×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Bending Retaining Wall Rebar Stubs
4

Bending Retaining Wall Rebar Stubs

Bending Retaining Wall Rebar Stubs

(OP)
I am working on a job where they have to drive a some equipment over retaining wall footings. As such, the contractor wants to epoxy in the rebar later. I am not a big fan of epoxying in rebar for an 8 ft. retaining wall due to the likelihood that it won't get installed properly. Is it OK if he casts in the vertical rebar stubs (#5 sticking up 30") and then bends them down until the equipment passes and then bend them back up? He is going to pack 6" of dirt over the footing for the travel lane. Any other ideas?

Thanks

RE: Bending Retaining Wall Rebar Stubs

IMHO, mechanical rebar splices on short rebar stubs (a few inches) would be better than bending, then rebending long rebar. Could be expensive. Here is one product, of many: Link

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Bending Retaining Wall Rebar Stubs

Be careful bending rebar. I remember hearing that a highway department approved such based on tests they had conducted. However, on the project it caused problems because in practice it was done using a pipe “cheater” bar for leverage, which proved to cause a shorter bending radius than the laboratory conditions. The point is that the method it’s bent could affect performance even if it would be acceptable otherwise.

RE: Bending Retaining Wall Rebar Stubs

(OP)
I imagine a cheater would end up being used. Maybe if he lays a 4x4 wood block down near the base, it would keep the radius big and also keep it from getting folded down flat by the vehicles?

thanks

RE: Bending Retaining Wall Rebar Stubs

I would just use epoxied bars. You can (and really should) require them to be installed by manufacturer certified installers (Hilti certifies installers) and get them inspected during install.

If you still don't want to do that, I'd use bar couplers rather then bending them. I've specified Leton's and the Halfen ones linked above.

RE: Bending Retaining Wall Rebar Stubs

Interesting - would retaining wall dowel bars to be epoxied in place be subject to the same permanent, creep-inducing load that overhead concrete panels induce in their epoxied anchors?
ACI allows field bending with restrictions.

RE: Bending Retaining Wall Rebar Stubs

I almost always design for long term load in situations like this unless the anchors are 'just there' for the sake of attaching things. There is a standardized test for creep (along with temperature), which results in capacity reductions.

If it was a retaining wall that would be ultimately supported at the top, I would ignore the creep since they were only in tension during construction.

RE: Bending Retaining Wall Rebar Stubs

I'd be fine with the bending given:

1) Small-ish bars.
2) One time rebend.
3) Rebend done to suitable radius using suitable equipment.
4) Rebend done at a suitably warm temperature.

Here's some info from a source that tends to be quite contractor friendly: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bending Retaining Wall Rebar Stubs

(OP)
This is a pretty small time operation. Probably have never heard of couplers and the stubs that get coupled to may still pop some tires.
I imagine all the bending will occur with a long pipe.
@Kootk - what would be suitable equipment? - a conduit bender?
I would also be concerned about how they will get the bar straight on the rebend. Seems it will be tough to control where it bends.
I might go the Hilti certified installer route as a compromise.

Thanks

RE: Bending Retaining Wall Rebar Stubs

Using splicers such as the ones SRE referenced would work. Bending will also work, provided it is done correctly with the proper radius. Your rebar supplier should be able to provide you with a field bender. I would in no circumstance allow glued in starters for a cantilevered retaining wall.

RE: Bending Retaining Wall Rebar Stubs

(OP)
Hilti has not certified installers in my area - aparently only in NY and PA

RE: Bending Retaining Wall Rebar Stubs

I like the epoxy idea in principal, but in design practice, getting the embedment required per ACI 318 Appendix D and ESRs is very difficult.
For cracked / seismic reductions, more than 24" of vertical embedment req'd just for #5 dowel. That's not to say that a shorter embed wouldn't pass a pull test.

RE: Bending Retaining Wall Rebar Stubs

Quote (XR250)

@Kootk - what would be suitable equipment? - a conduit bender?

I don't know much beyond what's contained in that link that I posted previously. Basically, not a pipe.

Quote (XR250)

I might go the Hilti certified installer route as a compromise.

