[TowerTalk] Guying a self-supporting tower - Yes

[Jim Lux]

----- Original Message -----
From: "David Robbins K1TTT" <k1ttt at arrl.net>
To: "'towertalk'" <towertalk at contesting.com>
Sent: Sunday, April 10, 2005 10:35 AM
Subject: Re: [TowerTalk] Guying a self-supporting tower - Yes


> > > 2) A more fundamental question would be is there any configuration
(i.e.
> > > combination of stiffness and strength) where the guyed column would
fail
> > and
> > > the canteliever beam would not, with the same lateral load.  This
would
> > > answer the "it can't possibly happen question", since all you need is
> > one
> > > example where a plausible structure will fail with guys but not
without.
> >
> > 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.
>
> Or take a piece of balsa or other light wood, put guys to the top of it
and
> tension to just about the breaking point, then give the wood a bit of a
push
> in the middle and it will snap... extend the wood above the guys a bit and
> give it a push up there and it will pivot around the guy point and break
> below the guys the same way.
>
> There are lots of ways to apply guys to something that can't handle the
> extra down force that will make it buckle and collapse.
>


That's the general idea.. but you need to consider the real question is not
static loads, but what happens when you put a side load at the top (and all
along the length). In the unguyed case, the cardboard tube or balsa just
bends (until it fails..)  In the guyed case, the side load turns into an
axial load (and a push in the middle, as well).

Presuming that commercial antennas aren't built with a huge design margin
(steel and labor and shipping are expensive, afterall), the real question is
how close to the breaking point of the tower are you with the design loads.

I think a better simple model might be a fiberglass fishing pole.  Very
strong in bending, but not real stiff.  the question would be whether a
steel (or aluminum) tower is more like a cardboard tube (stiff, but not too
strong) or a fishing pole (strong, but not too stiff).