Fully threaded anchor rods?

[Baffled Engineer]

Hello,

A contractor wanted to use fully threaded anchor rods instead of threaded ends only as shown below.
I have no concerns regarding the strength & net area of the threaded rod, but I'm not sure about the concrete bonding to the threaded portion.
Is it possible that the bonded concrete in the threaded portion of the rod will crack & possibly spall before the nut anchors can be engaged by tensile forces in the rod?

 

[JStructsteel]

It could do that, but how will you see it? Is this under a grouted baseplate?

 

[bridgebuster]

On NYSDOT projects we use fully threaded 15" rods for bearings. I don't see your case as a problem.

 

[SwinnyGG]

Is it possible that the bonded concrete in the threaded portion of the rod will crack & possibly spall before the nut anchors can be engaged by tensile forces in the rod?

Maybe, but why would you care?

You're getting bond strength to the shaft of the anchor that's at least as strong as a smooth section of bar would be.

 

[TLHS]

I see fully threaded rod a lot. Early in the F1554 days I was seeing rod coming through the steel fabricator custom threaded for the project. You can just buy pre-threaded rod a lot easier now, it seems, and I've seen it a lot more over the last five or maybe ten years. I think the increase in use of epoxy anchors has made it more common.

I'm not aware of problems relating to it. Just be really clear on what they're subbing. It's not as common now that people are (finally) used to F1554 but even just last year I had a contractor ask to sub in A307 rod and then when I asked for grade I got a shrug. Eventually they tell me it's grade A instead of the C you would want foranchor rods. I also once figured out after a bunch of questions that a contractor was trying to sub in hardware store redi-rod.

 

[dik]

Baffled... I use a similar detail, except only 1" threaded on the bottom end and the nut turned tight against that.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bobbyboucher]

for seismic country, there has been some research for baseplate assemblies suggesting possibly even to wrap the anchor rod in teflon tape as a bond break between the rod and the concrete to encourage yielding of anchors as an additional way to gain ductility in the base connection... don't think it's mandated anywhere, and it wouldn't serve any purpose in a connection that isn't specifically trying to achieve ductility

 

[OldDawgNewTricks]

Sometimes you also might also want a bond breaker in order to provide longer stretch length for pretensioned anchor rods.

 

[human909]

It is pretty common around here if not standard to use threaded rod. Why complicate the fabrication?

 

[JohnRwals]

I think this is good idea!
I wonder why construction industry has not adopted this detail to base anchor detail.
I know this issue was discussed in the plant industry...

 

[JLNJ]

Post-installed adhesive anchors use the full depth per ACI Chapter 17 (up to 20 anchor diameters) so that indicates to me that the threads don't cause an issue with shallow breakout, at least up to 20 anchor diameters. If you need more than 20 diameters for your bolt embedment you might think twice about allowing all-thread.

Of course, if you are relying on the bolt stretch to meet the seismic provisions, you don't want the threads. And if you had a tall vessel where you wanted to pretension the bolts, you don't want those threads engaging there either.

 

[dik]

I seem to recall an AISC publication on headed anchor rods and the development of them was approx 17 bar diameters... not ACI. I still encounter most projects where engineers are using 'L' or 'J' type anchors.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Baffled Engineer]

It could do that, but how will you see it? Is this under a grouted baseplate?

Yes it is under a grouted baseplate. My concern is that if the bonded concrete breaks near the top, then the connection is compromised for shear transfer even if no one sees it.

On NYSDOT projects we use fully threaded 15" rods for bearings. I don't see your case as a problem.

Thank you for your input.

Maybe, but why would you care?

You're getting bond strength to the shaft of the anchor that's at least as strong as a smooth section of bar would be.

SwinnyGG, for anchor design, we typically use the nut at the bottom of the anchor to transfer the tensile forces via bearing, we don't rely on the bond strength. Now that the threaded rod has more "bond" with the concrete, I'm not sure how much force would be transferred to that bond, and if that bond has sufficient capacity.

I see fully threaded rod a lot. Early in the F1554 days I was seeing rod coming through the steel fabricator custom threaded for the project. You can just buy pre-threaded rod a lot easier now, it seems, and I've seen it a lot more over the last five or maybe ten years. I think the increase in use of epoxy anchors has made it more common.

I'm not aware of problems relating to it. Just be really clear on what they're subbing. It's not as common now that people are (finally) used to F1554 but even just last year I had a contractor ask to sub in A307 rod and then when I asked for grade I got a shrug. Eventually they tell me it's grade A instead of the C you would want foranchor rods. I also once figured out after a bunch of questions that a contractor was trying to sub in hardware store redi-rod.

Thanks.

Baffled... I use a similar detail, except only 1" threaded on the bottom end and the nut turned tight against that.

Yeah that works as well. We sometimes tack weld the nut or use a double nut...but that's nothing to do with my question about fully threaded anchor rods...

for seismic country, there has been some research for baseplate assemblies suggesting possibly even to wrap the anchor rod in teflon tape as a bond break between the rod and the concrete to encourage yielding of anchors as an additional way to gain ductility in the base connection... don't think it's mandated anywhere, and it wouldn't serve any purpose in a connection that isn't specifically trying to achieve ductility

Sometimes you also might also want a bond breaker in order to provide longer stretch length for pretensioned anchor rods.

I think this is good idea!
I wonder why construction industry has not adopted this detail to base anchor detail.
I know this issue was discussed in the plant industry...

Yeah I thought of adding a bond breaker, but I was thinking that would reduce the corrosion protection afforded by the concrete?

It is pretty common around here if not standard to use threaded rod. Why complicate the fabrication?

I don't think it's that common for cast-in-place anchors to be fully threaded at least from where I'm from in Canada. We mostly use fully threaded rods for post-installed chemical anchors like Hilti HAS rods. I suppose the threaded rods are more commonly available than the threaded ends only, hence contractors often ask for this substitution to speed up lead times, but it needs the EOR's approval.