U-Bolt Moment resistance
U-Bolt Moment resistance
(OP)
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
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






RE: U-Bolt Moment resistance
I've seen dents left by Ubolts installed as antenna mounts.
RE: U-Bolt Moment resistance
RE: U-Bolt Moment resistance
RE: U-Bolt Moment resistance
BA
RE: U-Bolt Moment resistance
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
RE: U-Bolt Moment resistance
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.
RE: U-Bolt Moment resistance
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?
RE: U-Bolt Moment resistance
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.
RE: U-Bolt Moment resistance
http://www.americanradiohistory.com/Archive-Catalo.... 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.