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Cantilevered Retaining Wall

Cantilevered Retaining Wall

Cantilevered Retaining Wall

(OP)
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?
 

RE: Cantilevered Retaining Wall

I think it should be developed.  

RE: Cantilevered Retaining Wall

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.


 

RE: Cantilevered Retaining Wall

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

 

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

(OP)
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).

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

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

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

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.

 

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

Yes - agree with hokie's post.   

RE: Cantilevered Retaining Wall

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%20Test%20on%20Barrier%20Reinforcing.pdf
 

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

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.

RE: Cantilevered Retaining Wall

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.

 

RE: Cantilevered Retaining Wall

This one informs that the steel force is almost dissipated into the concrete at 15 diameters from section of application of the force to be developed. That said, hooks and 90º bends become favourable to shorten the required thickness for effective anchorage of some tensile force.

Since this is just a study on some tests, it is not meant to justify any reduction on any development length as required by the code (it is well known that codes ask for development lengths 2 to 3 times the rupture values).

It, however, explains why in some of the many cases where development lengths  are taken mistakenly or purportedly short from code values (as could be for rebar nascent from relatively thin slabs) are not generally showing evident distress.

http://www.vncold.vn/modules/cms/upload/10/New%20folder/vca/Proceeding/Session%20B/B06.pdf
 

RE: Cantilevered Retaining Wall

From the OP:
"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?"

The horizontal portion of the "O" bar is developed before it reaches the face of the wall, the location of maximum moment in the toe.  In the current Design Handbook (2009), page 14-11, under "Toe": "If the 'O' bars ... do not have enough available length for the tension development of the bar, then the bars must be hooked or headed."  

The issue should not whether the bar has been developed for axial strength of the bar.  The bar turns vertical and remains developed, and the vertical portion develops fully into the wall above the critical section at the base.

This condition is similar to a fully-developed corner bar through a wall to wall joint, or a bent bar which extends up from a wall and into a slab.  Detailing Corner in the Sept 2009 Concrete International, and Detailing Corner RFI 09-3 in the November 2009 issue, discuss reinforcing wall intersections which need to transfer significant moment.  The same discussion is relevant for the base-to-wall intersection of retaining walls.

There can be an issue with the strength of the embedment at the depth of the bar detailed.  I agree there should be no question where this depth exceeds ldh, but several of the values in the CRSI Design Handbook are less than ldh.  In the case where the depth from the base of the wall to the bottom of the hook is less than ldh, but greater than the value shown on page 5-23, Table 5-7 (2 inches above the tail of a code-compliant 180 degree hook), engineering judgement should be used.  The CRSI wall table values should provide at least the required Table 5-7 values.  Also note that in the last paragraph of the commentary to ACI 318-05 12.5, they say that the "minimum value of ldh is specified to prevent failure by direct pullout where a hook may be located very near the critical section."  There is nothing else with reference to the depth of concrete required to restrain a bend in a fully developed bar.  Lacking more detailed analysis, a compression strut can be postulated to exist between the inside of the bend and the compression zone at the face of the wall (and the compression face of the toe), with ties provided by vertical "O" and horizontal "P" bars.

The concrete around and above the "O" bars is confined and restrained by the wall above, and by "P", "D", and longitudinal bars.  These bars act similar to supplemental anchor reinforcement discussed in ACI 318 Appendix D for anchors, crossing the notional shear planes/cone.

I will see if a strut-and-tie and/or other analysis can be provided in the Design Handbook revision to cover this kind of question.

RE: Cantilevered Retaining Wall

TXStructural - everything you state may be true - I'm not sure I can use references to refute what you are implying - but my little picture's worth a thousand words in describing how I think about it....as for me and my designs....I'll require Ldh.

 

RE: Cantilevered Retaining Wall

Yup, bottom slab needs to 15" thick.  Has anyone contacted CRSI about this?

RE: Cantilevered Retaining Wall

Respect to all clever people posted above, but the joint between the stem wall and the front base slab is a typical closing corner discontinues joint. The vertical bar could be developed into the front base slab as long as the bearing inside the bent bars is not excessive, it may be desirable to increase the radius of this bend.
 
If the stem wall is thicker than the base slab the tension force in the base slab steel will be larger than in the stem steel, to avoid what JAE noted above stirrups might be required on the base slab. See attached strut and tie model that account for change in bar force in such a region. (Source RC book- by Mac Gregor)
 

RE: Cantilevered Retaining Wall

Teguci,
TXStructural is our own CRSI rep.

RE: Cantilevered Retaining Wall

First, thank you to all who contributed to this thread.  It has resulted in identifying some information that may improve our retaining wall chapter in the Design Handbook and other publications.  The material below is my reading of an article or two on some research, and has not been vetted through the Institute.  It will be placed into our formal process for editing and review prior to a CRSI Technical Note or article being developed.  

Nothing I have found conflicts with the information presented in the CRSI Design Handbook chapter on cantilever retaining walls.
--
Research was conducted into wall corner joint reinforcement, including specific work on retaining wall reinforcement details, at Chalmers University of Technology in Sweden, from 1965 through 1973.  A summary of this was published in the ASCE Journal of the Structural Division in June 1976.

