"Wes Stewart" wrote in message
...
On 9 Apr 2005 14:22:06 -0700, wrote:
Please don't get upset! I don't intend to actually do this!
All you antenna experts out there. If I have a tower say 60 feet tall
and
I can only put one set of guy wires up, should they go near the top of
the structure or near the middle?
What are the forces that the wind exerts on a tower and are these
forces evenly
distributed?
This is really a complicated subject and off the cuff remarks and
supposition are worthless.
The forces *can* be analyzed as uniformly distributed but usually, the
wind pressure is stronger near the top, particularly if there is an
antenna up there, which is the normal reason for a tower.
You might be inclined to say aha, then the top is the place for the
guys. But, guys turn horizontal force into vertical force that tries
to drive the tower into the ground. With the top and bottom
restrained, then the middle might be inclinded to deflect horizontally
under wind load. With it so moved, then the down force from the
weight of the tower and the additional force from the guys can cause
the tower to buckle.
I seem to remember from "structures" classes that the rule of thumb was
that if the solid diameter of a member was one tenth or more of the height
then the
member will break in normal compression mode. (Top loaded)
If the ratio of length to diameter was more then the member will buckle.
Seems like a good starting point to me. But then
that is the easy part since one would have to tension the guy wires to a
specific
force with multiple variables ! Note Tower manufacturers do not have to
introduce
safety factors in their calculations as would a crane or pulley manufacturer
where personel
are usualy in the area of use.Thus strength of material used must be on the
mark.
This is why I would guy at 30 feet and accept that movement at the top will
still happen
but with less moment of forces (wind loads) that would be around in the
event of no guys.
Art
Another concern is mechanical resonance. Top guying is going to make
the top resemble a string on a musical instrument with the wind trying
to pluck it. It has been my (sad) experience with a Cushcraft AV-80
tubular vertical that guying half way up, is equally bad. I had a
resonance set up and the tubing bent to yield and broke in two right
*under* the guys.
When I installed it I didn't have the data sheet but knew that it
needed guys. So I guessed at 50% up. Since then I've acquired the
data sheet and the recommendation is to guy about 2/3 up from the
bottom.
The bottom line is to listen to the manufacturer.