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[TowerTalk] Calculating dead weight stress on mast when tower iscranked

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Subject: [TowerTalk] Calculating dead weight stress on mast when tower iscranked over
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Date: Wed, 31 May 2006 16:24:25 -0400
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To all the antenna & mast engineers out there,

I have a 55' crankup / tiltover tower that uses a 24' aluminum mast 6061-T6511 
2" OD .25" wall.
My question is this: does anyone have a formula for calculating the dead weight 
load on the mast when the tower is horizontal? The mast extends 18' above the 
top of the tower (6' in the tower with two thrust bearings and rotor). At 3' 
above the top of the tower is mounted a HyGain TH7DX (75 lbs dead weight, 9.5 
square feet wind load). Ten feet above that is a homebrew beam for 30, 17 and 
12 meters that weighs 50 lbs and has a windload of approx 5 square feet. With 
the tower approaching horizontal, and the antennas just off the ground, the 
mast bends at an angle that really makes me think it is about to permanently 
deform, but so far it hasn't. I want a way to figure what margin of safety I 
have, if any, in lowering the antennas to make adjustments/changes. I probably 
need to change to a stiffer mast, but when I started this tower project, I 
didn't plan on adding the top antenna. The mast should be rated at 40,000 psi 
yield strength, but how does that relate to resistance to d
 eformation with a 50 lb load at 13' away from the tower along with a 75 lb 
load (TH7DX) at 3' from the tower?

I am also concerned about wind loading with the tower vertical and the mast's 
ability to handle that. For the time being, I keep the tower nested when I'm 
not chasing DX, so it isn't exposed to high wind gusts.

Any and all replies are appreciated. I searched the archives, but didn't see 
anything that fits this situation.

Thanks & 73,
Charles, K4ZRJ


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