Cantilevered Retaining Wall

[RyUIUC]

I am designing a Cantilevered Retaining Wall using the CRSI 2008 Manual as a reference. I have used the tables at the back of Chapter 14 as a starting point and do all the checks by hand. One comment I got from another engineer checking my work is that the vertical stem bars do not properly develop the ldh into the base slab. For example, if a #7 bar is specified for the stem flexural bars and these bars extend into the toe of the base, the base slab should be thick enough to develop ldh of the #7 before it turns into the toe. The CRSI manual does not seem to check this and their tables often specify bars that could not develop this length for the corresponding base thickness'.

I tried to rationalize with the checker that this bar is not really a 'hook' and that it is actually checked to fully develop beyond the face of the stem into the toe thus is actually being spliced into the base reinforcing.

Does anyone have an idea if the ldh really should be developed by these 'O' bars before the 90 degree turn into the toe or a good reason why its not necessary?

 

[ron9876]

I think it should be developed.

 

[ishvaaag]

What needs to be developed is the standing design (factored) force each side from the interface. The top side will be straight, and the bottom will include a quarter of a circle curve; its effects on the required projected horizontal length to be determined as per the code.

 

[JAE]

Should be developed but you can use As(required)/As(provided) as a reduction.

 

[TXStructural]

If the bar provides an extension longer than required for ldh, the longer extension continues to develop the bar. The reinforcement into the toe should be developed (ld) by the time if gets to the critical section for flexure in the base, and once developed, it stays that way. In essence, it will usually be developed by ld on each side of the bend.

I am in the process of editing that chapter of the Design Handbook as we type, and will make sure the particulars are addressed in the revision.

 

[RyUIUC]

TXStructural- Are you saying ldh continues to develop after the bend? For the purposes of flexure in the toe/heel I understand the base mat bars being developed on each side of the critical section, what I am concerned about is exclusively the vertical length of bar as the stem bars tie into the basemat.

For example, in an extreme case lets assume the 'O' bars extend into the basemat and bend immediately to lay just beneath the basemat top reinforcing layer providing an ldh of approximately 4 inches. Intuitively this would not appear adequate since the it can be assumed that as the stem bends toward the toe that these vertical bars would essentially be pryed out of the basemat through the top starting at the stem.

The CRSI manual tables in Chapter 14 commonly specify 'O' bars as large as a #7 in a 12" thick base. ldh(#7)=approx.12" for f'c=4ksi and assuming proper cover. Thus, when considering the bottom cover in the basemat, it would not appear to be thick enough unless the Asprov/Asreq provision was utilized (doubtful).

 

[ishvaaag]

Certainly for very thin slabs, rupture through disengagement rupturing the concrete at corners maybe an issue; and maybe codes are not very sensitive to this because uncommon with the sizes of the past, then not leading to particular specification. So my comment above needs be completed in that.

In the cases of dowels for columns nascent from slabs on both grade and at some elevation the question may have been somewhat tamed by the thicknesses required by punching shear, and the fact of service level loads not reaching the design factored level; so there may be a number of structures having such defect for actual factored level solicitations; and there must also be some failures observed and standing in the literature, for as in an application as this for earth retention the push may have well dealt with some too optimistically thin walls and foundations at their root.

 

[TXStructural]

Horizontal ends of O-bars should be above or below the bottom longitudinal bars, as deep as practical in the footing/base. Figure 14-10 in the Design Handbook shows this. These bars should be at the bottom to act for toe flexure in addition to providing vertical bars or dowels.

We specify that the base should be a minimum of 12 inches, which places these bars about 8 inches below the top of the toe. (3 inch bottom cover, 1/2 inch longitudinal bars, and 1/2 of a #6 or #8 vertical bar diameter.)

As to development of the bar, once a bar is developed by either ld or ldh (or as a headed bar), it is developed and is not expected to slip axially. In this case, the horizontal section develops in the toe and the vertical section develops in the stem/well. Since they are ends of the same bar, they are developed before they reach the bend.

 

[ron9876]

Yes but why wouldn't the distance from the top of footing to the bottom of the extension to the toe not have to meet the hook developemnt length.

 

[JLNJ]

I'm with Ron. You have to develop the 90 degree hook. It often dictates the thickness of the footing.

 

[dcarr82775]

I ran into the same thing maybe 10 years ago. I didn't think the 'O' bars were adequately developed for some specific instance. I agree with Ron and JLNJ that the footing needs to be thick enough to develop what is needed.

 

[JAE]

I'm pretty sure that ACI specifically states that extending the leg of a hook does you no good in developing that hook.

I do agree with TXStructural that the extension of the leg past the critical point of the base (face of stem) does indeed develop the bar for flexure in the base.

However, the flexure at the bottom of the stem won't be helped by the horizontal leg extension in my view. The vertical force in the bar will pry the concrete and bend the leg. Different mechanism.

 

[TXStructural]

The bar itself will be fully developed before entering the bend. Are you are suggesting that the bent bar has insufficient strength through the bend?

Other than bar pull-through, the failure mode seems to be concrete tearout as the wall tries to rotate toward the toe and pulls up on the hook. This seems unlikely since the toe face of the wall will be compressing the base and confining the reinforcement. This would put the face of the heel in shear. The "D" bars directly resist this shear. The "P" reinforcement in the top of the heel restrains the the wall from rotating and keeps any crack at the face of the heel tight so aggregate interlock is maintained.

I will do a bit more research on this for the next edition, and will post here if I find anything new.

 

[hokie66]

There has to be a minimum dimension from the top of the footing to the bottom of the bar bend. Otherwise, you could bend the bar into the top of the footing.

If the dimension is less than that required to develop a bar with a 90 degree hook, then the thickness has to increase or the bar size has to decrease.

 

[JAE]

Yes - agree with hokie's post.

 

[ishvaaag]

I also agree that Nokie's is the safe approach. It might be (maybe) unduly conservative for well detailed anchors of the kind, since no particular insistence has been placed on the matter, to the point that there are tons of columns nascent from slabs violating these things, and even CRSI might been overlooking this at some points. The matter seems to be still being investigated, will try to find more references

http://www.dot.state.fl.us/structures/ResearchProjects/Pullout Test on Barrier Reinforcing.pdf

 

[ishvaaag]

Park, Paulay's on hook anchors:

"the straight part following a hook is generally ineffective"

when bent around a bar gains of 10 to 30% can be obtained, but contact can't usually be warranted...

in general reinforcing the line of Hokie66 post.

 

[ishvaaag]

Section 3.10 within NCHRP 603

"Transfer, Development, and Splice Length for strand/Reinforcement in High-Strength Concrete"

http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_603.pdf

just adressing the standard hook in ordinary way, i.e., reduction of projected length plus adaptation to actual level of loading.

 

[SteelPE]

I can't ever say that I have ever had a wall where I was using #7 bars where the base of the footing was only 12" thick.

I usually use the CRSI tables for getting relative geometry and then run a bunch of the other calculations by hand. Of course, CRSI tables were passed down to me from my mentor and are probably pretty old. I guess if gravity changes some time in the near future I should throw the tables out.

 

[JLNJ]

My older CRSI has the footing depth equal to the stem width. For a 14' wall with #7s at 8 in the stem the depth is 19". Seems about right.