Anchor Rods with single nut 1

[Samwise Gamgee]

The erector told me that the anchors with double nut have an embedment of 10" which is the same depth as that of the pedestal and there is no wiggle room for error. He suggested using a single nut and tack weld the nut to the rod. Is this allowable ?

The intent for me to call our double nut was that I wanted to avoid the turning of the anchor. (And these are braced framed foundations)

 

[dik]

I don't know what the embedment of 10" has to do with the number of nuts. Often anchor rods have 16" to 18" embedment. I don't recall ever using double nuts on an anchor rod installation and don't know why. It's generally not good to weld nuts to anchor rods. If I require a fastener to remain in place, I do it with Loctite 'Red' or peening threads. Double nuts if used have to be different thicknesses. See link:

https://www.boltscience.com/pages/twonuts.htm

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

I don't see how a double nut would prevent it from rotating. It would just rotate out of both of them. I'm guessing you're going for a jam nut type of condition here? Pretty sure it won't work since you can't torque the jamb nut...

Either tack weld it in place (weld on the bottom of the nut) or upset threads to prevent it from moving.

 

[dik]

You torque the one nut against the other, restraining the top nut.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

A double nut and washer is my goto standard. When that is used, I'm fine with either tack welding or thread deformation to prevent nut travel. My concern if you're using a washer is that, without the upper nut, the washer might move out of place. I'd be inclined to just shorten your anchor rod assembly 1/2" so that everything fits with some tolerance. We're fabricating buildings after all, not pianos. If 1/2" would kill the design, it was the wrong design to begin with.

 

[dauwerda]

phamENG, The double nut is effective. When the two nuts are torqued against each other it has the same effect as if there were a plate in between them. There is not as much stretch length of the anchor material obviously, but it still creates plenty of frictional resistance between the threads of the nut and anchor rod. I have seen all of the above options specified - a double nut (often with washer in between nuts, but not always), a single nut that is tack welded (with the tack going on the bottom of the anchor), and upsetting the threads, and lets not forget, some manufacturers still might prefer the forged head option. The double nut option does require a longer overall anchor length if you are trying to maintain the same overall embedment depth.

 

[dik]

I misunderstood the problem... I was thinking the top fasteners... I almost never use washers or double nuts for the bottom of anchor rods... single nut and the end peened. If cast in concrete, there is no issue with them backing out. They place the anchor rod with nut in a vice with the nut providing the vertical restraint and mash the end of the anchor rod with a heavy hammer. Simple and it works well.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

dauwerda - I suppose I could see it if you put the assembly in a vice and then toque one against the other - but then it seems that upsetting the threads would be easier and less prone to error (unless the steel sub does it in the shop and then drops of the anchors for the concrete sub to place, I seriously doubt it'll get done).

The double nut with a washer is common, but the double nut is only for the reason that KootK mentioned - keeping the washer in place. I dropped the washer from my details a while ago - I read a pretty convincing article looking at the relative stiffness of the various elements and the contribution of the washer is typically very small compared to a heavy hex head alone. Of course KootK's detail shows a really thick washer, which I'm guessing is designed to counteract that.

 

[KootK]

I read a pretty convincing article looking at the relative stiffness of the various elements and the contribution of the washer is typically very small compared to a heavy hex head alone.

Makes sense. Could you point me to that article? I'd like to check it out.

 

[phamENG]

Sure. I'll see if I can find it.

 

[dauwerda]

My experience is that all anchors come from the manufacturer with the bottom nuts in place - they are not field assembled. But, I also work in heavy industrial and utility industries, where the anchors are assembled and galvanized, commercial practices are probably very different. Also, in the utility industry it is common for them (the utility) to want to use 3000 psi concrete and grade 55 or even 105 anchors. To preclude these anchors from having pullout failures, it is necessary to provide plate washers to provide a larger bearing area, typical practice is to include an area that projects past the nut no more than the thickness of the plate washer.

 

[Samwise Gamgee]

I typically use washer only when the anchor rods are of higher grades like Grade 55 and Grade 105. I remember being told that a plate washer is not needed is the anchor rods are Grade 36, I am not sure what is reasoning behind that.

I had concerns with welding the nut to the anchor rod as there are of different strengths. Wouldn't this cause any unwanted localized stresses due to the nut and anchor rod expanding at different rates?

 

[bones206]

PIP STE05121 Application of ASCE Anchorage Design for Petrochemical Facilities is a pretty good resource for recommended plate washer sizing.

 

[HTURKAK]

Typical practice in my zone is, if the anchor is manufactured from weldable material ,the nut is tack welded. If not, two nuts are provided and jammed together.

When the size of the single nut is not adequate to prevent crushing of the concrete, larger diameter round plate of
suitable thickness is used...

 

[phamENG]

No luck on the article, and I need to get back to work. Here are a few things, though:

AISC Design Guide 1 states:

The addition of plate washers or other similar devices does not increase the pullout strength of the anchor rod, and can create construction problems by interfering with reinforcing steel placement or concrete consolidation under the plate. Thus, it is recommended that the anchorage device be limited to either a heavy hex nut or a head on the rod. As an exception, the addition of plate washers may be of use when high strength anchor rods are used, or when concrete blowout could occur (see Section 3.2.2 of this Guide.)

The basic idea, as I remember it, is that the relative stiffness of the washer compared to the hex nut decreases as you move away from the shaft of the anchor (basic cantilever stiffness stuff). Therefore, the bulk of the resistance occurs in the immediate vicinity of the shaft, right where the head of the anchor is anyway. To engage any of the washer beyond the head/nut, it has to be stiff enough to satisfy deflection compatibility within the concrete. In other words, if the washer has to deflect more than the elastic strain of concrete can accommodate, you end up with a failure cone based on the the head/nut and then, once the washer has deflected, it engages the concrete beyond the cone, but it's more likely that you'll end up with a progressive failure here than the edge of your washer "catching" the anchor and preventing pullout.

You also have the potential issue of introducing a failure cone above your intended embedment. If your washer is not sufficiently stiff, then your embedment depth will be h-thickness of the top nut rather than h (depth to the top of the washer). This probably isn't all that critical, but it's something to keep in mind.

 

[phamENG]

dauwerda - I can get behind the no more than the thickness of the plate washer idea.

 

[Samwise Gamgee]

So can I allow a single nut to be tack welded to the anchor rod then ?

 

[HTURKAK]

When the anchor is manufactured from weldable material , YES..

 

[dik]

The washers can introduce a plane of weakness in some conditions... other than decades back, I've not used them.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Samwise Gamgee]

When the anchor is manufactured from weldable material , YES..

Anchor is manufactured from weldable material, can you please expand on that ?