Development length 8

[precast78]

Lets say you have a large unreinforced slab (infinite thickness) and you embed a rebar vertically ld deep. Do you think the bar will yield before the concrete breaks? Appendix D cone shear calc is only for headed stud or hook. I think the rebar will yield before the concrete breaks but my coworker thinks otherwise. Isn't that what development length is? It is the distance the rebar need to develop full strength? What do you guys think?

 

[TehMightyEngineer]

Assuming no edges near the dowel?

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

 

[BAretired]

I agree with your definition. Development length is the length of embedded reinforcement required to develop the design strength of the reinforcement.

BA

 

[JAE]

This:



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[slickdeals]

Rebar will yield. Epoxy adhesive manufacturers have minimum embedment lengths to develop reinforcing bars in epoxy.

 

[CELinOttawa]

Yield. You're right, and your colleague needs to know more about concrete detailing.

Download some of the ACI research reports, they are excellent.

 

[KootK]

Fundamentally, I believe that your colleague is right Precast78. I also believe this issue to be the biggest single misconception commonly held by structural engineers.

Development length guarantees one thing and one thing only: the tensile stresses in your bars will be transferred to the surrounding concrete without initiating a bond stress style pullout failure. Once those tensile forces find their way into the surrounding concrete, development length does nothing to preclude the tensile breakout of that surrounding concrete.

Once rebar tension is transferred into the surrounding concrete, there are basically two fundamental mechanisms by which one can preclude tensile concrete breakout:

1) Utilize the diagonal tension resistance of the surrounding concrete. This basically amounts to appendix D or something akin to it.

2) Pass the rebar tension to a concrete compression strut. This mechanism encompasses strut and tie models and lap splices and is really just what we know of as RC concrete design theory.

As for your specific question, precast78, I would say that it depends. For the sake of argument, lets say that we're talking about 35M vertical bars embedded Ld into a 20m x 20m x 3m thick raft footing. Consider these scenarios:

1) A single 35M bar located at the center of the footing. In this case, I suspect that the bar yields. I don't say that because the bar is developed however. Rather, I say that because I've run the numbers on this using Appendix D equations applied with some judgment.

2) A group of tightly spaced 35M bars located at the center of the footing. In this case, I believe that the bars may initiate a concrete tension breakout failure before reaching their yield strength. This is what Appendix D style equations tell me when I run the numbers. And this has real world significance. I do this calculation to see if my shear wall zones will pull out of the raft footings that hold them down.

3) A single 35M bar located 50mm clear from the edge of the raft footing. In this case, I believe that the bar will likely initiate a concrete tension breakout failure before reaching its yield strength. Again, this is what an Appendix D style check would indicate. Here, it's much worse than case #1 because of the eccentricity between the applied load and resisting mechanism. It's basically a breakout / pryout failure mode. I believe that TME was alluding to something similar with his comment regarding edge distances.

I think that it's also instructive to look at the set-ups used to test rebar development and described in ACI 430. They all supply a nearby concrete compressive strut that the rebar tension can be passed to.

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.

 

[KootK]

To me, it seems irrational to expect a reinforcing bar embedded Ld to be better anchored than a comparable headed anchor embedded Ld would be. After all, a headed anchor is just a reinforcing bar that develops much more quickly than traditional knurled rebar. The only drawback is side face blowout issues where applicable.

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.

 

[CELinOttawa]

Kootk: Maybe for an unreinforced blob of concrete, but not for any real world application.

I've done a lot of seismic anchoring and blast load testing. I believe this is also addressed in one of the ACI research "state of the art" summaries. I'll have to dig it up, but the answer is that the bar will yield.

 

[CELinOttawa]

Giving it further thought, you are correct to a degree; The load has to go somewhere, and that "somewhere" has to to be able to take the load. All of that doesn't change what Ld is, by definition. I quote ACI 408R-3 <<When used in design, development length and splice length are understood to mean the “length of embedded reinforcement required to develop the design strength of reinforcement at a critical section,” as defined in ACI 318.>>. If you are designing for yield strength to into an unreinforced concrete block, the material factors in the code formulae for Ld are there to ensure that the bar yields and does not overwhelme the concrete strength. You get some failures when you do this in the real world, but they are the outliers.

