On 12/9/2018 8:30 AM, Hans Hammarquist via TowerTalk wrote:
I did a calculation once regarding how far out from the tower you should attach the guy wires. The calculation took into account the springiness of the wires (Yes, a guy wire stretch like a spring when you pull on it.) I found (I don't remember the exact value) that if you attached the guy wire ~130% from the tower you had the smallest movement on the tower due to the spring action
Hans - N2JFS
IOW, you used Hooke's law for springs. That's a reasonable
start, but I believe you also have to take into account
the action of the catenary of the wire as a spring of
sorts. That's considerably more complicated than Hooke's
law, and depends on the tension and the weight of the
wire. We can probably accept 130% as an upper bound,
and probably better to stay well below that. Longer
is NOT better.
I can attest from experience that overly long guys get
very "bouncy" and even catch the wind, which builds
up the bouncing amplitude. The bouncing
can produce enhanced peak pull out forces on the guys.
The bouncing (again, hard to model) could be the sleeper
effect that brings down the tower. Reference: Tacoma
Narrows bridge, 1940.
Another non-intuitive aspect is that in the extreme
case of the guys being at infinity (IE horizontal),
then they actually do nothing to stabilize the tower
because as one guy stretches, the opposite guy
contracts with equal force, so that the net effect
is that the tower is free to move around in the
wind with nothing to stop it. IOW, the two springs
cancel each other out, until the tower moves far enough
so that the leeward guy goes slack. Think about it.
73
Rick N6RK
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