Anchors: Threaded Rod vs. Headed Bolt

I saw this as well and drew the conclusion that the code provisions were written for headed bolts/studs as you noted. It does appear to purposefully exclude threaded rods with nuts. However, I saw nothing that explicitly said a threaded rod with a nut welded to the end couldn't be used unless you call that a "specialty anchor" which are explicitly not included in chapter 17 (though I strongly inferred that this paragraph was meant for custom anchor and lifting inserts that obviously aren't covered by chapter 17).

All the research I've seen has shown that threaded rods with nuts welded or otherwise fixed to prevent the rod backing out were just as strong as a headed bolt or stud of similar size. I wouldn't have any issues with substituting a threaded rod and nut for a headed bolt in my designs.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

Thanks @TehMightyEngineer, I appreciate your answer.

This is what sparked my research:

I'm looking at a relatively old ASCE publication on Anchor Bolt Design for Petrochemical Facilities. They allow rods with heavy hex nuts. With some suggestions on treating the nuts... locking the nuts by counter punching threads, or tack welding (depending on rod material)

Though you probably want to find a more recent reference, my guess is you will find that it is allowed.

This was discussed a while back, here:
I have no problem using a smooth rod, threaded at each end for a short distance to accept a heavy hex nut, but personally never use a fully threaded rod.

shesanEIT,

All our drawings that go out that are cast in place are specified as a F1554 all thread rod with a heavy hex nut tack welded to the end as JoshPlum described. It definitely helps with pullout

The firm I work for also often specifies rods with threaded ends and a heavy hex nut tack-welded in place as the "head." Based on the comments here, it seems like a pretty common practice, and I find it odd that ACI doesn't seem to have anything to say on the matter. It seems to me like everything in Appendix D should apply as normal except for D.5.1., which never seems to control in the applications I'm familiar with, anyway. Why don't they just add threaded rods to their definition of cast-in anchor, add a modification factor to D.5.1 to preclude failure at the hex, and call it a day?

Once20036 thanks for the link. So I guess really the answer is it's not prohibited by the code, but not recommended by some.

CARunderscore, I agree.

Be sure to weld on the bottom side of the nut - not above the nut where you will cause a stress riser in the threaded rod exposed to tension. Isn't a stud by definition a bolt with threads on both ends, and it can be fully threaded at the option of the purchaser?

ACI318-14:
headed stud - a steel anchor conforming to the requirements of AWS D1.1 and affixed to a plate or similar steel attachment by the stud arc welding process before casting, also referred to as a welded headed stud.

We had a lunch-n-learn with Simpson Strong Tie's concrete anchorage group and they mentioned that headed studs and threaded rods with welded on nuts shouldn't be used interchangeably. I believe their reasoning went along the lines of: headed studs have to undergo testing to make sure the failure location is sufficiently far from the head, while threaded rods with nuts have no such assurance. So I suppose it has to do with your need for ductility.

A common way to avoid welding is use of two nuts, each heavily torqued toward each other to minimize tendency for rod to unscrew.

I don't feel welding the nut to the anchor rod is primarily because I don't want it to "unscrew". I ask specifically that the nut be shop welded, so that somebody can't remove the nut in the field for some reason related to installation sequence or convenience, and then forget to reinstall it. Anchor rod pullout is crap without the nut.

I used to put thick round plates on top of the nuts to increase the cone size and shear area. We were very specific to weld only on the underside of the nut to not change the metallurgy of the threaded rod. We were an older engineering firm that used lots of paper and colored felt-tip markers backed up with those new-fangled digital calculating machines.