Chad Kurszewski WE9V
Thu, 06 Feb 1997 09:28:46 -0600
> If this whole capacity calculation thing bothers you, why don't you just
>put up a bunch of 55G and quit trying to outguess Rohn specifications and
>other practical advice you have received from TowerTalk?
Are we getting a little short here, Steve??
It is not my intention to "outguess" Rohn, but to do several, iterative
calculations myself. Rohn tower applications as drawn/described in many
pages, are over engineered for safety. How much overengineered is one
of the questions I would like to figure out, along with WHAT is the
limiting factor? Moment? Compression? Guy strength? Anchor holding?
(Of course the real failure mode for Rohn/ham installations is TORQUE,
but can be combated with torque compensated antennas and tower torque
bars. Which then leads us back to the windload issue.)
Also, if you would perform some of the calculations yourself, you would
see that some examples are much, much more overengineered as compared to
the next example 10' taller, but list almost the same windloading capability.
The reason being that the top guypoint didn't change, and that the
moment increased by THREE times. (I am referring to to the examples
that have the top guy point at 5' below the top versus 15' below the top.)
In other words, you could put three times the square footage of antenna
on top if you keep the guy point close to the top. But then, if you'd
know how to figure this stuff out for youself, you would be able to
figure out that the moment isn't the biggest concern now, but now your
attention needs to turn to vertical compression of the tower and the
reactive force of the guy. Maybe that was this limiting factor the whole
time. But who here can tell me this?
Also, you could well exceed the published windload by placing any
sidemounted antennas near other guy points. Again, this is not a
moment issue, but one of compression of the tower and tension
on the guys.
To quote Howard, WZ6Z, in a recent post, regarding the
antennas on his tower: "I've varied this alot, ..."
And is one to hire a PE every time he changes an antenna?? Wouldn't it
be a lot better if you could figure it out for yourself? This is the reason
I do not want to get a PE involved at this point. I would like to try
on a few antenna configurations for size, see if they work, and then
when I change my mind, and experiment with antennas like a normal ham,
I will be able to figure out for myself if it is within the capabilities
of the tower, guys, and guy anchors.
And Steve, n0yvy, says:
>If you want professional advice, i.e. no handwaving...
No, I do not want professional advice at this stage. I want to correspond
with someone here on this reflector, that would know how to do this.
I thought hams like to do things for themself. Experiment. Did Hiram
hire someone else to do his work? Marconi? If you cannot do the
calculations, that's fine. I would like to speak TO ANOTHER HAM, who
does know how. Because I would like to learn how to do it myself. And
then I can pass the knowledge on to someone else as eager as I.
It appears to me that no one here, at least who have spoken up, knows how
to do this (Jay, you're excluded...thanks).
And Steve, what practical advice of tower talkians are you
referring to, with the exception of seeking a PE? Are you refering to
the guys who say, "I have a Rohn 25 packed to the hilt and it's been up
7 years, no problem." Is this the blind leading the blind? All I'm
asking is if there is anyone here who can do the calculations.
And the approach of putting up Rohn 55 just to play it safe because we
can't do the calculations and figure out that Rohn 25 will work fine is
silly. Yes, it's safe, but potentially overkill. But you'll never know
if you don't figure it out. Maybe that's why the government has $5000
hammers and $20,000 toilets.
>I'm waving and holding the TIA-222-F Tower Structure Standards.
Okay, does that doccument show the calculations??
Chad Kurszewski, WE9V e-mail: Chad_Kurszewski@csg.mot.com
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