[TowerTalk] Re: Tower Question

[David O. Hachadorian]

On Thu, 7 Aug 1997 10:10:03 -0400 (EDT) K7LXC@aol.com writes:
>In a message dated 97-08-07 09:25:42 EDT, w2up@itw.com (Barry Kutner) 
>writes:
>
>> Does it make more sense to have the top guys near your rotor, to 
>>  minimize the torquing (is that a word?) of the tower? For example, 
>if 
>>  you have a 20 ft mast, 8 ft inside the top section, put the guys 8 
>>  ft from the top. Or is it engineering-wise insignificant 
>difference?
>
>    Good question. Since I'm not an engineer, I don't have an answer 
>to that.
>TowerTalkians?
>
>73,  Steve  K7LXC

I think you're better off putting the guy wires as close to the
antennas as possible. Normal three way guying doesn't
do much to resist twist anyway, since twist is at right angles
to the guy wire, and the guy wires won't begin to exert anti-twist
forces until the tower gets twisted way out of shape. By that time,
you're in big trouble anyway.

I would be much more concerned with exceeding the bending moment
specs with a large distance between the antennas and top guy set.

As an example, consider an 80' Rohn 45, guyed for 70mph, no ice,
per the mfrs spec of guys at 35 and 65 feet. The tower is rated
at 23.3 sq feet. Now assume a 10 sq foot antenna at the 80 ft
level, and another 10 sq ft on a 10 foot mast, at the 90 ft level.

At 70 mph, the bending moment is:

(10sqft)(25ft)(20psf) = 5000 ft-lbs (top antenna)
(10sqft)(15ft)(20psf) = 3000 ft-lbs (bottom antenna)
(~3sqft)(~9ft)(20psf) =  540 ft-lbs (approx mast and 15 ft of tower)
                total = 8540 ft-lbs (no problem, since spec = 11,400)

However, if an 86mph wind gust comes along, the bending moment
goes up to 12,810 ft-lbs (8540ft-lbs)(30psf/20psf), and the 11,400
spec has been exceeded. The tower may fold at the top set of guys.

If the tower is instead guyed at the 40 and 80 ft level, the bending
moment at 86 mph becomes:

(10sq ft)(10ft)(30psf) = 3000 ft-lbs (top antenna)
(~1sq ft)(5ft)(30psf)  =  150 ft-lbs (mast, approx)
                         -----
                         3150 ft-lbs (not the final answer)

Since the fulcrum is now at the thrust bearing, the 3150 ft-lbs
has to be divided by the distance from the thrust bearing to the
rotor mounting plate. Assuming 2 ft worst case, the bending moment
on the tower is 3150/2 = 1575 ft-lbs, well within spec.


Dave Hachadorian, K6LL
k6ll@juno.com

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