A small quibble with your analysis, Dave.
It is impossible for the horizontal tensions at a
common guy point to be unequal, if the tower is plumb.
If they were, the guy point would move to a position
out of plumb, until the forces equalized (now counting
the force exerted by the out-of-plumb tower). If the
tower is plumb, it exerts no horizontal force.
As you said, what is important is how slack or tight the
guys end up being to achieve the specified (calculated)
Tom - N1MM
----- Original Message -----
From: Dave Hachadorian <firstname.lastname@example.org>
Sent: Monday, April 23, 2001 05:30
Subject: Re: [TowerTalk] Anchors unequal distance from base?
> On Sun, 22 Apr 2001 09:48:16 -0600 "Steve/n0tu" <email@example.com>
> > 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
> > anchor
> > 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
> > suggested
> > instalation specs which means all loading specs go out the window?
> > Right??
> > My short term intentions will be a lightweight tribander or several
> > 2el
> > 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
> Yuma, AZ
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