U-Bolt Moment resistance

[ENGINEER92]

I have been asked to analyzed an existing cell tower antenna mount for new antenna loads. The mount is adequate for the loads but I am unsure on the u-bolts rotating around the tower leg, it needs to resist around .226 k-ft. I have thought it through and came up with a possible way of checking this, but I would like a second opinion.

I have two half inch u-bolts that clamp the antenna mount to a 3" diameter tower leg.

1) Calculate the clamping force of the u-bolts
P=NT/(kD)
N=number of u-bolts=2
T=Torque applied to u-bolts=100 lb-ft
k=torque-friction coefficient=.2
D=u-bolt diameter=.5 in
P=2*100/(.2*.5/12)=24000 lb=24 k
2) Calculate the friction force on the tower leg
F=uP
u=friction coefficient=.7
P=clamping force=24 k
F=.7*24=16.8 k
3) Calculate U-bolt moment resistance
Mn=Fa
F=friction force=16.8 k
a=moment arm=1.5 in (this is the part I am unsure on)
Mn=16.8*1.5/12=2.1 k-ft
4) Apply Factor of Safety
Mn/FS=2.1/2=1.05 k-ft
5) Check if adequate
1.05 k-ft>.226 k-ft
therefore adequate

 

[Scratch3r]

Draw a picture of the cross section and show the couple. The friction factor is a tough call sometimes.

I've seen dents left by Ubolts installed as antenna mounts.

 

[ENGINEER92]

I believe I understand how to get the clamping force and friction force, but I am unsure on what moment arm I should use with the friction force.

 

[oldrunner]

You appear to have some pretty high loads coming from the dish. How large is the antenna? Or what is the wind speed and height on the tower? Some dishes have side struts to stabilize the whole unit. If the leg is actually 3" diameter, then it is some sort of tube. I don't know if the wall thickness is a problem. There are some u-bolt approvals on the State of California OSHPD website that might help you.

 

[BAretired]

Have you checked the plate for bending and deflection? With a clamping force of 24,000#, the moment in the plate is 12,000*3.5/2 = 21,000"#. I don't know whether the plate that comes with the U-bolts has sufficient strength to resist such a large moment. Also, deflection of the plate means that the nuts will not bear flush with the plate which brings into question your torque-friction coefficient.

BA

 

[klaus.]

A friction coefficient of 0.7 between steel and steel is much too much
it might be 0.2 to 0.3 ...but not more




best regards
Klaus

 

[KootK]

I'd estimate the moment arm at around 0.8 times the tube diameter. The effective bolt tension, and thus the COF may be difficult to estimate, however, because:

1) it will be affected by flexibility n the plate.
2) it will be affected by squash flexibility in the tube.
3) it will be affected by seasonal thermal fluctuations.
4) reliably quantified pretension in short bolts is difficult to achieve.

I'm not a fan of the connection for resisting moment. That said, perhaps the consequences of rotational failure of the joint are not a big deal so long as the dish doesn't actually fall off the pipe.

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.

 

[ENGINEER92]

Thank you all for responding, it has been very helpful. The more I look at it, I believe my problem is the way I modeled the mount in Risa. The way I modeled it, to make the model stable, I had to rely on the U-bolt resisting any rotation around the tower leg. The more I look at it, makes me believe that the mount's tieback is suppose to resist all rotation around the tower leg. So I am thinking my model is not an accurate representation of what is up there.

I attached a plan view of the mount. Does anyone have a recommendation on the proper way to model the mount where it attaches to the tower leg?

 

[ENGINEER92]

I have also attached another picture showing how I modeled it the first time and how I am thinking of modeling it now.

The image on the left is how I modeled it last time and the one on the right is how I am thinking of modeling it.

 

[oldrunner]

Download the Andrew Corporation Catalog 38.

http://www.americanradiohistory.com...anufacturers/Andrew-Catalog 38-2001.CV01.pdf.

Look on page 113.

This will have most of the answers including geometry for mounting. There are diagrams with the pipe connections as well as side struts. My printed copy is about an inch thick.

You didn't answer my question about the leg size - it is either a 3" tube which has a 3" OD or it is a 3" standard pipe which has a 3.5" OD. And it's a good idea to at least know what the wall thickness is.

Don't forget that you have to check the bending in the leg as well as the connections.