On 10/31/13 6:18 AM, TexasRF@aol.com wrote:
Hi Greg, this equates to a 42.5% guy radius vs ideal 80%.
I have seen (and owned) a tower with 40% guying radius. It is not
recommended but with proper engineering to calculate the lower wind load rating
it
certainly will work.
Note: real engineering calcs, not an on a napkin at the bar while sipping
Scotch analysis for this!
Yes.. coffee is definitely required..
Let's see, 200 foot tower, 18" on a side, assume flat face and Cd=1, so
cross section is 300 square feet. Assume 120 mi/hr, so wind load on
tower is 300*120^2/400 or about 11,000 pounds.
So horizontal load at the top of the tower is about 5500 lbs.
Tension due to the wind loads in the upwind guy, worst case, is about
14,000 lbs. Not including the static tension, the weight of the guy, etc.
let's choose 1" wire rope, which has a breaking strength of 83600 lbs,
and a safe working load of 16,700 lbs. This weighs about 1.7 lb/ft.
The guys are relatively short (about 220 ft), so the guy weighs 375 lb..
That's a small amount relative to the tension from the wind loads, so
you don't need a bigger guy.
Actually, you wanted a three level guying strategy, so you could
probably cut the guy strength down a lot. a) the angle on the lower
tiers is a lot more generous, so the tension for a given horizontal load
is less. If the bottom tier were at, say, 70 feet, that's more than 45
degrees if the guy anchor is 85 ft out.
Now, is that breakfast napkin design an optimum one? No. Is it something
I'd actually contemplate building? No, although I've done similar sorts
of things (two 50 foot tall square trusses about 5' on a side with a 80
foot truss between them holding a helicopter rotor), guyed with 1/2"
wire rope.
http://www.reelefx.com/index.php?c=effect.view&id=239
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