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Development length
7

Development length

Development length

(OP)
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?

RE: Development length

Assuming no edges near the dowel?

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

RE: Development length

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

BA

RE: Development length

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

RE: Development length

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

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

RE: Development length

2
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.

RE: Development length

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.

RE: Development length

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.

RE: Development length

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&source=web&...

RE: Development length

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

RE: Development length

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.

RE: Development length

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.

RE: Development length

(OP)
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).

RE: Development length

(OP)
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.

RE: Development length

3
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.

RE: Development length

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.

RE: Development length

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.

RE: Development length

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.

RE: Development length

@Lion06: Precast78's question centres around rebar embedded in to unreinforced concrete clearly not designed to be competent parent material. If the question was "can you yield rebar developed into a reinforced concrete member where a complete, properly detailed strut and tie load path has been provided?" then answer would obviously be yes.

@CEL:

Quote (Precast78)

I think the rebar will yield before the concrete breaks but my coworker thinks otherwise. Isn't that what development length is?

So... that does't suggest that Precast might be interested in discussing what development length is and is not? What it is not, taken on its own, is a guarantee that you can yield a piece of embedded rebar.

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: Development length

Agreed that the OP stated into unreinforced concrete, but he also stated that is was "a large slab of infinite thickness" to imply that it's not close to an edge. I don't see any way that breaks out before the bar yeilds.

RE: Development length

It most likely doesn't break out as I've stated myself. The important point is that, so far, no evidence has been supplied to indicate that is solely because the bar will be embedded a development length.

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: Development length

The bar will yield.

Assume a 45 degree concrete cone of depth say 20 bar diameters. (in reality it's much better than this).

The cone area provides ample concrete in tension to yield the bar. Really, this is the best case. In reality development length calcs are assuming an edge is nearby, and thus all your little "cones" along the bar don't add up to as much diagonal tension area as one humongous cone.


I disagree development length is simply designed to get stress into concrete at which point all bets are off. Concrete splitting failure is central to development length. You are increasing the length of concrete so that the concrete won't split along its length due to diagonal tension. that's what development length is.


RE: Development length

Here's an example that satisfies all of the criteria of Precast78's original post. Fortunatley, the bar is developed so we should be good to go.

And yes, I realize that this was not what Precast78 had mind. It is, however, an excellent device for illustrating the point that I've been trying to make: the parent material matters.

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: Development length

Hahahaa.... *sigh*

Right, now I have to be done with this thread.

RE: Development length

KootK - let it go. Cripes - the OP said SLAB - what you drew is not a slab.

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RE: Development length

I'm not letting anything go until Precast78 tells me that I'm on the wrong track JAE. And he may well do just that. As far as I'm concerned, I'm fighting the good fight here. And I'm not insisting that you or anyone else follow along if you're not interested.

As I explicitly stated above, I fully realize that my last example is not what OP intended. However, if readers approach my example with an open mind, they may well find that it brings the key issue, as I see it, to the fore: development <> ability to yield embedded bar.

Here's an example from my and CEL's concrete code. And it involves a big wide slab so we can all put our imaginations away. Why do they stress the strut and tie mechanism rather than just instruct designers to embed their zone bars Ld? Because Ld doesn't get the job done and many designers botch this, that's why.

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: Development length

Quote (KootK)

Here's an example that satisfies all of the criteria of Precast78's original post.

Quote (KootK)

I fully realize that my last example is not what OP intended.

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RE: Development length

So? There is no contradiction there. The extreme example does satisfy the criteria stated in the original post. And I do realize that Precast78 probably was thinking of mass concrete of infinite extent in all direction.

I intended my extreme example to serve as a logical device to demonstrate the fallacy that developed rebar = plastic pullout capacity. I also thought that it might be a bit humorous. So much for that.

I'm sorry that my latest contribution is causing so much angst. If I'd known that discourse was to be limited to "yes the bar probably yields", I would have passed on this one.

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: Development length

Koot-
That example illustrates the point I'm making. There is a main member check and a development check. Of course the parent material matters, but if the parent material can't handle the load then it will always before the bar being developed. That's just common sense. What you show in the sketch, however, is an I reinforced concrete tension member failing in tension before it can transfer the load into the bar. I don't believe that is at all the discussion here. Try putting that sketch on it's side and put a bar on both sides that stop 1" from each other. Now neck down that 1" length to a 5 sq in are and apply a tension to each bar. Of course the parent member will fail as a unreinforced concrete tension member. It doesn't mean that the bar is not capable of transferring the load to the concrete without breaking out.

RE: Development length

This thread is not behaving correctly in Firefox 35. When I can see the entire line, the text is too small to read, so I have to move horizontally back and forth for each line. Is anyone else having this problem?