Everyone on this thread should read this:

https://www.google.ca/url?sa=t&sour...IQFjAG&usg=AFQjCNH6SF7YRstugDIMbFu7xh1MFFJHAw

 

[CELinOttawa]

And, to be clear, by "blob" I mean unconfined, uncompacted concrete.

 

[TXStructural]

First, I'll preface my comment with an opinion: ACI development lengths are much longer than required. This is because of various complexities of the issue, and the belief that ever having a development failure outweighs the cost of a longer Ld. So you will probably find that in many cases the bar will yield, at least locally, before pulling out of a mass of concrete.

But, Ld is for fully embedded bars, where there is no open surface on the tension end of the bar. As others have pointed out, the compression struts formed at each deformation in the bar "push" on the concrete. Without restraint, a cone will theoretically pull from the surface of the concrete before the bar slips through the concrete, depending on many factors, including concrete strength.

You are both correct, and both wrong, at the same time, if that makes you feel any better.

 

[Lion06]

I see it this way. The simplest (theoretically) way to test this would be to cast a single bar in a large mass of unreinforced concrete large enough and thick enough to provide a full development length of the bar and not fail in flexure. Then lift the mass by the single bar and have the mass be heavy enough to be greater than the yield force of the bar. If this is done and the entire mass lifted by the bar, (so external forces are not providing struts to preclude breakout) I just don't see the bar breaking out of the concrete. I see the bar yielding.

 

[precast78]

KootK, if we assume the rebar has a head and embeded to concrete ld (no edge distance reduction), the capacity is way higher than when the steel start yielding (equation D-7).

 

[precast78]

If I calculate the shear cone similar to appendix D with headed anchor and assume the cone starts from the middle of ld, the concrete breakout is still higher than the steel yielding.

 

[slickdeals]

If concrete breakout is a controlling mode when edge distances aren't involved, then nothing in this report (based on testing) can be right

http://www.strongtie.com/ftp/bulletins/t-sas-setrbr11.pdf

 

[CELinOttawa]

The bar will yield. I have personally seen bars embedded to Ld yield in testing. This is a thought experiment gone wrong and is being over-thought.

 

[KootK]

I interpret Precast78's fundamental question to really be this: is rebar development, on its own, sufficient to guarantee that rebar will be able to attain its yield strength without concrete breakout? And the answer is no. In my opinion the fact that this is such a contentious issue makes it eminently worthy of discussion. Additionally, as my shear wall zone example makes clear, it is a matter of significant practical value.

Maybe I've interpreted Precast78's question incorrectly. Maybe he doesn't care about the nuances that make rebar development a distinctly different phenomenon from rebar anchorage. Maybe he really is just interested in knowing if a hypothetical piece of rebar with pull out of a hypothetical blob of concrete. I'll have to rely on Precast78 to let me know if I'm out of line here.

If the question du jour is whether or not bar development guarantees bar anchorage, then I believe that those quoting testing are on the wrong track. So what if testing indicates that embedded bars don't pull out before yielding? The tests don't tell us whether concrete breakout was precluded because development guarantees anchorage or simply because Appendix D style anchorage just happened to work for the setups tested. Based on the App D calcs that Precast78 and I have run, the latter case is entirely plausible.

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.

 

[Lion06]

koot- I have to disagree. The development length equations can be calculated with clear covers as low as ACI 318 permits (3/4" for slab) and there is no provision that it be developed into reinforced concrete, it is the engineer's responsibility to ensure that the parent concrete can resist the forces imposed (reinforced or unreinforced). There is a major fundamental difference between Appendix D requirements and that of a developed rebar, it's the very reason App. D exists.

 

[CELinOttawa]

That was *not* the question. If you want to start a thread to discuss/debate/determine good and bad details, problematic situations and the like, do so. I have done so in the past, and they are invariably of very great value. I would participate in the debate, and I think everyone would likely learn a few points from each other... HOWEVER:

The answer to the OP's question is that the bar yields.