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Using rebar for anchor bolts

Using rebar for anchor bolts

(OP)
I want to use rebar for anchor bolts, specifically, ASTM A615 Gr. 75. My reactions involve rather high moments and tension loads and I am therefore embedding the rebar into a drilled pier (and developing the bar). What failure modes do I need to consider? For tension, do I just need to make sure that I develop the bar and have enough steel area? For shear, can I just use the ACI shear-friction method to transfer the shear load to the drilled pier?

RE: Using rebar for anchor bolts

Be careful with this. Engineers often use rebar in place of anchor bolts because they think that it's a workaround for Appendix D. For the most part, it's not. The only major failure mode that rebar addresses is side face blow out. Other than that, it's just an anchor bolt. In fact, it's an anchor bolt that actually develops more slowly than a common rod with nuts or an anchor plate.

If you can manage to get your pier rebar to lap directly with your rebar anchor bolts, then you can skip the anchorage voodoo. That's pretty rare though.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

I got so focussed on the warning that I forgot to actually try to help. If you go with rebar anchor bolts. I'd recommend looking at it as a strut and tie joint. The end result will be long-ish rebar-bolts and some concentrated ties at the top of the drilled piers. Essentially a well thought out non-contact lap splice.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

(OP)
I don't see how you would breakout the concrete in tension; the rebar is developed fully into the drilled pier and is pretty close to the drilled pier rebar cage. As long as cover and development length requirements are met, you won't breakout the concrete in tension.

RE: Using rebar for anchor bolts

That's the classic misunderstanding smvk3: assuming that embedding the rebar Ld means that you've achieved anchorage to the concrete. You've only got anchorage to the pier in this scenario if:

1) You effectively lap the bolts with the pier cage as I've described above or;
2) You make the Appendix D style anchorage checks work (they won't).

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

I have often used reinforcing steel for anchor bolts, but never had to deal with "Appendix D". When using threaded deformed bars, be careful of the force used, as the outer skin which is removed in threading is the strongest part of the bar.

RE: Using rebar for anchor bolts

@Hokie:

1) You didn't use app D but you did take measures to effectively lap the bolts and the pier verts, right? And that required more than Ld? Before strut and tie, we would speak of a 35 to 45 degree failure surface with bars lapped on both sides of it. Sometimes, we still do.

2) I wasn't aware of the higher strength material lost at the outer surface of the rebar. How do we account for that?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

(OP)
I don't see how you need to lap the rebar with the drilled pier rebar cage; if you develop the rebar into the drilled pier, the load will be transferred into the drilled pier through the friction between the rebar and the concrete. What you are talking about is the method ACI appendix D uses (which they call "anchor reinforcement") where you have to lap an anchor bolt according to a a 35 degree tensile failure cone.

For deformed rebar, you don't need to do this as the force is transferred through friction between the rebar and concrete. The design tensile strength of the rebar (LRFD)would be 0.75*(Ane)(Fy) where Ane is the net area of the rebar (which takes into account the area removed from the threads).

RE: Using rebar for anchor bolts

(OP)
For shear transfer, I use the ACI shear friction method with a coefficient of friction of 0.7, and a rebar area of Ane, where Ane is the net area of the rebar (which takes into account the area removed from the threads).

RE: Using rebar for anchor bolts

@OP: I strongly disagree with your 23:31 post. Consider this extreme situation: Link. If simple anchor bolt/rebar development length got the job done, it would be fine. Clearly, however, it's not fine. The remedy is to run the vertical cage up so that the bars can be effectively lapped, as mentioned previously.

Your question about the use of shear friction here is an interesting one. I'm curious to see what others have to say about that. I don't know the answer.

Quote (smvk3)

What you are talking about is the method ACI appendix D uses (which they call "anchor reinforcement") where you have to lap an anchor bolt according to a a 35 degree tensile failure cone.

Not so. This is a long standing rational method that predates appendix D. See AISC design guide number one.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

@Hokie: thanks for that. It's interesting that ductility is compromisde when, often, that's exactly what designers are attempting to achieve with turned rebar anchor bolts. In a previous thread, didn't you mention that all AU rebar is weldable grade?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

(OP)
Kootk,

In that situation, yes, obviously you would not fully transfer the tensile load into the pier. However, what I am saying is that as long as you develop the rebar into the pier cage (which would have the cage extend up to the top of the pier in your picture), you would fully transfer the tensile load into the pier.

RE: Using rebar for anchor bolts

KootK,
The statement about ductility probably being compromised is, I think, a reference to the stress risers created by the threads. I didn't realize that anyone was attempting to achieve ductility by using threaded rebars. That would be misguided. Yes, the Tempcore 500 MPa bars are weldable. However, there is some imported stuff which is not as good, so care is required.

