Thread embedment required for bolt in tension

[EngDM]

Hi all,

A simple question that I have never really thought about, but if you have a bolt in tension are there any checks as far as the threads are concerned? Is there a codeified minimum number of threads (or length) to engage before getting full capacity of the bolt in tension? Is it just to the end of the standard nut size (I.e. that is how the nut lengths were determined and standardized)? Is there an equation in the event that you are not fully into the nut?

This question is moreso for tension requirements, not the intersected vs not intersected shear plane checks.

 

[jhnblgr]

https://www.aisc.org/globalassets/modern-steel/archives/2014/12/si.pdf

https://www.aisc.org/globalassets/aisc/publications/standards/a348-20w.pdf

 

[SWComposites]

the general rule of thumb is min of 3 full threads.
and the nut material/type needs to be strong enough to fail the bolt in tension.

 

[Tomfh]

Codes often require at least one thread beyond the nut.

One thing I've noticed in practice is that if a nut and bolt aren't well machined that the nut can slip anyway, yielding the threads before yielding the bolt.

 

[HTURKAK]

I remember for structural applications (Research Council on Structural Connections Specification requirement is that the end of the bolt be at least
flush with the face of the nut).
Common practice at least 2 threads after nut.
There are previous threads for this subject.
One of them,

Standard for how many threads to be out from a nut​

 

[BulbTheBuilder]

I remember for structural applications (Research Council on Structural Connections Specification requirement is that the end of the bolt be at least
flush with the face of the nut).

yes, point 4 of chapter 8.1 of the RCSC spec. attached by jhnblgr

"The joint shall be installed to the snug-tight condition with sufficient thread engagement" pg. 16.2-60
where "Sufficient Thread Engagement. Having the end of the bolt, not including the spline of a spline-end bolt, or the available bolt threads extending beyond or at least flush with the outer face of the nut; a condition that develops the strength of the bolt."

The threads of bolts are constant and are dependent on the bolt size - SCM Table 7-14. Try to get more projected thread length means increasing the bolt length which isn't recommended. I just read Specifying Bolt Length for High-Strength Bolts by Charles J. Carter (this past Tuesday. He talked about the jamming of the nut with increased length.

In my opinion, sufficient thread engagement >=flushing is adequate to develop tension of bolts.

 

[EngDM]

We are trying to come up with a detail for supporting a threaded rod in tension, and due to material availability we are going the route of tapping our own holes in a solid square stock.

the general rule of thumb is min of 3 full threads.
and the nut material/type needs to be strong enough to fail the bolt in tension.

I guess the square stock should be a high strength steel as opposed to 44W then? Or could I leave enough net tension area around the bolt to exceed the capacity of the bolt and be fine. This assembly would have a larger tension area than your typical A325 bolt nut.

 

[icebloom]

See file at link below which discusses the strength of partial nut engagement for bolts, which may be helpful. Obviously code requirements are always for full engagement for a given nut but from a theoretical POV it may help.

https://www.aisc.org/globalassets/modern-steel/archives/2016/09/steelwise_2.pdf

 

[dik]

In response to your question...

Not at all, but I'll check.
Reply back....

"To weld to CSA W59, a weld procedure qualification will need to be completed prior to welding the structural parts. That being said, it shouldn’t be an issue welding with 7018/ ER70."

 

[Lomarandil]

We are trying to come up with a detail for supporting a threaded rod in tension, and due to material availability we are going the route of tapping our own holes in a solid square stock.

I guess the square stock should be a high strength steel as opposed to 44W then? Or could I leave enough net tension area around the bolt to exceed the capacity of the bolt and be fine. This assembly would have a larger tension area than your typical A325 bolt nut.

Structural nuts have hardened threads. You won’t match that capacity by merely tapping a hole. Might work for very low loading.

 

[human909]

We are trying to come up with a detail for supporting a threaded rod in tension, and due to material availability we are going the route of tapping our own holes in a solid square stock.

I guess the square stock should be a high strength steel as opposed to 44W then? Or could I leave enough net tension area around the bolt to exceed the capacity of the bolt and be fine. This assembly would have a larger tension area than your typical A325 bolt nut.

You would likely need an engagement length of at least 2x the diameter to start approach full strength in "standard mild steel". If this is a highly loaded item I'd be very careful in specifying the required engagement length, the steel and the tapping.

You can also use weld nuts or a captured nut design.

 

[EngDM]

You would likely need an engagement length of at least 2x the diameter to start approach full strength in "standard mild steel". If this is a highly loaded item I'd be very careful in specifying the required engagement length, the steel and the tapping.

You can also use weld nuts or a captured nut design.

The load isn't huge, but it is for a critical piece of the structure (supports a cast concrete balcony). I was going to make the tap like 2" long (1"Ø threaded rods), but if there is no real straightforward procedure to determine thread strength for standard mild steel I may have to think of a different approach like welding a couple nuts together.

 

[Lomarandil]

I think there is, but it's in the purview of mechanical engineering.

 

[Stick.]

For just determining the engagement length for a tapped hole, I generally follow this method: https://roymech.org/Useful_Tables/Screws/Thread_Calcs.html

Once you have the engagement length determined, you should also consider whether tolerances, chamfers or other factors will reduce the actual engagement and then increase what you specify to account for that. I doubt you'll need to adjust anything based on that if you're already considering a 2 in tapped hole for a 1 in fastener, but it's good to check/consider.

 

[jhnblgr]

Maybe contact a manufacturer and see what they have. Some times they stock it or they can make what you need https://www.portlandbolt.com

Haydon Bolts Inc - Our Products

Haydon Bolts Inc, Est 1864, is the industry leader in rod and headed bolt manufacturing, as well as carrying one of the largest inventories of Hex and TC or Twist-ff Bolts, in a variety of grades and finishes. Call us to see to see what we can do for you, today!

haydonbolts.com

 

[EngDM]

Maybe contact a manufacturer and see what they have. Some times they stock it or they can make what you need https://www.portlandbolt.com

Haydon Bolts Inc - Our Products

Haydon Bolts Inc, Est 1864, is the industry leader in rod and headed bolt manufacturing, as well as carrying one of the largest inventories of Hex and TC or Twist-ff Bolts, in a variety of grades and finishes. Call us to see to see what we can do for you, today!

haydonbolts.com

I think the reason the fab is looking to in-house it is due to lead times and increase costs to import it to Canada, and they can't find it locally. Thank you for the suggestion nonetheless.

For just determining the engagement length for a tapped hole, I generally follow this method: https://roymech.org/Useful_Tables/Screws/Thread_Calcs.html

Once you have the engagement length determined, you should also consider whether tolerances, chamfers or other factors will reduce the actual engagement and then increase what you specify to account for that. I doubt you'll need to adjust anything based on that if you're already considering a 2 in tapped hole for a 1 in fastener, but it's good to check/consider.

I will look into this, appreciate the link!

 

[jhnblgr]

you likely can find similar companies within CA, i know these guys typically have quick turnaround unless you need a really long continuous threaded rod.

Typically when someone tells me they can’t find a product they never actually even checked.