Using rebar for anchor bolts

[smvk3]

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?

 

[KootK]

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.

 

[dcarr82775]

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.

 

[dcarr82775]

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.

 

[KootK]

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.

 

[KootK]

@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.

 

[KootK]

Oops. Here's the attachment.

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

 

[smvk3]

Thanks for the input.

 

[TXStructural]

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.

 

[KootK]

@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.

 

[TXStructural]

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.

 

[Ingenuity]

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".

 

[jfmann]

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

http://www.structural101.com