----- Original Message -----
From: "Michael Tope" <W4EF@dellroy.com>
To: "David Robbins K1TTT" <k1ttt@arrl.net>; "'towertalk'"
<towertalk@contesting.com>
Sent: Sunday, April 10, 2005 1:22 PM
Subject: Re: [TowerTalk] Guying a self-supporting tower - Yes
> ----- Original Message -----
> From: "David Robbins K1TTT" <k1ttt@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.
>
I think the other wrench in the works is whether it's really a strength of
materials question or a slender column buckling question. If you're talking
about free standing Rohn 25 vs guyed Rohn 25, the stiffness is quite high
(so that it doesn't buckle when axially loaded), so it comes down to
strength of materials, and I suspect that the limit for free standing Rohn
is where the bending loads reach the structural limit.
Someone who has the engineering package for a Rohn installation could look
and see if the analysis was based on column buckling or bending moments.
(there's also a third possibility.. where they analyze for local buckling,
which is where a segment exceeds the bending limit)
The real question is when you want to guy something that was originally
designed as free standing (and may be quite flexible, and subject to
buckling at relatively low loads). There's also the practical detail that
most people put the guys part way up, not at the top, so the top part is a
bending moment, and the bottom is axial loaded, plus a bend propagated from
the top.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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