TT: Note Mike's msg, below, and my comments below that. N2EA:
Date: Tue, 28 Dec 2004 20:26:38 -0800
From: "Michael Tope" <W4EF@dellroy.com>
Subject: Re: [TowerTalk] Rohn 25g ennginnering spec. 130 mph
Actually a 40ft R25 tower bracketed at 8' would probably
handle 130 MPH with no antennas installed. When I ran
the calcs for my 34ft R25 tower bracketed at 8', it came
out to 150 MPH wind survival (6700 ft-lbs overturning
moment at house bracket) with no antennas installed.
With an Cushcraft A3 tribander on top, it derated down
to ~85 MPH. Nevertheless, if James wants to maintain the
130 MPH rating and actually put an antenna on the tower,
he's either going to need guy wires or a stronger tower.
73 de Mike, W4EF..............................................
It's one thing to do those calculations, Mike, it's quite another
thing to certify wind survival ratings with your engineering license
at peril. Or your insurance premiums at risk, as Rohn had.
This is why you can hear of Rohn installations which have survived
serious storms, and are obviously loaded and installed way beyond Rohn's
catalog specs. They had a built in safety factor in the specs.
I had rohn 45G up for several years in VT, house bracketed at 6' and 18'.
5 sections plus 10' of mast, with an 3 el lpda @ 55', and an r7000 above
Had 2" of radial ice and 75 mph winds more than once. Never budged.
Used a custom house bracket, which was much more robust than Rohn's.
This was in the don't ask, don't tell department. If it came down,
I doubt my insurance would have paid a claim on it.
I do not advocate anyone doing what I did. But my load calculations were
like Mike's...and the bracket system had 2x safety @ 100mph. The
tower isn't manufacturer-rated for the way I loaded it.
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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