Lots of assumptions are made doing things like this. Sometimes relatively
reasonable assumptions only colored a bit by economic considerations result
in disaster.
Winter of 1960-61 Cqprock New Mexico. Worlds tallest structure at the time
I think (some height record) a TV transmitter tower. An
economic-engineering decision was made to forgo the optional de-icing
equipment. During a "freak" ice storm (region declared a National Disaster
and Army sent generator trucks in to electrify the town of Tatum, New
Mexico) the ice buildup on overhead power lines was as much as 5-6 inches in
diameter. Power to Tatum was from Lovington, 22 miles away. nearly all the
22 miles of power poles snapped off a few feet above ground due to weight of
ice covered wires.
The TV tower fell down. One guy on duty at the base of the tower in the wee
hours dived under a sturdy metal desk and survived with minor scratches.
Without knowing the tower specs, earth characteristics, etc. regarding the
tower starting this discussion it is impossible to determine how good or bad
or risky the installation is. I am repurposing the bottom 40 ft of a 100 ft
tower that was free standing with a good sized wind generator on top. At
ground level the 4 inch tube legs (1/4 inch wall) are 14 feet on centers.
Unfortunately gyroscopic forces were not properly accounted for thus no
engineering to accommodate them. The generator and tower crashed to the
ground. in light to moderate wind after several bolts failed that held the
welded-on pipe flanges that joined each 20 ft section. Enough components (20
ft tubes with welded on flanges and angle iron X braces) survived to
reconstruct the bottom 40 ft. I will use this 40 ft as a base for a crank-up
tower.
Patrick AF5CK
-----Original Message-----
From: Jim Lux
Sent: Thursday, October 31, 2013 8:50 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] minimum guy radius
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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