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On old bridge plans I have occasionally seen a note to "burr threads after installation". This was done to keep the nuts of bolted connection from loosening all the way and falling off of the bolt.

An example of this practice can be seen in this Google Books excerpt from Bridge Engineering, Volume 2:

Nuts on the floorbeam hangers have a tendency to work loose; this we generally remedy by putting on check nuts where the thread is long enough to permit; if not, we burr the thread after adjusting.

That book is from 1916, but I have seen this on much more recent bridge plans. Sometimes the note is along the lines of "intentionally damage threads ...".

The practice of damaging the threads is still an option per this AISC response.

This isn't such a big issue for normal bolted connections where the bolt could be cut (or otherwise destroyed) and replaced with a new one. This is a problem for anchor bolt connections where one end of the anchor bolt is embedded in concrete. These bolts can not be easily replaced.

The intention of damaging the bolt thread is to keep the nut from working loose. The expectation seems to be that with enough torque the nut would effectively re-tap the bolt as it is removed.

  • Is it feasible to back the nut off of the bolt without destroying the bolt?
  • Would there be any confidence that the anchor bolt could be reusable after this procedure is complete?
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  • $\begingroup$ Layman's comment: Nuts can be replaced if you don't want originality - so destroying the but leaves you a thread that can be addressed with a die nut or thread repair tool. This may well be possible with nut in place. If you have enough time, patience, skill and luck you may be able to "repair" a thread to a reasonable extent with manual tools - especially on larger diameter bolts. | Backing off a nut over a damaged thread of unknown badness is liable to damage the nut and, worse, may cause the nut to strip the so far undamaged bolt threads it is attached to. .... $\endgroup$ – Russell McMahon Mar 11 '15 at 23:01
  • $\begingroup$ ... I'd be inclined to do anything that risked damaging already undamaged thread. If a die nut or similar cannot be used I'd expect that splitting the nut would be attractive. In the absence of good thread repair tools, if you have excess nuts then a high tensile nut can be butchered to form a makeshift die. Chasing up the threads with a hand file and perhaps drift over the first few turns may give you enough starting purchase to gently progress BUT something more elegant would be preferred. || Disclaimer: I'm an EE, much experienced in breaking things and hoping to know better next time :-). $\endgroup$ – Russell McMahon Mar 11 '15 at 23:05
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    $\begingroup$ I don't have an answer for you other than "I wouldn't try, and I haven't seen anything in code to support the idea that you can." But as a point of interest, AISC still recommends the practice as an option. aisc.org/… (FAQ 6.5.1.) Of course double nutting would result in a similarly reliable connection and not cause damage to the rod. $\endgroup$ – Ethan48 Mar 12 '15 at 0:09
  • $\begingroup$ What's wrong with simply cutting off the damaged part of the bolt, and then removing the nut? Presumably, the damaged part was not being used to actually apply tension to the joint. Obviously, this means that you can't use the same technique a second time, but there are other ways to secure nuts that don't depend on having exposed thread on the bolt. $\endgroup$ – Dave Tweed Mar 12 '15 at 11:58
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    $\begingroup$ Where I work, we call this 'farmer's loctite' and use it on lots of connections in machines. Certainly in practice, most of the time the nut can physically make it back off and you can even re-use both parts. But My understanding is that the question here is specifically about anchor rods, which are both more critical and also bound by code(s.) There's potential to cause damage to either or both parts, if not to strength then to coatings. It will also obviously change the torque/pretension relationship when that is being used. $\endgroup$ – Ethan48 Mar 12 '15 at 13:22
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I asked a friend who is a highly competent EE by day - but who restores super sized steam engines by night and collects olde heavy metal engines etc and has much experience with old large corroded items. His comments:


It really depends on the specific situation.

For mild steel bolts in good condition which have not been too excessively deformed, and where the nuts are in reasonable condition, it is often possible to "re-form" the damaged threads by applying sufficient torque to back the nut off. In some case you can encourage this to happen by carefully grinding the deformed end of the bolt to get it back to something close the original nominal thread diameter. You can also increase your chances if you apply a penetrating lubricant (and any lubricant is better than none). Applying heat may also help if that is possible - and if you can apply heat and then apply lubricant at a temperature where it is not broken down the lubricant will tend to be drawn into the thread as further cooling occurs.

If the nut is in poor condition or the shaft of the bolt has corroded and become "waisted" your probability of success will be pretty low. If just the nut is corroded you may be able to split the nut with chisel, but you then have the problem of how to reform the threads. If sufficient bolt end exists you may be able to use a die to cut new threads, but you will then have to use a bush under the new nut to take up the length of the remaining old thread as the new thread is unlikely to run into the old properly.

In some cases you may be able to reduce the diameter of the bolt and cut a new thread at the reduced diameter. If you are willing to make you own nut, you have full freedom as to the diameter you choose - at the expense of a non-standard thread for your successors to deal with when its their turn.

For high tensile bolts your job is much harder and I think the chance of success pretty low - and you will probably need more specialised equipment. A carbide shell type cutter which fits over the bolt thread may allow the nut to be cut away and perhaps a die grinder could be used to clean up the thread so it can be re-cut (but even HSS dies will struggle with high tensile steel).

There are special repair kits available that work a bit like concrete anchors. They have a nut with a segmented sleeve which has sawtooth ridges on the inside and is slightly tapered. You slip it over the (suitably prepared) bolt end and then tighten the nut. As the nut tightens it compresses the sleeve which in turn bites into the bolt. Don't ask me what they are called or who makes them.

There is also some process used in civil engineering that allows a failed steel tie in a blind hole to be repaired in-situ. It must be some kind of welding process but I have no idea how it might work. I believe that it was used to do some repair work on the bridge over the Grafton gully [in Auckland, New Zealand]. I think it is also be used to extend existing concrete bridge structures when the roadway underneath the bridge must be widened.

Source: Ken Mardle.

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