RE: Using rebar for anchor bolts

Quote (Hokie)

I didn't realize that anyone was attempting to achieve ductility by using threaded rebars.

@Hokie: Usually only in the sense that designers are attempting to circumvent brittle concrete breakout failure modes. That's all I was getting at above.

Quote (smkv3)

I don't see how you need to lap the rebar with the drilled pier rebar cage; if you develop the rebar into the drilled pier, the load will be transferred into the drilled pier through the friction between the rebar and the concrete.

Quote (smvk3)

what I am saying is that as long as you develop the rebar into the pier cage (which would have the cage extend up to the top of the pier in your picture), you would fully transfer the tensile load into the pier.

@smvk3: In my opinion, these two statements contradict one another. If the first statement is true, then it wouldn't matter we stopped the pier cage in my example. If the second statement is true, then it implies some form of load transfer between the anchor rods in tension and the pier vertical reinforcing. And that load transfer would obviously be something akin to a lap.

The only thing that development length accomplishes is to prevent a reinforcing bar from pulling out from the concrete. Development length does not prelude the concrete cone surrounding an embedded reinforcing bar from breaking out from the main body of the concrete element. If you only embed your anchor rods Ld, and those rods are not directly lapped to matching pier verticals, you will have failed to develop the full capacity of your anchor rods. If you don't believe me, check out AISC design guide one. Or Widanto's now famous paper.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

(OP)
Kootk,

Per your original picture, the failure cone would not be at a 35 degree angle (since the rebar does not have a bolt at the end). It would propagate straight towards the edges of the drilled pier. So if the pier rebar above this crack is developed, how does the concrete separate?

RE: Using rebar for anchor bolts

Quote (smvk3)

Per your original picture, the failure cone would not be at a 35 degree angle (since the rebar does not have a bolt at the end). It would propagate straight towards the edges of the drilled pier.

I don't agree with this. In my sketch, the failure plane was horizontal because, given where I stopped the pier cage, the failure mode would be direct tension, not breakout. Breakout style failures happen at an angle because they represent concrete compression struts that are not sufficiently restrained. With headed anchors, which develop almost instantaneously, we assume that the struts travel at 35 degrees which is fairly aggressive. With rebar, the angle must be assumed to be steeper, on the order of 40-55 degrees per ACI strut and tie provisions. This is why I claim that it often makes little sense to use turned rebar in place of anchor bolts. The turned rebar only addresses side face blowout. Other than that, turned rebars are just slow developing anchor rods. Turned rebar generally requires deeper embedment.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

(OP)
I guess we will have to agree to disagree. I am well aware of widanto's method of anchor reinforcment, however, I don't believe you can treat rebar and headed anchor bolts the same way. You would not have such pronounced diagonal concrete struts with the rebar; the tensile force is transferred through the bond stresses between the rebar deformations and concrete throughout the length of the rebar. For a headed anchor bolt, all of this transfer occurs at the top of the anchor bolt nut, which creates the concrete strut shown in the picture above.

RE: Using rebar for anchor bolts

Good call smvk2. I was getting ready to suggest that we stop at a gentlemanly disagreement as well. For what it's worth, the sketch below shows what I think ought to be done here. It's really just basic reinforced concrete design: passing tension from one bar to another.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

I think it can work. You need to extend the anchor bars far enough into the pier such that a 35 degree (App D) line off the bottom of the anchor bars intersected the pier verticals at a location to provide at least ld embedment on the pier verticals above that line. Thus you are providing steel to resist the breakout cone per Appendix D. Anything less than that and you are into Appendix D or something akin to it. I have seen a couple posts on this website where people think rebar cast in concrete doesn't have to worry about concrete breakout. I couldn't disagree more on that.

RE: Using rebar for anchor bolts

Good to see you on this one Dcarr. It absolutely can work. Whichever assumptions are made regarding the breakout surface, the end result will only be plus or minus 18" on the anchor rods. No big deal.

Got any thoughts on the shear friction part of the question? I've wondered about it myself. Is it still shear friction if the concrete isn't actually cast against the steel? I'm tempted to say yes as it should be mostly dowel action anyhow. I'd start my own thread on it but I'm trying to curb some compulsive behaviour at the moment.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

I'm just stalking you Koot.

I glazed over the shear friction part. I would go all Appendix D on it for shear and not shear friction. Then get around it by providing a ton of ties at the top to prevent the breakout. I am a little fuzzy on that part, but I gather it is how to transfer the shear component into the pier.

I have not researched it, but ACI318-12 give the same same 1.5:1 failure plane for epoxy rebar which I would think has a force transfer method closer to embedded rebar than a H.A.S. So I am not sure you have steeper angle, but maybe you do.

