Tack Welding Structural Nuts 1

[connectegr]

Can you tack weld structural nuts? A325 or A490

I am aware of the AISC note "Anchor bolt material that is quenched and tempered should not be welded or heated". But does this prohibit the tack welding? The heat applied is minimal and applied only to the nut. Does this actually impact the properties of the material?

Obviously this is due to an access problem. I have seen this done throughout my career. As you all know there are bolting conditions where there are no other options. Prior to erection, the nuts are tack welded to the inaccessible inner ply of the connection. The bolts are then inserted from the outside and "snug" tightened by turning the head.

IS THIS ALLOWED?
(technical references are appreciated)

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[dhengr]

connectegr:

As you suggest, I wouldn’t expect small tack welds on A325 or A490 nuts to significantly affect the material properties of the nuts near the threads. The further the tack is away from the thread, the better, thus probably less affect on large sized nuts, than on small ones. And, maybe the tack weld shouldn’t be at the center of the flats on the nut. Be sure there is good contact btwn. the plate and bearing surface of the nut before tacking. I’ve seen it done too, and didn’t disallow it. But, I often wondered, given the Q&T material and a quick weld, and no preheat, how effective the weld was. And, if they weren’t just causing a crack prone HAZ at the nut surface. Maybe use a softer filler metal, so the weld is more likely the weaker material. We do pre and post heat Q&T materials, within restricted limits, for normal welding, so I’ll bet the real issue is excessive heating or the arc strike type stress raiser.

I have not read the AISC note on Q&T anchor bolt material, and welding or heating on them, but it doesn’t surprise me. I wonder if it wouldn’t be read to mean: any excess heating will likely change the Q&T and thus the mechanical properties; and welding is more likely than not to cause a stress raiser, notch effect, crack starter across the highly stressed rod.

I suspect you won’t find any literature or testing specifically approving or rejecting tack welding of nuts; and that you may find a few inspectors who might not allow it. Maybe AISC , AWS or the RCSC would respond to this question.

 

[Ron]

No.

 

[connectegr]

Thanks dhengr

I did ask AWS and I am waiting for a response. I will ask RCSC at out committee meeting in June. AISC said "it is done, but not recommended". I was hoping for a more conclusive answer. I like a why or why not kind of answer. I wonder if any research has been done.

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[dhengr]

O.K. Why not.

 

[sdz]

Do not even tack weld quenched and tempered bolts. I've seen the results. They snap.
Even the nuts is a problem due, I believe, to sulphur content.
You could try to get weld nuts.

 

[Ron]

Both the nuts and bolts are heat treated (quenched and tempered) to achieve the required strength and hardness. When you strike an arc for a tack weld, the localized temperature is extremely high..much higher than the transition temperature of the steel. This causes a metallurgical change in the material that will either further harden it or anneal it. Neither is good on a localized basis. Even with low cycle stress, the potential for cracking is high.

If the electrode happens to stick when first touched to the bolt or nut, the temperature of the whole assembly can rise rapidly. Further, most welders will "drag" or flare the electrode to prevent such sticking. Dragging creates an intermittent arc strike, prone to creating localized hardening, thus increasing the crack potential. Flaring the arc creates a high localized temperature and uncontrollable results on the heat treatment of the bolt or nut.

Further, AISC says don't do it...see the notes in Table 1-C of the AISC 9th edition (p.4-4) "Anchor bolt material that is quenched and tempered (heat treated) should not be welded or heated.”

 

[racookpe1978]

OK.

But think of where the "tack weld" is and where the strutural stress actually.

The tack weld is on top of the nut, at the accessible end (visible end) of the bolt (or rod) where that (unstrained) end of the bolt is sticking out of the nut. Further, the tack (heat-affected zone) is physically located downstream of the nut - that is, past the point where the rod or bolt has been stretched and is holding the load.

The tack only affects a small point (small area) on the outside of one 1/6 of the threads: the whole shaft thickness is not heat-affected, and the area that is deformed (melted + heat-affected-zone around the melted area) is a small part of the threaded area.

Since the threads being welded are not carrying load, the center of the threaded shaft at the tack is not carrying load, and the top of the nut being tacked is not carrying load, what is the threat?

The tack is to prevent the nut from coming unthreaded - a greater danger than the local-damage to the bolt threads outside of the load path.

 

[connectegr]

racook...
In this case none of the bolt or threads are welded. A wide flange column will be erected against a concrete wall. The wall blocks access to the column web on the far side. At the column splice the erector would like to tack weld the nuts (before erection) to the inside of the column flange. The bolts will then be install from the outside.