BA

RE: Development length

I tried it in Internet Explorer 11 too...same problem.

BA

RE: Development length

I basically agree with Lion06s comment re. member failure vs stress development failure.

In the original case the stress development failure would be a just that - a bond shear splitting failur. By definition this won't happen as we've anchored it to the development length.

The member failure in that case is a plain concrete cone failure, which a simple calc shows exceeds the bar strength.

I.e. the bar will break.

Are there member failures which will happen PRIOR to stress development failure or prior to bar break - OF COURSE!

RE: Development length

Quote (Lion06)

That example illustrates the point I'm making.

I know. Other than the intent of this thread, I've agreed with every single statement that you've made. In fact, I would have sworn that we were on the same side here save the fact that you specifically said that you disagreed with me above.

Quote (Lion06)

if the parent material can't handle the load then it will always before the bar being developed. That's just common sense.

I'm afraid that it is anything but common. There are at least two very experienced engineers participating in this thread that seem to believe that development = bar anchorage. In my personal experience -- and I've asked pretty much everyone that I've ever worked with -- this opinion is held by the majority of engineers. Much of the detailing that you see in reputable publications also seems to suggest that bar development = bar anchorage. And maybe it does; it's not a settled issue for me by any means. Personally, I've been seeking resolution to this issue since about 2009, to little avail. What I can tell you for certain, is that many engineers will get very defensive very fast when challenged on the notion that development = anchorage. That assumption is endemic to many people's concrete detailing.

Quote (Lion06)

What you show in the sketch, however, is an I reinforced concrete tension member failing in tension before it can transfer the load into the bar. I don't believe that is at all the discussion here.

Clearly, we'll have to wait for Precast78 to jump back in on Monday and clarify the intent of his question. I believe that he's curious about the fundamental nature of development for two reasons:

1) Precast78 and I have discussed this very issue at some length in prior threads.
2) The "does the bar pull out if the parent material is properly designed using RC concrete principles" version of the question seems wildly trivial.

Quote (Lion06)

Try putting that sketch on it's side and put a bar on both sides that stop 1" from each other. Now neck down that 1" length to a 5 sq in are and apply a tension to each bar. Of course the parent member will fail as a unreinforced concrete tension member. It doesn't mean that the bar is not capable of transferring the load to the concrete without breaking out.

That's exactly what it means. In my extreme example, the rebar is capable of transferring its tension to the surrounding concrete without failures of the bond stress or concrete splitting variety. However, once that tension is in the surrounding concrete, that concrete is not capable of providing the necessary resistance and it "breaks out". The pure tension failure in my example is just one permutation of the concrete breakout phenomenon. There's considerable breakout capacity when the surrounding concrete is infinite. There's very little breakout capacity when the surrounding concrete is almost non-existent. And everything int between is... something in between.

There are some "in between" examples shown below.

@Tomfh: know that I've been following your comments with interest. I've chosen to only address Lion06 directly because you two seem to be saying pretty much the same thing.



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: Development length

Anyone who hasn't tried to assess this in the context of shear wall zone anchorage really should (detail D in my previous post). It's basically the same problem as Precast78's original post, just with a ton more tension and almost the same breakout frustum. The snippet below is taken from one of NEHRP's Seismic Design Technical Briefs and shows the current thinking in the US. It mirrors the sentiment of the Canadian code snipped that I posted above.

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: Development length

*sigh* Last try. No one has said "bar development = bar anchorage", other than you as your interpretation of what others have said.

Anyone who has done strut and tie work will know that development does not mean anchorage. As esoteric as we can make this thread, it will not change the simple fact that this isn't what was asked.

The OP asked a simple question, and it was answered quickly. Several posters even provided further reading. The question was specifically laid out to preclude problems with the strength of the element into which the bar is anchoring.

RE: Development length

koot-

I think we generally agree, too. What we disagree on is the terminology. What you call a breakout is what I'm calling a primary member failure. If we now take that necked down section of concrete between the two ends of the opposing bars and enlarge it such that the unreinforced strength of the concrete member in tension exceeds the yield strength of the bar then the bar will yield before that member failure happens. So to me, this is about the primary member's ability to carry the load, not the ability of the development length to transfer the load into the primary member.

RE: Development length

@Lion: I'm going to shelve this until I hear back from Precast78 regarding the direction that he'd like to steer things here. I've got my own V-day plans to set in motion...