RE: Using rebar for anchor bolts

SMVK3,

Be careful on threading the rebar. You will want to specify your minimum area at the threads. I have found some fabricators will for example, turn a #5 down to a #4 then thread it, leaving you with essentially a #3 bar. Given the tolerances, your #5 isn't really the full 0.625" so the threads are not fully engaged. That is why they turn the bar down a size to get a solid full diameter shaft and then cut the threads onto it. I just go with Williams and Dywidag bars when I need threaded bars.

RE: Using rebar for anchor bolts

With 318, you could go as acute as 25 degrees with a strut and tie model. So it need not be steeper than 35 degrees. Shallow would just mean a greater tie demand. NZ has you extend non-contact spices 1.5X the distance between bars. Even if I use a different method, I usually pay homage to that arrangement. It looks about right and there's seldom much to be gained by doing less.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

@smvk3: check out the attached excerpt from ASCE 10-97 that I stumbled upon by accident. It provides support for the concept of using shear friction in this situation (even for regular anchor bolts). It's a bit dated but still better than nothing.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

(OP)
Thanks for the input.

RE: Using rebar for anchor bolts

I'm late to the thread, having just returned from the long days at ACI and CRSI fall conferences.

First, please do not try to thread a piece of rebar. As other have said, the mechanical properties of common bar rely upon the full section, as the processes used for making rebar may result in varying metallurgy through a section. The correct way to do this would be to use a coupler and threaded dowel, or possibly a factory-constructed deformed bar anchor with a threaded rod attached (similar to a piece of weldable rebar welded to a threaded rod, but the fabrication involves special processes.)

As far as development goes, it is perfectly acceptable to lap an anchor with a rebar cage. The same rules would apply as for non-contact lap splices. Presumably the anchor rods would be inside the confined core of the foundation, so most of the limit states from App D are restrained by the surrounding reinforcement, i.e., "supplemental reinforcement." Appendix D is typically applied to reduce the embedment depth to less than a typical Ld. Also, many anchors are higher capacity than a similarly-sized rebar, making a strict lap a bit difficult.

I would encourage you to use a headed anchor rod instead of a deformed bar. We know very well the behavior of anchor rods, and the rods are a commodity item which will most likely cost significantly less than a specially designed anchor.

RE: Using rebar for anchor bolts

@TX: two questions for you:

1) Can rebar couplers be trusted to reliably grab a smooth dowel? Or a threaded dowel for that matter? I've never seen any manufacturer's recommendations for that.

2) On the rare occasion that I've used threaded rebar, I usually go with Dywidag threadbar or Williams products to ensure better quality control and predictability. The problem that I have there is convincing myself that the threads on those kinds of bars bond to the concrete at least as well as rebar knurls. Once upon a time, I found a spec for what constitutes a rebar knurl and satisfied myself that a Dywidag thread was better. That's was just a judgement thing though. As Northridge has taught us, judgement is no substitute for testing. Any thoughts?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Using rebar for anchor bolts

A field-installed rebar coupler is typically rated for deformed bar only. So I would not put a tensile demand on a coupled smooth bar. One would need to use a coupler with cut threads or another form of engagement.

A spiral-deformed bar (such as trade named Threadbar) is usually sold as an ASTM A615-compliant product, just like rebar. The deformations have to comply with the same requirements as common rebar and are simply rolled in a pattern to allow thread-on couplers.

As far as a better deformation pattern, for years, people have been trying "high relative rib area" deformed bars and other shapes to make the bars develop better. CRSI and our members routinely support research into this, including recently completed and ongoing projects at a couple of universities. The current deformations seem to work because they develop the bar over a length/area such that the concrete can take the load. Trying to develop in too short or over too long a length leads to excessive slip or bond failure, concrete crushing, difficulty in production, handling, or fabrication, or similar problems. One other disadvantage of other patterns, the rebar must be rolled heavier because more of the steel is in the deformations, leaving less for the main bar cross section.

RE: Using rebar for anchor bolts

Not directly related to the OP subject, but interesting none-the-less. #4 threaded rebar dowels for LENTON/ERICO FORMSAVERS failed in an application across a building construction joint on multiple floor locations, resulting in a 1/4"+ "crack".

RE: Using rebar for anchor bolts

Interesting .........other than potential problems with threading end of rebar (as discussed in one post, at least).......it is reasonable to consider top of pier as "cold joint".....with flexural reinforcing extending across the joint. On column side, "development" is considered to be mechanical connection. On pier side, use basic design for development of reinforcing bars.

John F Mann, PE
www.structural101.com

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