Nested Pipe Moment Capacity

[Azz90]

I am trying to determine the moment capacity of a nested pipe connection. For example,a 3" schedule pipe fitting inside a 3.5" schedule pipe. I would put a single bolt through both pipes to resists any axial loads. If you have ever used a cheater bar you know that there is some moment capacity for this connection.

I have played around with some AISC HSS equations for concentrated loads on the face of round sections, but I am not sure the appropriate length of the concentrated load (the normal application for these formulas would be a stab plate welded to the face of the section).

Are there any formulas for this type of connection? Are there any rules of thumb regarding the splice length to develop full moment capacity of the members? Any additional thoughts?

Thanks in advance for any replies.

 

[ToadJones]

My initial thought would be that the moment at the interface of the two pipes is the same, so the smaller pipe has to carry the same moment as the larger.
also, I would also think that the "pin" or single bolt will develop shear stresses as the two pipes carry the moment...somewhat analogous to shear flow.

 

[rb1957]

if the pipes are a tight fit then they'd transfer load like a socket and the bolt is there as a "chicken" fastener. in this case, i'd say the the two tubes are working together (like a single tube), the stress at the ID of the larger pipe = the stress at the OD of the smaller one.

if the pipes are a loose fit, then the bolt is transferring moment from one to the other and at the interface between the pipes, the two pipes can have different stresses. The maximum moment transferred into the smaller pipe is probably limited by bolt bending. The secondary loadpath into the smaller pipe would be as the larger pipe bends around the smaller one, eventually there'll be contact and a couple would load up th esmaller pipe.

 

[Azz90]

Thanks for the replies so far.

To better clarify,let's assume a loose fit between the pipes and the moment is transferred by contact of the end of each pipe into the wall of the other pipe.

An easy fix would be to use two bolts and check the appropriate AISC formulas for shear, bearing, etc...

However, this application would be for a high volume manufactured part and I want to avoid any manufacturing and installation tolerance issues by minimizing the number of holes that need to be aligned.

 

[rb1957]

does the moment start in the larger pipe and end up in the smaller one ? then you know the distance between the contact points, and everything follows.

maybe the larger pipe is reinforcing the smaller one. now i think you have to calculate the moment that causes the pipes to contact. then after contact the pipes would share moment in proportion to thier I, moment transferred to the larger pipe will cause it to deflect so there needs to be more moment in the smaller pipe to contact the deflected larger pipe.

 

[Azz90]

I agree that once I know the distance between the contact points I can calculate the forces required to resist the moment.

I guess the biggest issue I am struggling with is not the capacity of the members themselves, but rather the localized stresses caused by the contact of the two pipes.

I'm not sure what the contact stresses look like (for example, how much area of each pipe is in contact). Even when I do know this stress pattern, it's not obvious to me what formulas to use to justify the stresses are below the allowable.

 

[paddingtongreen]

The bolt may get a small kick depending on it's location, but it may also be zero. The small pipe is inside the large pipe for a short distance; as a moment is applied, one side of the end of the small pipe bears on the inside of the large pipe while the the opposite side bears on the inside of the end of the large pipe. Those are line loads that would be infinite except that the material deforms. There will be a small triangular bearing area at each contact point, varying from zero to a maximum near the initial contact point.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[racookpe1978]

I have not ( I emphasize!) run equations on this, but I have bent several "cheater bars" exactly like this while bending hand rails and ballasters for circular stairways and ornamental iron balcony rails.

If the moment (force) is applied (or resisted) from the small (inner) bar to the outer, regardless of how far the two overlap, the crushing force will begin failure by crimping and bending right at the point where they touch. The only time the inner bar did not fail first I was trying to straighten a very thin-walled square tube, misjudged the force used and tore through the wall of the 1-1/2x1-1/2 tube steel.

I have never felt any noticeable shear force when I pull on this type of cheater bar, but then again, I've been careful to pull perpendicular to the bar, so I wouldn't expect to.

 

[Ron]

I have done this with aluminum pipe. In that case we shimmed the space between the two pipes to get load transfer between them (using same material as pipe for the shims). The shims were full length, but welded only at top and bottom of the outer pipe. You can weld them all you want to the inner pipe and then insert the whole assembly in the larger pipe.

Since these were exposed architectural/structural columns, there was also a cap welded on top of the larger pipe where it ended below the top of the smaller pipe (the smaller pipe is taller by several feet than the larger pipe...done for the architectural effect. The larger moment in this case is at the bottom of the column, due to lateral wind load.

I treated as a composite section and analyzed accordingly.