A thought exercise about guy wires
 

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?

Thanks for your help.

wrote in message
ups.com...
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?

Thanks for your help.


AS you said , not recoomeded for only one set at that heigth but the top set
of guy wires always go near the top of the tower.

I would put them at the center
as I see buckling as the worst danger.
I have also done this with a 60 foot boom antenna
with no problem. I was forced into this position in the first
place because I have a fold over tower and eventualy stopped
using the top guy wires thru lazyness.
Art




"Ralph Mowery" wrote in message
ink.net...

wrote in message
ups.com...
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?

Thanks for your help.


AS you said , not recoomeded for only one set at that heigth but the top
set
of guy wires always go near the top of the tower.

wrote in message
ups.com...
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?


If the tower is 4 feet across at the base, you may not have a problem,
otherwise - If you do that, be sure the tower is more than 60 feet from your
house - so it will miss the house when it falls.
Most of the water towers you see are a hundred feet or higher, but have no
guy wires, because the base is pretty wide.
guying at the top only may cause it to bow in the middle during a windstorm
to such a degree as to collapse.
I would guess the tower manufacturers would have wind and static loading
data available.
You have wind loading and the static loading due to gravity - which begs to
become dynamic.

try practicing with half-inch pvc water pipe - see where the guy wires do
the most good.

wrote:
I can only put one set of guy wires up, should they go near the top of
the structure or near the middle?

According to Rohn specs, a single set of guy wires limits
your tower to 40 feet maximum height - sorry about that.
--
73, Cecil http://www.qsl.net/w5dxp

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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.

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.

"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.

Did you ever take any ME courses Art?

Simplest back of the envelope approximation is 2/3 is best, ignoring
toewr resonance and lower wind as you get closer to ground level, so,
lower points would put more torque on the tower. With an antenna
mounted, it would move the optimal single guy point upward.

It's possible I may have this cimpletely wrong, if so, correct away folks.

tom
K0TAR

wrote:
I would put them at the center
as I see buckling as the worst danger.
I have also done this with a 60 foot boom antenna
with no problem. I was forced into this position in the first
place because I have a fold over tower and eventualy stopped
using the top guy wires thru lazyness.
Art




"Ralph Mowery" wrote in message
ink.net...

wrote in message
roups.com...

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?

Thanks for your help.


AS you said , not recoomeded for only one set at that heigth but the top
set
of guy wires always go near the top of the tower.

That wasn't the question.

"Cecil Moore" wrote in message
...
wrote:
I can only put one set of guy wires up, should they go near the top of
the structure or near the middle?

According to Rohn specs, a single set of guy wires limits
your tower to 40 feet maximum height - sorry about that.
--
73, Cecil http://www.qsl.net/w5dxp

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CW wrote:
That wasn't the question.

No, but that was the answer AFAIAC.

"Cecil Moore" wrote:
According to Rohn specs, a single set of guy wires limits
your tower to 40 feet maximum height - sorry about that.
--
73, Cecil http://www.qsl.net/w5dxp


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