[TowerTalk] Guying a self-supporting tower - Yes

[Michael Tope]

----- Original Message ----- 
From: "David Robbins K1TTT" <k1ttt at arrl.net>

>> This is the question I'm interested in. Because if it turns out that
>> it's difficult to design even a ridiculous structure that is weakened
>> by guys, some good light will be shed.
>
> I am good at the ridiculous..
>
> just use the example of a very weak tube, like a cardboard paper towel 
> tube.
> It will stand upright very nicely by itself... but then add a couple of
> strong cords pulling down at a steep angle and it will crush even without 
> an
> added load of wind.

One element of this debate is whether or not you can generally
say that for any free standing tower structure there exists a
specific guy wire configuration that will result in an increase in
the overall  wind load capability of the tower structure. My guess
is that the answer is to that question is almost alway "Yes".
The second element of the debate is a question of how sensitive
any given tower structure is to the parameters of that optimum
guy configuration. Your cardboard tube example illustrates that
there do exist guy configurations that will result in a lowering of
the overall load capacity of a tower structure. The question is
how far from normal Rohn 24/45/55 guy parameters (tension, angle,
breaking strength, etc) do you have to stray before you get into
that territory with a typical self-supporting tower?

I think a good starting point to answer this question would be
to compare the compressive force in the stressed tower leg
(leeward side) of the self-supporting configuration at maximum
rated wind load with the compressive force induced by the guy
wires at no wind load. If the compressive force due to guy
tension with no wind load was a significant portion (say > 5%)
of the maximum compressive force in the leeward leg of the
max wind loaded unguyed tower, then I would begin to worry that
I was sailing into dangerous waters where additional structural
analysis would be warranted to make sure that the supplemental
guys weren't degrading the mechanical integrity of the
free-standing structure.

73 de Mike, W4EF..................