U-bolt crushing pipe 1

[LeonhardEuler]

Can anyone help me with predicting the local strength of a round handrail pipe being clamped by a U-bolt. Also help predicting the moment strength of the u-bolt from the predicted clamping force

 

[LeonhardEuler]

I basically want to make sure I don’t crush the handrail by tightening the bolts too far. Also my specific question about predicting the moment capacity of the friction connection is how do I go from the clamping force to a lateral force that is dependent on contact area?

 

[JStephen]

How I would look at it- assume the tension in the U-bolt translates into a uniform radial force around the bolt, then look in Roark's Formulas for Stress and Strain at the cylindrical shell equations.
Or go to the hardware store, buy a piece of pipe and a U-bolt, tighten the heck out of it, probably quicker.
If it's standard weight steel pipe, I don't think you'll do much damage with a U-bolt. Aluminum or plastic pipe or lightweight steel tubing, maybe so.

 

[oldestguy]

As JSTEven says do a little experimenting. then you can establish a safe torque for the wrench when tightening in the field. So you get some yielding of the pipe. Makes a better grip in case of temperature caused dimension distortions down the road. So they break a few U clamps on the job. Cheap.

 

[LeonhardEuler]

The parts only going to be installed in one place for a short time. It is an aluminum pipe hat it is being secured to. It’s supporting a 2’x2’ flat shape that can experience about 40mph wind. I just want to make sure that it can theoretically hold that force with the specified nut torque and not crush the pipe. Testing it would be the best method, but I’d have to get my company to buy extras which they won’t want to do for the small application

 

[Ron]

I will assume you are using an aluminum U-bolt since you will have a corrosion issue with a steel U-bolt. If your pipe wall thickness is 1/8" or thicker no are not likely to collapse the wall with the low torque you are using.

The force on the pipe will not be radial. It will be top and bottom at the U-bolt plate and radius contact points. U-bolt plates have slotted holes so the sides of the bolt can slide. Only if you deflect the pipe such that the sides bulge will you get a side reaction on the bolt.

You only have about a 20 lbf load on the plate. Just tighten the bolts so that the assembly doesn't move under reasonable hand push and move on. Don't overthink this!

 

[LeonhardEuler]

I am actually using a steel u-bolt, but the piece will only be assembled for about a month so there should not be enough time for significant corrosion. I believe the handrail will be the anode here and is much larger, so I should see less? I agree just tightening it to where it sting enough to take a push, but I am not allowed to do the physical work in a union state and have to closely spec how they should tighten it and I can’t seem to elegantly put in words “make it tight enough” without giving a torque spec. If I give a torque spec I want to make sure they aren’t crushing the pipe

 

[LeonhardEuler]

Found answers if anyone else is looking for them.

Decided to specify snug tight, which is defined in AISC 360-10 as a the full force that an iron worker can apply with a wrench.

Section F8 of AISC 360-10 provides local strength formulas for round HSS, or pipe.

 

[12345abc6ttyui67]

Where in F8 of AISC 360-10 is this case covered? Unless I am mistaken F8 is for bending and buckling only. No discussion of crushing.

 

[JStephen]

I don't think that "snug tight" definition applies to small nuts like you'd have on a U-bolt for 2" pipe (3/8" size or so?) I'm thinking you'd just twist them right off.

 

[Ron]

Agree with JStephen. Snug tight is defined for high strength structural bolts such as A325 or A490 bolts. It is the effort of the ironworker using a spud wrench, not a socket and ratchet. You would probably strip these if you put that much effort. An ironworker can easily apply 150 to 200 ft-lbf torque on a high strength nut.

 

[LeonhardEuler]

Well... these are good points.

It looks I’m back to square one...

F8 covers the local buckling strength of round HSS, so my thought was that it would a fair approximation. Anyone have a better reference???

Not sure how to go about specifying the torque since I cannot be the one installing the system. I want them to do something along the lines of. “Torque until clamp cannot be moved by reasonable hand force” as suggested earlier, but isn’t a very “official” way of specifying torque.

I invite suggestions!

 

[Ron]

LE...is this project in Florida? If so, Florida is not a "union state". It is a merit shop state. Besides, you can do a performance test on site whether it is union or non-union for the project. That requires engineering input and licensed engineers are not subject to union rules.

If you can, at least, go to a Lowe's, Home Depot or hardware store and buy the parts to do a small scale test. That can be done in a few minutes.

 

[Tmoose]

Hi LeonardEuler,

By "crushing" do you mean permanent deformation maybe .1" deep or less right under the u-bolt clamp, like happens to car exhaust pipes?

 

[LeonhardEuler]

Ron Project is on a government facility that has their own rules. No wrench for me

Tmoose you’re correct. I don’t want to plastically deform the pipe

 

[Ron]

LE...then take the Lowe's/Home Depot route. Get a piece of pipe, a u-bolt and crank it down so that it doesn't rotate. If you don't want to do that, have the contractor submit a sample of the pipe and the U-bolt, then do your own test in your office. Won't take much. If you're near Jacksonville, bring it to my office and we'll do it there!

 

[LeonhardEuler]

Ron good advice. I suppose you would need to do this with a torque wrench to make it repeatable.

I find it really shocking that there are no AISC design equations based on test data for local failure of a hollow pipe. Perhaps it is an idea to include in my own grad research

 

[Ron]

Yes, you would need a torque wrench. It only needs to be the auto mechanic's variety, not a structural torque wrench.....your torque will certainly be below 150 ft-lbs.

You might also consider requiring a mock-up in your specification.

What is your location?