In a bearing bolt connection, how much stress is actually in the nut? No tension, just shear in the bolt.

I have seen this done several times. But, this time approval is my responsibility. I was hoping to provide a more solid response than "in my engineering judgement..."

http://www.FerrellEngineering.com

 

[connectegr]

A little more information

The welds are not intended to be structural or transfer any force. Beyond the resistance needed to provide a snug-tight fit of the connection material.

Another confession, is that I have personally done this tack welding, as a young fitter in a fab shop. (A little knowledge, and simple instructions) My instructions were to make sure there was enough tack to secure the nut (simple). The worst case is if the weld breaks before the bolt is tightened. What is the fix for a "less than snug tight bolt with no access to the nut"?

Mechanics of Solids and Metallurgy came later in my education and career.

http://www.FerrellEngineering.com

 

[Ron]

Why not use a nut that is not heat treated? It's a bearing connection, so stresses are low and will be concentrated as shear in the bolt.

racookpe1978...while I agree with you, the fact is that if a failure of some type occurred in the future, the fact that this was done will be an issue....it might not have anything to do with the failure, but will become a significant red herring if nothing else.

 

[connectegr]

Ron
You struck my concern on the head. I DO NOT believe the tack welds will impact the design of the connection. But, it will become a "red herring" if something happens. Even if there is no failure in the nuts or bolts.

I am hoping to find a firm position from which to base my approval. (maybe this is my conservative nature)

But, I am leaning towards NOT APPROVED, based on lack of information on this specific condition.

(is that hypocritical based on my personal involvement with this procedure in the past)

http://www.FerrellEngineering.com

 

[dhengr]

Ron has the best idea of the day. Since it’s a bearing connection do you even need the Q&T nuts? Probably not, but now there is another issue. Since you have tacked randomly located nuts (tolerance wise wrt bolt holes), over oversized bolt holes how do you even get a bearing connection until you fail all the tack welds in shear. And, this leaves a nasty surface for the nut to slide on as the bolt moves to bear on the steel plate/bolt hole.

You will probably still get Q&T bolts, but that should be O.K. The nuts are applied in the shop under some control as to spec., but the A325 or A490's are all they have on the job site, and you certainly don’t want to start mixing bolts out there, or Ron can add another item to his liability list.

Unfortunately, he has the right thinking liability wise too, although I don’t like the direction that issue it taking these days. As he does, I do a fair amount of forensic work, and I spend half my time debunking these kinds of dumb side issues. I suspect that’s why connectegr brought the question up too.

 

[dhengr]

connectegr:
There are thin gage stl. locking/holding devices that get tacked to the stl. pl. over the bolt holes, positioned like a washer. They have two or three ears on them which are bent up the flats of the nut and slightly over the top, to hold the nut in place.

 

[connectegr]

I am waiting on a response from AISC and RCSC, but I leaning towards a NO, based on lack of information available.

I do think this should be addressed specifically by RCSC or AISC. The fact is, this does happen in structural steel fabrication/erection often. And in most cases nobody asks for the connection engineer or the EOR's approval. On the structural drawings these erection issues are not always clear. So they get address on the fly, as the erection schedule pushes the decision along.

I plan to bring it up at the RCSC Committee meeting this summer.


ron & dhengr
concerning the hole diameter and substituting A307 nuts. The holes are standard for bearing bolts. Pre-tension for slip-critical connection and oversized holes would create a bigger problem and more questions. Combining A325 bolts with A307 nuts, is another of the issues that lack supporting information.

Thanks for all the responses and feedback.

http://www.FerrellEngineering.com

 

[connectegr]

dhengr
WELDABLE NUT HOLDING DEVICE!!!

More, more, more information please...

Great solution, but I have never heard of this.

http://www.FerrellEngineering.com

 

[MintJulep]

These are designed to snap in, and for square nuts, but you can imagine a weldable type for a hex nut.

 

[connectegr]

Is there a trade name for these? I had no luck with Google.

http://www.FerrellEngineering.com

 

[MintJulep]

"cage nut" or "floating nut"

 

[connectegr]

Not exactly intended for structural applications. But this is definitely moving in the right direction.

Maybe we need to patent our own version. Similar to a structural washer with wings to hold the nut.

Thanks for the help. I will let you know when the money starts rolling in.

http://www.FerrellEngineering.com