I have a spreadsheet here for the 34' 25G tower that I had
in Florida. My tower held a CC A3 (4.7 sqft) and was bracketed
at around 8'. If I use this model and change the windload to 2.5
sqft (per the proposed installation specs), the overturning moment
reaches Rohn's maximum (6700 ft*lb) with at a windspeed of around
100mph. The model assumes that the wind load is given in a flat plate
equivalent projected area (Cd = 1) and assumes the wind speed/pressure
relationship given in Leeson (P=.004*v^2).
Seems like a pretty conservative installation.
Mike, W4EF...........
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From: Bill Coleman AA4LR[SMTP:aa4lr@radio.org]
Sent: Monday, July 26, 1999 12:55 PM
To: wa4dou@excite.com; towertalk@contesting.com
Subject: Re: [TowerTalk] Housebracketed Rohn 25G
On 7/25/99 9:05, wa4dou@excite.com at wa4dou@excite.com wrote:
> Am building a housebracketed Rohn 25G for my 6 meter yagi.
>It will only be 31 ft. high. The yagi will be a couple feet higher. My
>bracket is at 10-11 ft. off the ground. Windloading will be about 2.5 sq.
>ft. I'm guessing it should be safe at 80-90 mph.
I'm no mechanical engineer, but memory serves that Rohn 25 should be
practically self-supporting up to 40 feet (with no antenna). 2.5 square
feet isn't a really large antenna.
> Anyone noticed that Rohn never specifies these towers below 40 feet, and
>they always call for 2 housebrackets. The tower above the top bracket is
>either 10 ft.higher, 14 ft. higher, 24 ft. higher or 34 ft. higher, as the
>tower approaches 100 ft.
I've scrutinized these drawings myself. You'll notice really quick how
the allowable antenna area diminishes with the height of tower above the
bracket.
> Am i correct in my assessment that below 40 ft. one must draw conclusions
>based on inferred info found in the data mentioned above and in the
>freestanding data and interpolation of same?
Rohn always shows two brackets, but the brackets are always more than 10
feet apart. Given the rigidity of one section of tower, I doubt that a
second bracket in a 11 foot span is going to do much. So one bracket is
probably all that's needed.
The main question is how safe is the 31 foot installation with 2.5 sq
feet of antenna? Steve's already given you the wind speeds for your area.
I'd approach it like this. Think of this short-bracketed installation as
something between a free-standing 31 foot tower and a free-standing 20
foot tower (minus the 11 feet below the bracket).
I don't have a catalog, but it seems like Rohn 25 ought to be able to
hold 2.5 sq feet of antenna in a free-standing 20 foot height at an
appreciable wind speed. Your installation would be somewhat less than
that.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
-- Archibald Asparagus, VeggieTales
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