[TowerTalk] Re: Tower Question

[Barry Kutner]

On  7 Aug 97, David O. Hachadorian <k6ll@juno.com> wrote:


> 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 - I'm not sure you're correct:
>From the thrust bearing, to the rotor plate, there is a mast. This 
mast is not movable in the Y and Z planes as it is pinned at the 
thrust bearing and the rotor itself (short of extraordinary force 
moving it vertically, shearing bolts, etc.) It can only rotate in 
the X plane. Therefore, the bending moment and resistance to buckling 
this "reinforced" section of tower, between the thrust bearing and 
rotor plate, will take is much much greater than the tower as a 
whole and its square foot rating.

Please correct me if I'm wrong...
73 Barry

--
Barry Kutner, W2UP              Internet: w2up@itw.com
Newtown, PA         FRC         alternate: barry@w2up.wells.com

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