Tube Slip Joint Moment Connection

[BS2]

I have a moment connection that I need to make work that consists of two tubes (snugly fit) nestled one inside the other. I have looked all over for information about what design checks may be applicable, but to no avail. The closest I could find was nested tapered utility pole calculations that used a linearly increasing distribution of the contact stresses. Does anyone have insight into how this issue might be calculated?

For specifics, the connection has a 2.875" diameter x 13 ga tube surrounding a 2.625" diameter x 11 ga tube with a 12" overlap. Additionally, the connection has two 3/8" through bolts.

Thanks for any insight that you may provide!

 

[MacGruber22]

To me, this wouldn't require a calculation as the size of the tubes/pipes imply relatively small load. 12" of lap seems like plenty to provide bearing/friction transfer. Now if you needed to check that 3" of overlap could work, that sounds like another story. Also, the change in section doesn't seem remotely large enough to warrant using non-prismatic equations.

What you describe sounds a lot like the style of connections that are used in stage lighting rigging pipes.

 

[CANPRO]

I agree with MacGruber22, I wouldn't be concerned about the moment transfer with that much lap length. Not sure how to show that on paper, but having a lap 4x the diameter of the tube seems like plenty. If you come across a decent reference for this I would be interested to see it...I spent a bit of time on google with no luck.

The title of your post says this is a slip connection. Does the connection have to function as a slip connection and a moment connection at the same time? Seems like a conflict of interest.

 

[SAIL3]

12" looks like more than enough overlap.....awhile back I designed a similar conn...while I could not find any published lit on it, I finally concluded that the stress distribution from the moment causes the inner tube to oval @ 90 degrees from the outer tube so the resulting deflections tend to fight each other...I concluded that the weakest point was where the outer tube terminated....it was an engineering decision..

 

[BS2]

This connection is essentially for a semi-permanent tent like structure. The larger tube will be permanently installed in a pier while the upper portion can be removed in the winter. Because it is near the ground and part of a cantilevered column system, it will see a large moment (relatively). Intuitively, I thought 12" would be enough lap, but I wasn't sure how to show this in a calculation. I will post here if I find any relevant references.

Sorry about the confusion on the title of my post. Because of the removable nature of the connection, the tubes need to remain capable of "slipping" past each other once the two bolts are removed. A more appropriate title may have been "Tube Sleeve Joint Moment Connection".

@SAIL3: Those were my thoughts exactly. The tubes will deform into each other and hopefully transfer 100% of the moment. You are correct - this is definitely an engineering decision/opinion.

 

[KootK]

I'd like to check out that utility pole document. Can you share the title BS2?

As a lower bound, you could just use the capacity of the two bolts.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BS2]

The document is titled "The Slip-Joint Connection - Alternative Connection Between Pile and Tower". Refer to page 7. It specifically applies to tapered poles and has a discussion about friction effects. However, there is also a section on the bending moment without friction.

I agree, the capacity of the bolts could be used as a lower bound.

PS - When I googled that title, it was the first hit. I am not sure what the policy is about uploading or posting a link to the PDF, but if you can't find it and it is allowed here, I will post it.

 

[msquared48]

The shorter the splice, the higher the shear due to the moment seen, and the higher the shear to the sidewalls at each end of the splice.

I question the pipes being disassembled without grease in the splice area. This implies not relying on any interstitial friction in the splice joint.

Mike McCann, PE, SE (WA)

 

[DamsInc]

If I understand correctly, you are telescoping one tube into the other? A typical cold formed tube will have a weld on the inside of the tube which could prevent telescoping if it's not accounted for.

 

[MotorCity]

Unless the tubes are 100% in perfect contact with each other and are fully bonded together, there will be some wiggle room between the two. I would resolve your moment into coupling forces that are separated by the overlap distance. Then check each tube for the resulting forces.

 

[JAE]

Here's someone's thesis on the topic. Might take a month to decifer:

http://bura.brunel.ac.uk/bitstream/2438/7380/1/FulltextThesis_2.pdf

Check out page 6 of this magazine:

http://issuu.com/pumasalient/docs/eis_31_web_pdf

A Florida DOT document on round tube slip connections - see page 37 and after on the document (page number not pdf page)

http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_STR/FDOT_BDK75_977-32_rpt.pdf

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[BUGGAR]

Once had a removable pipe railing that fit into a sleeve cast in a slab at the edge of a 15 foot drop. The railing movement when someone leaned against it caused more than one near heart attack as the rail leaned outward, only to stop at the last minute. The local cowboy drove some of those tapered horeshoe nails into the gap and the problem was solved. This was in an area where there were more horses than people (Bonny Doon, CA) so somebody always had a horseshoe nail handy when the railing was re-insatalled.

 

[SAIL3]

I suspect that the bolts do not come into play in resisting the moment....IMO, it is predominately a bearing mechanism....the wall of the inner tube bearing against the wall of the outer tube...

 

[msquared48]

Yup...

Mike McCann, PE, SE (WA)

 

[KootK]

I absolutely agree that there is a bearing mechanism. That's why I pitched the bolt couple as a lower bound solution above.

The nice thing about the bolt couple is that it's reliable and easy to calculate. I like a theoretical discussion as much as the next guy but, truly, if this crossed my desk as production work, I would start with the bolt couple. If that worked, I'd move on; if it didn't, I'd start looking at other mechanisms. Additionally:

1) Why two bolts if not to establish a couple? Redundancy? I would hope that there isn't enough axial load in a system like this that it couldn't be handled by a bolt in double shear.

2) For a structure with human occupants, I'd feel better if the bolt couple worked as a back up mechanism for moment transfer.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

BS2:
I would see this as the inner tube being in initial radial compression and the other tube being in initial radial tension, due to the fit-up of the two. The magnitudes of those forces and stresses will be a function of the class of fit btwn. the two. The two bolts through the joint essentially maintain the fit btwn. the two tubes and should not have any play or the fit-up loosens very quickly. These holes should be match drilled or bored and a very tight fit. They really only carry the vert. loads above the joint. Then, you superimpose a moment which varies from max. compression to max. tension over the length of the overlap, or max. compression on one end and side to max. compression on the other end and other side. That couple is approx. 2/3rds of the lap length (assuming triangular pressure distribution) times bearing pressure resultant vectors/reactions. The max. bearing stress btwn. the two pieces of tubing is likely not exactly linear over the length of the overlap. But rather, likely zero to max. fairly quickly at each end of the overlap, curved in shape, and then fairly linear over the mid part of the overlap.

 

[KootK]

If this were true HSS I'd consider a cap plate on the embedded pole. I suspect that would help to lengthen the effective moment arm some. I don't imagine that's too practical with a light gauge member however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.