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[TowerTalk] guy tension

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Subject: [TowerTalk] guy tension
From: (Steve Sawyers n0yvy)
Date: Fri, 02 May 1997 20:55:13 -0500
Edward Hancock wrote:
> I read with interest the various suggestions about guy wire tension.
> But the more I read the more confused I become, because of what is
> probably a simple misunderstanding.
> If I have a tower at 60 feet and use .25" guy at 10% of its rated
> strength and it holdes the tower per specs why do I need to increase the
> absolute tension lbs. if I just go up to a larger diameter guy?  If 600
> lbs held the tower with a .25" guy to go to 800lbs just because I am
> going to increase the guy to .365" just doesn't make sense to me. (NOTE:
>  all sizes and tensions are presented for example only.  Somebody please
> explain
> --
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As you correctly noted, the tension on guy wires is to hold a tower in
place as a load - wind or otherwise is applied.

I will try to explain this without going into a lot of math.

Any structure, when a load is applied, will move.  There is no such this
as an immovable object. 

The tower support at a guy anchor has a lateral spring effect resisting
any and all side loads at the guy cluster. The stiffness of the lateral
spring is a function of number of guys  (we are used to 3 or 4, but more
are possible) times the stretchiness of the guy material (yes steel guy
wire is stretchy to an engineer) divided by the weight of the guy
material. The steepness of the guy wires enter into it also.

For a given tower and guy material, the higher the tension divided by
the weight, the closer to vertical the guys will hold the tower for a
given load. If you just increase the weight of the guys without
increasing the tension, you allow the tower to move farther down wind.
This increases the bending on the tower section. Tower sections are
designed to handle compressive load equally on all three or four legs.
They do not do well when you add bending stress caused by a tower that
has slack guys.

As you increase guy tension, you do increase the load on the tower, so
to much of a good thing is not always good. So there is a balancing

Another consideration is the effect of wind on the guy wires them
selves. If you get them too loose, they start a slow whip and move the
tower back and forth ie. induces fatigue in the tower. If you get them
really tight then they hum like a fiddle string and fatigue the guys at
a high cycle rate.

People smarter than me, have done a lot of "what if" analysis and
determined that for all the trade offs, the guy range should be in the
8-15% of tensile strength to keep the tower straight and the guys from
flapping or singing.

My rule of thumb is 8% if the guy is out at 100% of tower height, 10% if
at 80% of tower height (standard Rohn drawings) and up to 15% if the
anchor point is at 65% of tower height. You loose a lot of wind load in
this last type of installation.

If you are really interested in this get a book called 'Cable
Structures" by Max Irvine. Max was (is?) a Professor of Civil
Engineering at the University of South Wales in Australia. The book was
orginally published in 1981 by MIT Press (they have some knowledge of
engineering) and was republished by Dover books in 1991. Mine is the
later. ISBN is 0-486-67127-5. The price printed on mine was $8.95 - best
money I ever spent on a book.

It is a bit of slow read - he starts out with the analysis of stone
arches and inverts it for cantenary cables. But I found it very
understandable. The math is algebra, geometry and trig including
hyperbolics, but not a lot of fancy calculus until you get into the
analysis of the dynamic reaction of a cable structure and you need it
there. I do suggest you make an equation sheet of variable definitions
as you go through it with page references as I had to flip back and
forth to pull some things together. Not exactly night table material but
good lunch hour brain food.

de n0yvy steve

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