail towards the open side (away from backfill), but that is all about practice. I don't care the hook tail in wall footings, and always place the tails outward for columns to avoid congestion due to overlapping, which is a quality killer.

 

[BAretired]

ret13,

I do not dispute your placement of hook. But suppose you discovered the contractor had made a mistake and placed the hook pointing toward the backfill on a long retaining wall. Would you let it go or make him rotate all of his vertical bars?

BA

 

[r13]

Let it go, and have a cup of coffee and find a place to relax down

 

[BAretired]

Any more coffee and I would become a nervous wreck!

BA