Force required to strip threads

[CWatters]

Not homework and I'm not a Mech Eng. I was trying to follow a discussion elsewhere about a building foundation that involved 2.75" diameter steel bolts. Two nuts are locked together on the bolt and embedded in concrete. Threads of the bolt itself are sleeved so only the nuts are anchored into the concrete. I was curious how you estimate the max load (tension) the bolt could withstand without stripping the threads.

Are there standard charts for this sort of thing or do you calculate it based on the shear strength of the material the nuts/bolts are made of each time?

If the nuts are locked together does this mean that only the threads in one nut are carrying load?

 

[dlgoff]

Not homework and I'm not a Mech Eng. I was trying to follow a discussion elsewhere about a building foundation that involved 2.75" diameter steel bolts. Two nuts are locked together on the bolt and embedded in concrete. Threads of the bolt itself are sleeved so only the nuts are anchored into the concrete. I was curious how you estimate the max load (tension) the bolt could withstand without stripping the threads.

Are there standard charts for this sort of thing or do you calculate it based on the shear strength of the material the nuts/bolts are made of each time?

If the nuts are locked together does this mean that only the threads in one nut are carrying load?

You might want to download the PDF file form here:
https://www.academia.edu/7435715/Machinerys_Handbook

 

[Baluncore]

This is actually a case of identifying what will fail first. My guess is that the threaded rod is the weakest link. The effective diameter will be the bolt diameter, 2.75", less twice the thread depth.

The bolt will usually stretch first. Nuts are designed, so there is sufficient thread contact to handle the bolt tension.
What is the thread profile? How thick is the nut?

There should be a steel plate above the top nut, to increase the area of the concrete being crushed. The area of the top face of the nut or plate presses against the concrete. At what bolt tension will the concrete be crushed?
If there is no plate, then what is the AF size of the nut?

If the nuts are locked together does this mean that only the threads in one nut are carrying load?

When nuts are locked together, the threaded rod between them is under tension. The nuts are therefore seated against the thread, such that pulling on the bolt, or pressing down on the top nut, will relieve contact pressure on the thread of the top nut, transferring the thread contact force to the bottom nut as the bolt tension exceeds the pretension between the nuts. Then, as the bolt stretches, the tensile force is applied to both nuts.

Can the bolt be stretched and the thread deformed? Is the material a soft steel, or a high grade steel that will shatter like glass?

 

[jack action]

Are there standard charts for this sort of thing or do you calculate it based on the shear strength of the material the nuts/bolts are made of each time?

Manufacturers usually do the calculations for each of their bolts for their clients because, yes, it is based on the yield strength of the bolt material.

When you bolt two pieces together, the idea is to create the maximum friction force between the threads to prevent the bolt to unscrew itself. Obviously, this maximum value should be low enough as to not exceed the yield strength of the bolt.

Reference: https://mechanicalc.com/reference/bolted-joint-analysis#preload-values

If the nuts are locked together does this mean that only the threads in one nut are carrying load?

Yes. Pulling on the bolt will pull on the lower bolt and the reaction will be the concrete pushing on the upper bolt. These two forces working against each other on the nuts will reduce the preload on them. If the upper nut wasn't locked (in concrete in this case) and the preload was reduced to zero, it could unscrew by itself.

 

[Baluncore]

If the upper nut wasn't locked (in concrete in this case) and the preload was reduced to zero, it could unscrew by itself.

That could only occur if axial or rotational vibration was present, without concrete.

In general, the Tan(thread angle) is always designed to be less than the friction coefficient between the nut and bolt, so tension alone will not unscrew the thread.
That explains why PTFE (Teflon) nuts always have a fine thread.

Nuts and bolts are usually employed to pinch or clamp two materials together, so friction between those materials prevents movement. The bolt is not in shear, nor does it carry a live load in tension.

The use of a threaded rod as part of a rag-bolt assembly in concrete, leads to a more dangerous field of use that requires a deeper failure mode analysis.
It is usually the case that an "above-footings" steel structure will be attached to the rag-bolt with a single nut with a lock washer. That top nut then becomes the critical threaded joint, not the nuts locked together in the concrete.

If the threaded rod turned, as the nuts below loosened, the nut above the surface would tend to tighten and compensate.

 

[Lnewqban]

... I was trying to follow a discussion elsewhere about a building foundation that involved 2.75" diameter steel bolts. Two nuts are locked together on the bolt and embedded in concrete. Threads of the bolt itself are sleeved so only the nuts are anchored into the concrete...

It seems to me that it would be impossible to turn the head of that bolt. Should that be a threaded rod instead?
In that case, the single top nut deserves the analysis.

Please, see:
https://www.metalconstructionnews.com/articles/the-facts-about-foundation-anchor-bolts

 

[CWatters]

Thanks for the replies.

Yes I said bolt but the actual situation was a threaded stud. I hadn't thought the difference mattered but can see it could.