See the attached pages. It's a pretty tricky connection really.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bending Retaining Wall Rebar Stubs

Not trying to advertise for Hilti but their latest adhesive product (Hilti HIT-HY 200) removes a lot of the issues that used to plague post installed anchor installation. I've been specifying it exclusively for most applications. The contractors like it as well because it's faster and easier than the old stuff.

They use a special drill bit that does most of the hole cleanout that used to be conveniently "forgotten" by the installer. Gives me some peace of mind that I'm not relying on some random laborer (nothing against them) to remember a convoluted brush-blow-brush-whatever procedure to get full capacity on the anchor.

RE: Bending Retaining Wall Rebar Stubs

I think the issue with adhesive anchors is not so much whether they function as advertised but, rather, that this is the wrong application for them. In a cantilever retaining wall the tension steel needs continuity with the tension steel in the base. An adhesive anchor will not achieve that.

RE: Bending Retaining Wall Rebar Stubs

Or you could just backfill with soil cement and make the wall redundant.

RE: Bending Retaining Wall Rebar Stubs

can you sidestep the entire problem and give him a width (6', 8', etc.) of no vertical bars that he can use and then design the wall to span horizontally with add'l vertical at each side of this zone. In that corridor you can drill/epoxy later or bend/rebend etc. but you'd have suspenders for the operation.

RE: Bending Retaining Wall Rebar Stubs

Check out ACI 318-08, section 7.3 and commentary. They have some good notes on field bending.

They say a bar can be bent and straightened with the approval of the EOR (wonderful...). There are some stipulations on bend diameter and what to do in the event of cracking (heating the bars). They also refer to a document, "Bending and Straightening of Grade 60 Reinforcing Bars" which I don't have. Maybe that text will have information on what type of tools are required and other industry practices, though it's 30 years old.

RE: Bending Retaining Wall Rebar Stubs

(OP)

Quote (Archie264)

I think the issue with adhesive anchors is not so much whether they function as advertised but, rather, that this is the wrong application for them. In a cantilever retaining wall the tension steel needs continuity with the tension steel in the base. An adhesive anchor will not achieve that.

I thought I would just have to embed it deep enough to develop the design tension in the bar?

He is going to have equipment moving all over this thing as they are suspending a house 10 ft. above the footing so I don't think I can give him a path.
I might end up going the bar splice route as they seem to be readily available here and only require a 4" stub.

RE: Bending Retaining Wall Rebar Stubs

Well I'm no expert on anchors or anchorage. However, it was pointed out on these forums a while back (sorry, I can't find a link to it) that to achieve continuity the doweled-in bar has to be in close enough proximity to the bar it's to interact with that the forces transfer to each other through the concrete. And that's in just a straight-forward lapped bar situation, not relying on epoxy. To drill the holes for the epoxied-in bars close enough to the cast-in-place bars for the forces to transfer...well, it would make me nervous. There would be a lot of concrete disturbed, cracked, fractured, whatever, by that drill bit. It's a lot to hope to get right.

RE: Bending Retaining Wall Rebar Stubs

The European Hilti document that I attached above deals with all of this in detail. It's a good deal more than simply developing the bars.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bending Retaining Wall Rebar Stubs

KootK, fair enough, I should have opened that up before responding. And now that I *have* opened it up, all I can say is I'm having trouble following it completely. Are they saying that the embedment on the tension side need to be deep enough that there remains embedment below the cracked cone? If so it sure seems like a lot of equations to just say that. I suspect there's much more to it than that but at the moment I'm not going to invest the time to try to chase it all out. Hopefully the document will help the OP (XR250), though, and thanks for posting it.

RE: Bending Retaining Wall Rebar Stubs

Quote (Archie)

..I should have opened that up before responding. And now that I *have* opened it up, all I can say is I'm having trouble following it completely

I find all of the eurocode stuff in that document pretty hard to follow myself. Ever since I found it, I've been dreading having to convert it to Canadian. Essentially, it says pretty much what you said previously: what is required is not just bar development but, rather, a true moment connection with the footing. Hilti's said it with strut and tie logic; you said it with common sense and a fundamental understanding of concrete detailing.