Specific reinforcement detailing recommendations for retaining walls included:
- Area of main reinforcement is "designed on the basis of the moments and normal forces" in the adjacent members, ignoring inclined bars.
- Bars are placed such that they are as close to surfaces as cover requirements permit, and extend to the ends of the heel and toe, preserving the same cover.
- Area of steel of inclined ("D") bars should be approximately ½ the larger area of main reinforcement.
- Toe length should be the same or longer than the wall stem thickness, and should be long enough to provide anchorage of reinforcement.  Bottom reinforcement ("O" bars) should extend continuously through the length of the toe.
- Observe minimum bending rules and spacing limitations for bars.
- Inclined bars ("D" bars) are used to limit corner cracking through the joint at the back of the wall.  These are not strictly required, but will improve performance and increase the capacity about 25%.
- Reinforcement percentage should not exceed 1.8% (fy=57 ksi) or 1.2% (fy=80 ksi).  (Based on 4300 psi concrete cube strength; the article says that other values may be interpolated and extrapolated.)
- Bars are to be shaped and spaced so that the design is readily constructable and "concreting is possible."
--
Based on a read and re-read of this summary article, it appears that it is unnecessary to provide embedment to the depth required to develop a standard hook, the issue was not mentioned, but there is a requirement to develop the bars in the toe before reaching the critical tension region (red in the attached sketch.)  I am attempting to get a copy of the full research report and see if it is addressed.

I have attached a sketch that shows a standard reinforcement layout and joint (taken from the CRSI tables), and on that I have superimposed the truss idealization and an approximation of the location of the concrete tensile stresses, as taken from the report mentioned above.  The research summary discussed a theoretical model where walls and footings were the same thickness (said to be more easily computed.)  The research experiments were conducted on samples with 8 inch (20 cm) thick walls and 10 inch (25 cm) thick footings. (By observation, the "d" value for the wall stem and footing are approximately the same due to increased cover in the footing.)  Similarly, the CRSI tables typically list a footing thicker than the wall stem.

(Reinforced Concrete Corners and Joints Subjected to Bending Moment, Nilsson and Losberg, Journal of the Structural Division, Vol. 102, No. 6, June 1976, pp. 1229-1254)
 

RE: Cantilevered Retaining Wall

TX,
On a bit of a different subject, I hope you also emphasize what Nilsson says about opening corner joints.

RE: Cantilevered Retaining Wall

Hokie, since it is a serviceability and durability issue, I'm sure we will cover that.  

Hetgen, ironically, the article I discuss above says that a primary thing to avoid is ties/stirrups in the footing for constructability.  I figured that was one reason they looked strictly at footings which were thicker that the wall stem.  Also, the research found that as long at the toe was wider than the thickness of the wall stem, and the "O" and "P" bars are properly developed, the behavior was different from a regular "L"-shaped corner.

RE: Cantilevered Retaining Wall

TX,
It is also a strength issue for opening corners.  Nilsson says that the typical arrangement (with bars in the inside faces going across and bending into the outside faces) is only 10 to 30% efficient.  With the European practice of using hairpins, 80% efficiency can be achieved.  For more, it is necessary to provide stirrups, including on the diagonal.

This is mainly applicable to rigid concrete frames, and to wall corners in concrete tanks.  

RE: Cantilevered Retaining Wall

Just want to have a go from the legal standpoint:  
Per ACI 318 12.1.1, we are allowed to consider development as a combination of hook, length and mechanical device, but, per 12.12.1, negative moment reinforcement (appropriate for cantilevers) must be developed "into or through" by only one of these methods.  In our case, the bars are fully developed by length "in" the toe (I am concerned with ACI's use of "into or through" since it implies a limitiation of orientation).  Also revealing, 12.10.1 allows tension reinforcement to be fully developed by making it continuous with reinforcement on the opposite face of the member.  This is what we are doing but doesn't necessarily jive with 12.12.1.

From an engineering standpoint, I am still concerned.  If we are OK with developing the bar into the toe could we get away with #11s instead of #7s?  What becomes our basis for footing depth?  How do we verify the confinement of the hook in the footing?

RE: Cantilevered Retaining Wall

Hokie, I agree with your observations, including that special detailing and "D" bars apply most directly to corners rather than "T" intersections.  I did not discuss the case of a minimal toe or "L" configuration, since these are not typical in cantilever walls, and must follow different design schemes.  The article I mentioned covers these in some depth.  Do you design any "L"-configured walls?

Teguci, ACI language is "shall be anchored in or through..." which says that a bar, once developed remains developed.  We are developing the "O" bars in the toe, then making a bend to vertical.  In the original JAE sketch, there was a "block failure" as the whole bar and compression strut rotated and lifted.  The "P" and "D" bars should prevent this action.

Interestingly, in ACI 318 Figure R12.21 (a), you would likely get better frame action by providing full development in the vertical before turning the bar into the beam.  This would more correctly engage the beam reinforcement with the column reinforcement.  The fact the we do not see problems in the short hook configuration demonstrates how conservative our design requirements are.

To your point about bar proportioning, I think you are correct that a poorly-proportioned reinforcing layout could be a problem, and it might be useful to have some logical limits, (i.e. As= 1.56 sq. in. = #8 @ 9" v. #11 @ 18", in the same wall/footing combination.)  

I will certainly discuss this with a few authorities on development and bond at the upcoming ACI convention.

RE: Cantilevered Retaining Wall

TXStructural,
The "D" bars do not cross the block shear plane so I doubt that they will help in the failure mechanism of my sketch.  They do add to the As tension reinforcement though.

The "P" bars would have to work in shear friction and be fully developed on either side of the block plane.  Right now, the CRSI manual only requires the "P" bars to extend ld from the back face of the wall while the shear block plane is located perhaps mid-way across the stem thickness.  A subtle difference but one that should be considered.

Also, the "P" bars, if looked at as a type of shear friction reinforcement, are located only 2" clear from the loaded face side which to me is questionable in their efficacy to do the job.

In your further research and discussions you might consider the above.  I'm curious to know how the individual ACI requirements (hook development, shear friction behavior, etc.) can all be technically violated yet still work in a retaining wall as you suggest.

Keep us updated - thanks.

 

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