At 07:40 PM 4/20/03 -0500, Steve Narducci wrote:
>This is a homemade aluminum crank up (made by a toolmaker in a machine
>shop 15 years ago).
Do you know anything about the design? Rated load at what wind? Presuming
it was up before, what load was on it and for how long? Worst wind speed
encountered?
It looks very nice, but intuitively, I'm a little concerned about the small
section (18" at the bottom, 9" at the top). I'm no engineer -- this is just
by comparison to other towers I've seen, and they have all been
steel. Aluminum is considerably more elastic than steel, so it's
conceivable that the whippiness you're seeing is normal and expectable, a
combination of the tolerances between sections and the normal flexing of
the aluminum.
>It is 96 ft to the
>Thrust bearing when erected. I notice what looks to be about 8-12 inch
>sway at the top in about 15mph winds with nothing but the
>bearing, rotor and mast on it. I think I have come to the conclusion
>that I am going to put one set of guys on it 75% of the
>way up. Here is a link to show you the entire installation this week
>showing the tower and all.
>http://www.yorktownhouse.com/crankup.htm
If I were in your shoes, I would not guy it until I put an antenna on it
and tested it. In the guyed tower study presented on K7NV's web site,
guying even 5 feet below the top of the tower tends to put enough
additional load on the cantilevered section so that, when modeled with
finite element analysis software, the tower fails above the guys almost
every time. The other sore spot is the base, which, like yours, is rigidly
mounted in concrete.
Unless you know a lot more about the engineering of this tower than you
have told us so far, or are prepared to hire a PE to analyze it, then I
think you need to proceed very cautiously. I would find an inexpensive
tribander somewhere (maybe a TH-6 or 7), mount it on the mast, and begin
with the tower fully nested. Observe it in gusty winds. Then, if you're
comfortable, run it up another 10 feet or so and observe again. If the
only way you can rest the tower sections on metal rather than on the cable
is to run it all the way down, I would run it down between tests.
Whatever you do, don't climb the tower when it is other than fully down on
the stops. The horror stories on Towertalk are legion and scarey! since
it nests to 36 feet, you will probably have to climb it or hire a
cherry-picker to install the antennas -- I'd urge the latter, by the way.
73, Pete N4ZR
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