"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.

"(A)What are the forces that the wind exerts on a tower and
(B)are these forces evenly distributed?"

A. More than you might think!
B. No. Depends on the particular configuration of the tower's load,
and the strength/speed of the wind. Infinitely variable.

You already know that there's no 'good' solution for one set of
guywires. The 'best' solution I've heard so far is an excess of
insurance and a gulible claims adjuster.
'Doc

Thank you all for the information. I knew I could find good"stuff" out
there.
The wind generator (not an antenna) I was planning has limited
information available as to loads. ie. No Flat Plate or FPE numbers.
The only statement is that the tower must take a 200 pound lateral
thrust and not deflect more than six inches with winds of 200 mph. The
Trylon T-200 tower specs. for my maximum wind speed area (from the
chart at their web site),
shows that the T-200 will take a 42 pound lateral thrust at winds of 85
mph.
If 120 mph winds cause the wind generator to exert 200 pounds of
lateral thrust, what speed of the wind will yield the 42 pound maximum
lateral thrust that the tower will hold? Please show your work.

fcrumpler,
Figured as a simple ratio, about 25-30 mph. No idea if
that's accurate...
'Doc

Dear "fcrumpler" no call, no location:

Theoretically, the force produced by moving air impacting on a flat
plate is proportionally to the square of the air's velocity. However, in
the real world where one should appropriately deal with wind gusts and
variability of materials, things are not so simple. Safety factors should
be used. Accommodation for the significant vibration to be caused by your
proposed "wind generator" must be made.

Numbers supplied by some vendors involve no safety factors.

You need to retain a Professional Engineer licensed in your state or
province who is experienced with vertical wind structures. This is
certainly the case if the proposed structure is able to damage other
property or endanger living things.

It is not safe for you to scale the numbers that you have. Too little
information. Too much risk.
Mac N8TT
--
J. Mc Laughlin; Michigan U.S.A.
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