Hi Steve. I just wanted to congratulate you on a very precise
explanation of the need for guy tension differences at different heights
Your reasons are clearly explained and easy to understand. Not an easy
project for a potentially complicated subject. I, for one, have
printed it and filed it away.
73, Jerry K0GUG
********************
On Fri, 02 May 1997 20:55:13 -0500 Steve Sawyers n0yvy <sawyers@inav.net>
writes:
>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 point. Another
consideration is the effect of wind on the guy wires themselves. 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 originally
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 catenary 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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