On Sun, 11 Feb 1996, Stan Griffiths wrote:
> One topic that seems to be missing from all the tower discussions I see here
> lately is the fact that it is not simply the number of square feet of
> antenna that loads the tower, but how those square feet are distributed on
> the tower as well.
>
> Everything I see published from Rohn shows the ENTIRE antenna load
> concentrated right at the top of the tower in one place. Certainly,
> mounting antennas on a mast that sticks above the tower loads it
> differently, but how differently? I have two towers with antennas above the
> top by about 10 feet. How did I determine it was safe? I didn't! I just
> guessed and I've been lucky for 25 years. I must have made a good guess.
> There has to be a better way . . .
All of these discussions on tower/antenna loading have pretty much
assumed a steady-state constant max wind velocity and max windload
from which you can compare resulting stresses with the manufacturers' specs.
Certainly you need to do this and be in compliance with such limits.
But that's only half of the story.
Go out and watch your stuff in a windstorm. If it's anything like around
here, in a good storm you are gettting hammered by big gusts every few
seconds from directions which can change maybe 30-50 degrees in direction
with each gust. And the timing between gusts can vary. All of this can set
up twisting, stretching,and bending dynamic oscillations in the
tower/mast/guy system. Don't forget slop in the rotor. All this sort of
behavior is virtually impossible to model particularly in the general form
it would require to produce the kind of computer program being discussed
in this thread.
If all these perturbations manage to combine themselves
to locally overload something briefly, you can
kiss your big signal *goodbye*.
It can happen at wind speeds well below the peak wind speed for which you
thought you designed your system. My guess is these effects account for many
of the lost systems due to windstorms.
A classic case of this is the first Tacoma Narrows bridge which was most
certainly designed to withstand a bumper-to-bumper traffic jam of
fully-loaded 18-wheelers in both directions in combination with *static
loads* from some peak windstorm. Yet it failed when lightly loaded in a
rather moderate garden-variety windstorm. For those who have never seen
the video of what came to be known as "Galloping Gertie," the wind set up
a swaying/twisting action in the main span that increased in amplitude
until the whole works deposited itself in Puget Sound.
If you have a big stack, and particularly with some side-mounted
goodies with their interesting torsional loads applied to the tower, with
two or three sloppy rotors banging the structure intermittently,
you may have inadvertently created the ham-radio equivalent of the above.
Good luck...
Big Don
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