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
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
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Using rebar for anchor bolts
RE: Using rebar for anchor bolts
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
RE: Using rebar for anchor bolts
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
2) See the last page of the attached.
http://www.reinforcing.com.au/~/media/OneSteel%20R...
RE: Using rebar for anchor bolts
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
RE: Using rebar for anchor bolts
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.
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
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Using rebar for anchor bolts
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
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
@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.
@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
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
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
RE: Using rebar for anchor bolts
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Using rebar for anchor bolts
RE: Using rebar for anchor bolts
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 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
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
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Using rebar for anchor bolts
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Using rebar for anchor bolts
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Using rebar for anchor bolts
RE: Using rebar for anchor bolts
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
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 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
RE: Using rebar for anchor bolts
John F Mann, PE
www.structural101.com