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: Development length

(OP)
Thanks guys! It was just one of those engineering discussion during lunch break. The reason I said infinite slab was so we can throw edge distance out of the equation. I also said infinite thickness to make sure the concrete wont flex. I also said it is unreinforced so we are not transferring any load from the rebar in question to rebar in the slab. It was just a theoretical question. I wish appendix D talks about straight anchor instead of only headed anchors or hook anchors. If I do use appendix D with the straight bar, where do you start drawing the 35 degree angle to figure out the cone area? I dont think it is fair to say it starts from the bottom of the rebar. I dont think it is fair either to draw the 35 degree angle from the end of the development length.


RE: Development length

Okay, so now you're talking about something that most engineers can relate more closely to when discussing rock anchors and that type of application where different zones along the anchor are expressly calculated.

I think what you're saying is that you expect there to be a length over which the bond fails and the bar pulls out cleanly, then an area where the bond holds and the cone develops above this.

You're right to think this happens, but wrong to think that Ld doesn't address it. Development length gives you the full length upon/over which the bar gets developed, regardless of which of the multiple failure modes actually occur. As you get shorter you'd have failures that would range from strain hardening overtaking the strength of the theoretical "cone" and having a very flat inverted volcano pop-out all the way down to failures of the bond and a straight "pop out" of the bar.

Some of the very early work on reinforced concrete did indeed involve such lengths being calculated, particularly early French and German codes. This simply isn't the case anymore...

When we get PASS the regular bar development into the idea of where the stresses in the bar go we get to Kootk's varied concerns... Until that point the bar plain old holds and yields.

RE: Development length

In broad terms, you need two things to ensure that the theoretical rebar won't pull away from the theoretical concrete before it yields:

#1) The bar should not slide out from the concrete in which it is embedded by way of bond stress failure / local concrete splitting.

#2) A chunk of concrete should not separate from the main body of the parent material and come away with the rebar.

So far, this ought not be fodder for debate. It's just statics. So, analytically, how do we make sure that #1 and #2 don't happen? Like this:

#1) We embed our bar by a development length and;

#2) We employ some calculation procedure to assess concrete breakout.

So, in my opinion, bar development is necessary, but not sufficient, to ensure that our hypothetical reinforcing bar can be stressed to yield before initiating a concrete breakout failure. This makes sense to me for a couple of reasons:

1) If development were sufficient on it's own, then it would be sufficient for all cases where the code definition is met, including situations with small edge distances etc. And we all seem to agree that concrete breakout will be an issue where edge distances are an issue. For what it's worth, my understanding is that

2) As shown in the test setup sketches that I posted above, the testing for Ld involves a concrete strut that the bond stress forces can push back against. As TXstructural pointed out, development length was never intended for situations where the bond stresses would push against a free edge.

I too have long been frustrated with the lack of an Appendix D provision for cast in rebar used as anchorage devices. The ACI 318 provisions on post installed adhesive anchors are coming tantalizingly close. In Eligenhausen's book, which was the primary source for the 318 App D provisions, he explicitly differentiates between situations where concrete breakout applies and where development length is the name of the game (snipped below).

For what it's worth, my understanding of Eligenhausen's work is that bond and concrete breakout are both incorporated into the Appendix D. As such, it's debatable whether or not it's even appropriate to be using the term "development length" in the context of our problem in this thread. In this situation, bond and breakout are inextricably intertwined. This is consistent with Precast78's intuition that a 35deg cone emanating from the bar end might not be "fair".

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: Development length

Kootk: That simply isn't true. You're overcomplicating the issue. Where you have full development length, without cover issues, you will be able to transfer the stress into the surrounding concrete. Whether the concrete can handle the loading is the next question, not part of the problem.

RE: Development length

Quote (Precast78)

Do you think the bar will yield before the concrete breaks? Appendix

That was the original question CEL. How do you propose that we address it without considering whether or not the concrete breaks? OP even referenced App D by name in the problem statement.

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: Development length

The OP mentioned that monstrosity in excluding it only. No reference to any content was made.

I think we may need to agree to disagree on this one. The alternative is to keep running around in circles as the conversation appears to be endless so long as you keep redefining what we should be discussing.

I do have to say that the two examples you give from Eligehausen (no 'n', a very common mistake) are obvious to the point of being moot. Of course development length is insufficient to anchor a bar at the far end of a beam; where would the stress go? Strut and Tie, as is implied by the stress field, have very particular requirements for good reason. As for the lapped and spaced bars, no code that I know allows a row of laps like this. And who would ever do this? Tie the bars together and deal with this as a lap splice...

Nor am I beyond the academic interest. I worked in seismic anchor development, and had the privilege of working with the good Dr-Prof-Ing Eligehausen when he acted as an expert witness for us in a patent suit. I just don't think you're right in your thinking.perhaps it is a simple matter of terminology, who knows?

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