On Sun, 22 Apr 2001 09:48:16 -0600 "Steve/n0tu" <firstname.lastname@example.org>
> I'm placing a modest 40' Rohn-like tower guyed at 20'/38' on a small
> hill-top treed (35') location behind the house. First off, my guy
> points of attachment will be below the elevation of the towers base
> and at
> slightly different heights. My county wind speed rating is 85mph.
> But more of a concern, because of natural obstacles the guy anchors
> are at
> different distances from the tower's base (say about 10%)? Looking
> at Rohn's
> guy-anchor layout drawings showing perfectly flat no-obstacle
> enviroment I
> wonder if my setup is structurally sound or flawed?
> Or more importantly, I'm concerned I'm not able to meet the MFG's
> instalation specs which means all loading specs go out the window?
> My short term intentions will be a lightweight tribander or several
> monobanders (20/15) max! ~ Steve/n0tu
You can make small adjustments in the tension of the unequally
spaced guy wires, and/or unequal height guy anchors to reach the
same horizontal tension recommended
by the tower manufacturer. The horizontal tension component should
be identical for the three guy wires at each guy level.
For example, a 40 foot tower normally has the guy anchor
at a distance of 32 feet (80% of height). The top guy wire
makes an angle of 38.6 degrees (arctan 32/40) with the tower.
If the wire is 3/16" EHS, the manufacturer recommends an initial
tension of 400 pounds. 400 pounds at an angle of 38.6 degrees applies
a horizontal component of 250 pounds (400 sin 38.6). Try to
achieve this 250 pound horizontal force with your three guy wires
at this guy level.
If one of the guy anchors on this tower has to be placed at a
distance of 42 feet from the tower, the guy angle becomes 46
degrees (arctan 42/40). At this angle, the horizontal component
is 72% of the total tension (sin 46), so to get 250 pounds
horizontal tension, you need 347 pounds total tension (250/.72).
So this one guy wire would be tensioned to about 350 pounds.
There are limits to this, of course. If you go too far out with
the anchor, you get excessive sag in the wire. If the anchor is too
close, the tension in the guy wire and compression in the tower
leg become severe in the wind.
Dave Hachadorian, K6LL
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