Welding Nuts 1

[Ussuri]

A steel fabrication drawing has just passed my desk with a detail which involves welding a nut onto a SHS member. This seems like a poor detail to me, and I have concerned about the structural integrity and strength of a connection like this.

Colleagues have told me this is a commoon detail, but I have never seen it before. Have any other members come across this in the past?

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Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.
Archimedes

 

[henri2]

Ussuri,I personally have never seen such a detail used in structural steel construction, albeit its applicability has been discussed in other fora.

A few questions.

1. Do they want to weld the nut on the inside surface of the SHS? If so, there may be alternate solutions.

2. Are we talking about tack welding here?

3. Is the nut part of an HSFG bolt assembly …what is the BS-EN standard for the nut? If the %C is relatively high it is a bad sign for welding and could lead to a needle-like brittle martensitic microstructure.

4. If this is an HSFG assembly, is welding of nuts permitted by provisions of the design standard BS5950 and applicable BS/EN standard for high strength bolts? Sometimes it is prudent not to stray too far from regulated and established practice. I was never a bold designer...always conservative and played it safe.

5. Is this a detail they have to use?

I am sure the welding code you use stipulates that welding procedure specifications are required (prequalified or qualified by testing) prior to welding. However, I seriously doubt if the governing welding code would permit this.

BTW, some companies manufacture “weld nuts” and there is a discussion about “weldnut” in one of the forums but it’s not related to your situation.

http://www.eng-tips.com/viewthread.cfm?qid=121772

 

[Ussuri]

Henri2

Thanks for your input. I didn't like the detail so I have had it changed, but its still an interesting point. The application is as follows:

An outer 200x200x10 SHS with a 180x180x10 SHS inside. The two members are free to move relative to each other to take up some slack on site (site here is the bottom of the North Sea). When in position the two members will be match drilled and bolted through.

The nut is welded to the outside of the 200SHS, to basically provide a thread. This will be used to grip the internal member during drilling to hold everything together (think of a G clamp type detail).

The detail was for a fillet weld all round the nut.

We are not looking at HSFG here, and the nut was be a grade 8 to BS6392. I dont have it to hand so could tell you the %C content.

I have replaced the nut with a welded and tapped boss manufactured from normal structural steel.

I was wondering how common welding nuts actually was, because I had never come across it before.

 

[miecz]

For large utility boilers, back in the 1970s, the standard "locking procedure" at our firm was to tack weld the nut of hanger rods to the supporting member. These furnaces were hung from the roof level by hanger rods from 1 to 4 inches in diameter. The rods needed to accomodate the lateral expansion and vibration of the boiler. Another method to ensure the nut didn't loosen was to use a double nut. The the tack weld was the preferred method, as I remember it. At the lower end of the hanger assembly, the rod was tack welded to the forged clevis.

 

[jechols]

sounds painfull

 

[Ron]

Under static loading with no need for high strength bolted connection, welding would not be a big deal. If you have cyclic loading or a need for a high strength connection, welding a high strength nut will compromise either.