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Erecting Long Masts

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Subject: Erecting Long Masts
From: (David O. Hachadorian)
Date: Thu, 19 Dec 1996 15:45:29 PST
On Wed, 18 Dec 1996 19:18:51 -0700 Terry Dunlap <>

>I am curious as to the need to have so much mast buried in the tower.
>Wouldn't the rotator be just as isolated from side forces with say 
>half as
>much embedded mast as long as the intermediate support was still 
>there?  I
>understand about erring on the side of caution but isn't this huge 

Interesting question, here are some numbers.
The way to figure this is to regard the bearing at the top of
the tower as the fulcrum of a lever. All of the bending
moments above the bearing must be equal in magnitude to
the bending moments below the bearing.

Here are several different ways to use a 20 foot mast, with
22 square feet of antennas concentrated 2 feet below the top
of the mast, 71 mph wind (20 psf). This is for a stiff, non-
bending mast.

a. 2 ft rotor to bearing, 16 ft bearing to antenna.

bending moment = 22x20x16 = 7040 ft-lbs.
side force on rotor = 7040/2 = 3520 pounds.

b. 5 ft rotor to bearing, 13 ft bearing to antenna.

bending moment = 22x20x13 = 5720 ft-lbs.
side force on rotor = 5720/5 = 1144 pounds.

c. 10 ft rotor to bearing, 8 feet bearing to antenna.

bending moment = 22x20x8 = 3520 ft-lbs.
side force on rotor = 3520/10 = 352 pounds. 

Once you have those numbers, you can determine whether they are
within the bending moment specifications for your mast and tower
top, and within side force specifications for the rotor.

Dave, K6LL

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