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?

 

[KootK]

@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:

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.

 

[Lion06]

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.

 

[KootK]

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.

 

[Tomfh]

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.

 

[KootK]

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.

 

[CELinOttawa]

Hahahaa.... *sigh*

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

 

[JAE]

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

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[KootK]

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.

 

[JAE]

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

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

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[KootK]

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.

 

[Lion06]

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.

 

[BAretired]

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

 

[BAretired]

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

BA

 

[Tomfh]

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!

 

[KootK]

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.

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.

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.

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.

 

[KootK]

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.

 

[CELinOttawa]

*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.

 

[Lion06]

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.

 

[KootK]

@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.

 

[precast78]

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.