I just rechecked my calculations and I found an error. I used
2 sqft/section instead of 2.9 sqft/section (I was going from
memory). According to my updated numbers this derates
things down to about 106 MPH. Perhaps there is a height
factor or something in EIA-222-F that accounts for the rest
of the difference between your calculations and mine (your
numbers seem to be about 50% higher than mine). I am
assuming a wind pressure of P = .004*v^2 (e.g. 45 psf at
106 MPH, 68 psf at 130 MPH).
BTW, I should have been more careful to emphasize that
a 40ft R25 tower with an 8 ft house bracket is not a good
installation for an area with a 130 MPH requirement. My
only point was that the tower might actually survive a 130
MPH wind storm provided that no antennas were installed,
but even that is questionable now that I have corrected the
projected area of the R25 sections in my calculations (with
the corrected projected area, my numbers show about
10,000 ft-lbs overturnng moment at 130 MPH). At best a 40ft
R25 tower bracketed at 8' with a small tribander on top is
a 60 to 70 MPH kind of installation. Perhaps even less when
you properly apply EIA-222-F.
I had a 34ft R25 tower bracketed at 8ft when I lived in
Florida. I had calculated that it wasn't good for more than
85 MPH with the antenna installed, so my plan was to pull
the antenna down if any hurricanes were headed my way.
73 de Mike,
----- Original Message -----
From: "Hank Lonberg" <firstname.lastname@example.org>
Sent: Wednesday, December 29, 2004 11:09 AM
Subject: [TowerTalk] 130mph Rohn 25G
> I am really curious as to how this works.
> When I calculate the bending moment for a 40 foot section of 25G with a
> bracket at 8' up using TIA/EIA-222-F specifications for 130 mph design
> velocity, I get a moment of 15,872 ft-lbs at the bracket. This is over
> the allowable as specified by Rohn. This for the tower section alone with
> antenna loading.
> When I use 80 mph design wind velocity I get 6,042 ft-lbs which is just
> than the 6,720 ft-lbs of allowable moment as specified by Rohn for 25G.
> Again this is without any antenna loading. This would equate to having
> 1 sf of allowable antenna capacity at the top.
> Can some one explain their calculations to me as it appears that I am
> something incorrectly!
> Lonberg Design Group, Ltd.
> Hank Lonberg, P.E.,S.E. / KR7X
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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