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)

http://www.FerrellEngineering.com

 

[MintJulep]

I know that I've seen such things, but I can't find any pictures on the web.

 

[Ron]

Cage nuts and tab nuts (for welding) are usually not available for high strength applications or for large sizes.

If you want to accomplish the same thing, and assuming you don't have a bezillion of these to do, buy a sacrificial socket of the size to fit the nut....insert the nut in the socket, using a foam spacer if the nut is shallower than the socket. Weld the socket to the beam...now the nut is captive in the socket and will not turn when the bolt is placed and tightened.

You could design a variety of retaining devices for the nut based on similar applications for smaller nuts.

 

[racookpe1978]

Use a pant leg washer.

Tack weld one leg to the base metal, bend the second leg up and then tack it to the nut.

ALWAYS torque the nut and bolt before tack welding: Don't use the tack weld to hot the nut against turning when you are torquing the bolt.

 

[EngineerTex]

If we're just throwing out ideas, I would talk to a machine shop about tapping an appropriately-sized axial hole in a piece of (annealed) 4130 keyed shaft. Then, I would weld a piece of (easily welded) 1018 keystock to the backside of the beam. You may also need to add a retaining wire to the tapped "nut" to hold it in place until the bolt is inserted.

In this case, you have no risk of the load trying to pass through the weld -- an extraordinarily bad idea as stated in other posts above. As long as you make the "nut" deep enough (and prove it), it shouldn't need to be much thicker than a normally-sized nut.

Engineering is not the science behind building. It is the science behind not building.

 

[TXStructural]

I understand you need to restrain the blind nut from turning during installation. However, you need to avoid keeping the nut from translating as the connection settles into place. The bolt holes allow the connection to move at least 1/16", but if the nut can't move, the connection won't work as designed. You may end up without proper bearing.

You might weld a nut to a washer, and the washer to the base, giving the system enough movement.

ASTM A563 covers structural nuts, not A325 or A490. There are 8 grades of nuts under A563, and some may be weldable. Be aware that the weld will be creating a stress riser in the nut, particularly if the connection is susceptible to fatigue cycling.

 

[connectegr]

TX
This is a practice that occurs. However, I cannot document an acceptable procedure. I agree that the weld must crack for proper bearing to occur. But will the crack occur in the weld or the HAZ of the nut? I am also aware that there are other structural bolts, such as A307, which are weldable. But, RCSC does not allow combination of bolts and nuts.

There are more problems than solutions. But as I said this procedure is used by fabricators and erectors, when no other solution can be found.

I have also realized another problem. If the welds must crack to achieve bearing, how does the designer explain the "bolt banging" when the building is in service.

Thanks for all the responses. I plan to follow up after the RCSC meeting in June.

http://www.FerrellEngineering.com

 

[paddingtongreen]

Could you use the TC bolt, with the head on the blind side? on the viewable side, you would be left with the nut and a short stub of the thread.

If you do weld the nut, I suggest you put a bolt in it and pull it up tight before you weld, take out any space due to high spots.

I was told not to tack high strength nuts by a couple of level threes, they told me that some Q&T nuts cracked open at weld time and some later when the bolt was tightened. Even a tack weld heats and softens the material, it tries to expand but can't because it now has a lower E than the rest of the nut, so it is compressed. When it cools it tries to shrink, causing some considerable moments and forces in the nut.

It is as though you took a steel ring removed a piece and then glued the ends together.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[paddingtongreen]

I woke up with a start, in the middle of the night. Why did I say it has a "lower E" when I meant "yield". Sorry about that.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Guest102023]

A small weld is the most dangerous kind, which runs counter to the intuition of folks who don't know enough about steel hardenability and think they can get away with it. I call this the 'sneak up on the steel before it notices anything' theorem. By the same logic a welder might also unilaterally decide to forego preheat.

Ferritic fasteners generally have higher carbon content than weldable structural steels, as well some have alloy addition. This is a recipe for cold cracking.

 

[QC1971]

Why not attach them with an adhesive or epoxy? More than enough strength to hold the nut in place but would yeild much sooner than the steel.

 

[ToadJones]

When referring to high strength quenched and tempered anchor rod material, what materials are you referring to?
ASTM ?????

 

[Guest102023]

NO.
Think of it from the liability point of view. In case of an incident the standard or code essentially becomes law, and in this case the AISC clearly prohibits welding.

Now the technical aspect: a small weld is insignificant in size but constitutes worst-case conditions for hydrogen (or cold) cracking. Non-welding engineers and some welders with ignorance of hardenability concepts think they 'can get away with this'; these are the same folks who also wouldn't think it necessary to preheat, another no-no. But it does often get done by impromptu self-certified engineers in the field without benefit of even the back of a napkin to do calculations on.

If you must tack, use a nut of weldable material (provided it can handle the stresses). In general, preload and friction and not gimmicks are what keep a nut tight.