Quote (Archie)

Are they saying that the embedment on the tension side need to be deep enough that there remains embedment below the cracked cone?

I think that they're just taking the development length from the end of the bar so as to produce the most advantageous strut and tie model (the one in which the effective lever arm is reduced the least). There's a blurb about flexural concrete cracking near the embed surface in there as well.

Quote (Archie)

I suspect there's much more to it than that but at the moment I'm not going to invest the time to try to chase it all out.

Yeah, there is. The reduced lever arm business is the most significant aspect in my estimation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bending Retaining Wall Rebar Stubs

Is it weldable rebar? That is not too difficult.

RE: Bending Retaining Wall Rebar Stubs

(OP)

Quote (KootK)

I find all of the eurocode stuff in that document pretty hard to follow myself. Ever since I found it, I've been dreading having to convert it to Canadian. Essentially, it says pretty much what you said previously: what is required is not just bar development but, rather, a true moment connection with the footing. Hilti's said it with strut and tie logic; you said it with common sense and a fundamental understanding of concrete detailing.

That is pretty complicated. Seems they could make some simple tables out of all those equations. Since I am not about to go thru all of those calculations at this time, the bar splice seems to be the best solution.

Thanks everyone!

RE: Bending Retaining Wall Rebar Stubs

Hilti's got some new software that will do this for you, presumably in an ACI318 environment. I haven't had a chance to tinker with it though.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bending Retaining Wall Rebar Stubs

(OP)
Unfortunately, i am having problems getting hilt's stuff to install on my machine. Simpson works fine, however.
I guess I don't really understand that document. If I use hilti's tables to embed the rebar enough to develop the strength of the bar, does that not take into account the cone failure of the concrete? If so, then I have the tension side of my force couple.

RE: Bending Retaining Wall Rebar Stubs

Send Hilti your details and have them design it for you. They live for this stuff. Might even come back stamped by some German PhD.

The crux of the document is that the T/C lever arm is likely less than that associated with your stem flexural design. As such, you don't really know what tension to anchor for without considering the reduced lever arm. While the tension demand may be higher than expected, the individual bar anchorage is probably better than indicated in the tables. Part of the tension will react against the wall compression block.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bending Retaining Wall Rebar Stubs

That's a neat little document.

I take from it that the main difference between this and the standard pullout situation is that flexure cracks will form around the anchored bar from having tension/compression fields in such close proximity - this then reduces the length of bonded epoxy to bar (worse than hiltis tables). However, they then proceed with a strut/tie analysis (i.e. using the reinforcement in the footing to help with confinement, which I don't think the standard hiltis tables do), which would help significantly. But THEN, using the footing reinforcement to help with pullout results in higher tension forces in the footing top reo mat than there would be if you resolved the tension by taking the standard footing lever arm. I think.

Christ if you worked for Hilti you really would know absolutely everything about absolutely nothing.

RE: Bending Retaining Wall Rebar Stubs

At the risk of stirring the pot, I think that the Hilti stuff has the shock value that it does because we are, in effect, comparing these two things:

1) a post-installed rebar connection that is carefully engineered and;
2) a CIP rebar connection that generally goes un-engineered.

Terminating the dowels with a bend makes for a better joint but not necessarily an adequate one. If we were checking the wall to footing connection rigorously for the cast in place dowel condition, I suspect that a lot of the same issues would crop up.

Just to be clear, I'm not doing this any better than anyone else at the moment. I'm not running strut and tie models on my wall to footing connections. However, I do:

1) Install the diagonal bars per the CRSI manual and;
2) Have the decency to feel guilty in the morning.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bending Retaining Wall Rebar Stubs

@XR250,
If you do not have the confidence that the contractor can properly install epoxy-set rebar extensions, why would you have any confidence that he could properly straighten out a bent bar, given that is a trickier proposition than the other, and fraught with all sorts of mistakes (assuming it was properly bent to start with)? I think that the suggested option of using mechanical rebar couplers on short stubs makes more sense to me that any of the other, trickier suggestions. As for puncturing the tires: cover the travel lane with 8" of gravel, then put down a 1/2"-thick steel, temporary cover plate over that. That should allow for traffic flow, without sacrificing any expensive tires.
Hoping you come up with a good, workable scheme, smile
Dave

