Thread Pitch and Size 2

[Boiler8019]

I'm looking at a 10" diameter tower crane tieback with an in line adjuster composed of a 4" diameter threaded rod that screws into 3" thick end plates. So the tieback looks like this:

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The -'s represent the threaded rod

This rod will therefore take all of the 200kip of axial force. How would one specify the thread pitch and type on a connection like this? Is there any way to verify the axial capacity of the threaded connection?

 

[271828]

Unless there's another source, you could use the 13th Ed. AISC Spec. Chapter D. You might also find Table 7-18 useful.

 

[Boiler8019]

What is the title of that table? I dont have ASD books anymore in my office.

 

[271828]

Which Manual do you have? I think I can tell you where to look in anything >1989.

In the new 13th Ed. LRFD/ASD Manual, Table 7-18 is "Threading Dimensions for High Strength and Non-High-Strength Bolts." It would give you the net area to use in the Ch. D rupture calcs.

 

[Boiler8019]

Sorry, thought you meant the old green book. Yes, I looked into that table in the LRFD 3rd ed and it's the same.

Do you apply rupture limit states to thread area? I want to determine if the bolt will strip out of the 3" deep tapped hole.

 

[271828]

The rupture check is through the bolt's cross-section, at the smallest diameter.

I don't know any way to check a shear limit state in which the threads shear off, which I think is what you're envisioning. If I wanted to check that, I would probably figure out the shear plane area. Divide the load by this area and see what kind of stress I come up with. Hopefully it will be very small.

 

[cap4000]

I have the Beer-Johnston Statics 5th edition book and they use a "Block on an Inclined Plane" analogy involving dry friction. You will need to know lead or thread pitch, angle of the threads and the AISC bolt tensile value. You can then calculate the torque on the nut required to obtain your axial value. Hope this helps.

 

[Boiler8019]

Yea, I just want the strength of the threads. Not the required torque. 200kips acting only on 3" of threads is questionable in my mind. Cross section of the bolt/rod is more than enough.

I dont know why AISC doesnt incorporate this into the book since it would apply to stripping leveling nuts on anchor rods during construction. Doesnt seem to be much info about it out there.

 

[271828]

Boiler, I think there is a lack of info because it probably can't control over rupture through the bolt cross-section at the threads.

3" of threads is a lot! Still worth a calc, though, if you're up for it.

 

[miecz]

I would check a Mechanical Engineering Forum. I checked the "Guide to Design Criteria for Bolted Joints" and found nothing. The approach here is to make the threads long enough so thread stripping does not govern the design. It's simply a matter of standardizing the nut height, and then forget about it. You need to find the shear strength of the threads. Seems like the kind of thing that can only be determined by destructive testing.

 

[271828]

"Seems like the kind of thing that can only be determined by destructive testing. "

Yeah, this is one of the reasons for my last post. I've seen plenty of bolts break during destructive tests, but never one that broke by shearing off the threads. From that, and the lack of mention of it in the AISC Spec., I've pretty much written it off as something that won't control for normal conditions with a full nut, etc.

 

[JedClampett]

The thickness of a heavy hex nut is equal to the bolt diameter. And those are of a special super hard material made to be threaded into. You're probably threading into a A36 grade plate.
My point is, to be safe, I'd like 4 inches minimum of threaded material thickness to carry a 4 inch rod.

 

[271828]

JedClampett, that is an excellent point! It isn't the bolt threads that we should be worried about, but the plate, duh.

I don't even know if 4" to carry 4" really proves anything, though because Fy and Fu for the plate are so much less than Fy and Fu for a nut.

 

[unclesyd]

Thread shear stress sites.

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

When using the following site set the input and output to high

http://www.turula.com/bolt.html

 

[unclesyd]

Here is one more approach to your problem. It gives a little
more practical advice.

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

Here is fastener system you may want to consider.

http://www.superbolt.com/index.html

 

[unclesyd]

Slowly rounding up the references.

In lecture 28 by Dr. Robert Lemaster at UTM (University Tennessee, Martin) there is a very good reference, as we used to say with pictures, to Thread Shear Stress Area on page 21 of this paper.

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 28.pdf