Bending Retaining Wall Rebar Stubs 4

[XR250]

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

 

[KootK]

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.

 

[Archie264]

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.

 

[KootK]

..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.

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.

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.

 

[Brad805]

Is it weldable rebar? That is not too difficult.

 

[XR250]

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!

 

[KootK]

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.

 

[XR250]

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.

 

[KootK]

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.

 

[bowlingdanish]

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.

 

[KootK]

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.

 

[thaidavid40]

@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,
Dave

Thaidavid

 

[XR250]

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.

 

[XR250]

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.

 

[jayrod12]

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.

 

[sandman21]

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.

 

[XR250]

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.

 

[BUGGAR]

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.

 

[KootK]

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.

 

[XR250]

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.

 

[Teguci]

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.