Thaidavid

RE: Bending Retaining Wall Rebar Stubs

(OP)
Thanks ThaiDavid.
I def. don't have much confidence in him.
I think i will go with the splice route.
He is also the mason, so hopefully he gets his bars in the right place.
They are also in a hurry, so waiting 7 days to drill in some anchors and then a few days for the RE500 to cure is not likely going to be acceptable for them.

RE: Bending Retaining Wall Rebar Stubs

(OP)
Hilti has some new tables within the last month that show the required embedment to develop a bar. Check their ESR Reports.
Apparently, you need over 14" of embedment for a #5 bar in 4000 psi concrete.

RE: Bending Retaining Wall Rebar Stubs

And I believe that is considering installed in a large chunk of concrete. Once you have to start worrying about concrete failure modes like break-out and blow-out that embedment depth increases.

RE: Bending Retaining Wall Rebar Stubs

If you are trying to post install and use develop reinforcing use Profis Rebar. You can use either 200 or 500SD, the procedures and limits are different than using App. D, so if you worry about the contractor not being familiar with couplers I would be very worried about post installed solutions.

RE: Bending Retaining Wall Rebar Stubs

(OP)
So back to this job. The contractor made the footings too small for the retaining wall I designed and did not install any of the rebar in the footings except the normal longitudinal stuff. He ended up hiring another engineer to tell him how to fix it. This is what he came up with (LOL) (see attached).
Fortunately, I think the owner is going to go back to me to get a proper detail which will likely involve designing a buttressed wall.

RE: Bending Retaining Wall Rebar Stubs

If you're using the right grade of washed gravel backfill, you could probably pour a thin grout into the backfill and make the backfill a self-supporting mass. Pour it in low lifts so you don't overturn the contractor's wall. Of course this will frustrate any future digging in this area.

RE: Bending Retaining Wall Rebar Stubs

Has this "fix" been plemented yet?

I love having to compete with the incompetent in a space where there are rarely consequences. Last month I looked at a big RTU add on OWSJ. The contractor wanted to just put a post right under the RTU so as not to have to reinforce the joists. I explained that solution would do nothing to shelter all the neighbouring jousts from code mandated drift loads. We got the joist supplier to detail the reinforcing and everything. I was on site last week and, low and behold, there's the damn post, with no joist reinforcement. And stamped sketches for it from an alternate engineer. Now I've got an ethics problem to boot as the contractor is a good client in many respects.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Bending Retaining Wall Rebar Stubs

(OP)
The contractor wanted to do it this weekend. I told the owner if he went with this fix then I am off the project.
I think the owner wants me to stay so he told them to hold-off.

What are you going to do about your ethics problem?

@Buggar - never heard of doing that. I want to stay reasonably conventional on this job as there is alot of question on the competancy of the contractor. I already have my liability limitation in my contract and will over-design everything by about 100% to allow for his impending mistakes.

RE: Bending Retaining Wall Rebar Stubs

Is this a basement type or a cantilevered type retaining wall? For a basement type wall (as sort of shown in the sketch), you only need the footing to wall reinforcement for shear transfer and I wouldn't be as concerned with the development of the bar (I'd probably space them at 16" oc though). I would typically move the bottom bars to the middle to inside face of the wall to promote a theoretical pin connection and to prevent the bottom of the wall from blowing out due to the shear load (increase d). Of course you would want to detail the reinforcement on the tension face of the wall as well.

If your top diaphragm can take it, I would assume a bottom pin for calculation and keep your footing small and simple.

If you want a cantilever type wall then the connection at the top will need to be detailed for slip.

RE: Bending Retaining Wall Rebar Stubs

(OP)
It is a cantilevered retaining wall.

RE: Bending Retaining Wall Rebar Stubs

Quote (XR250)

It is a cantilevered retaining wall.

Oh well.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources