At 10:55 AM 3/19/98 -0500, Pete Brunet wrote:
>I got a chance to look at a Rohn catalog. In the 45G section there is one
>windload chart that shows max wind loads of 32, 23.8, and 17 sq. ft. for 70,
>80, and 90 mph. Then later on separate charts for 70, 90, and 110 mph, for a
>70' tower they show wind load figures of 15.9, 15.2, and 9.2 "allowable proj.
>area (sq. ft.). Is "proj. area" on these later three charts the same as an
>"antenna area" on the first chart. If so, why the apparent discrepancy? We
>are considering 20.1 sq. ft. of antenna (see next paragraph).
I don't know the two different sets of charts you are referring to,
but I can comment on the "proj. area".
In the newer Rohn book, in the drawings, each tower configuration
(wind speed and tower height), there are two loads shown. On is
in a square box, the other figure is right next to it with a circular
"box".
If you read the notes below, they define the boxes.
Square box is allowed projected area for flat member antennas.
Circular box is allowed projected area for circular member antennas.
A common misconception is to use the circular numbers since ham
antennas (beams) are round membered antennas (and the circular numbers
allow a larger wind load). BUT, you have to read the words before
circular member. It says PROJECTED AREA.
Projected area is defined as the (sums of the) diameter of the element
times the length. A 2" x 15' mast (for example) has a projected area
of 2" times 10' (or 2/12 feet * 10 feet) = 1.67 sq.ft.
The actual wind load for that same mast is only 67% of that (for round
membered antennas), or 0.67 * 1.67 = 1.11 sq.ft.
If you notice, the square box figure is roughly 67% of the circular
box figure.
Antenna manufacturers do not give their antenna areas in "projected area",
but rather actual wind loading. Maybe Tom, or any other antenna
manufacturer, can back me up on that. After all, if you're looking to
buy a Force 12 Magnum 3/3Z, you don't want the shock of seeing a projected
area of 37.3 sq.ft. Much more palatable is 25 sq.ft. wind loading.
Projected area is NOT wind load for circular membered antennas.
So when you're looking to put something on a Rohn tower, you have to
use the square box "projected area for flat membered antennas" figure
because that is the WIND LOAD that the tower is rated for. That
figure is apples and apples with antenna manufacturers "antenna wind
load" figure.
>With a 15' chrome-moly mast with a TH11 (wind load 12.5) at the bottom and a
>F12 Magnum 340 (wind load 7.6) at the top does the torque associated with the
>force of the top antenna on the 15' mast need to be factored in? I did
not see
>any mention of this in the Rohn catalog.
I think that the loading specified is directly at the top of the tower,
not above it. I'm not saying you can't do it (who doesn't?), but when
you put something above the tower, you not only have to consider
wind loading, but torque on the mast, as well as the tower, as you
eluded to.
As an oversimplification, torque on the tower is a function of how much
tower extends above the last guy set. (There is also torque below
the guy set, but I won't get into that.) The specification for allowed
torque is on the mechanical drawing page. On that page is a chart
of dimensions, characteristics and strengths.
If you followed me to this point, you must really be interested. Let
me conclude that, if you were to perform the calculations yourself,
you would be amazed at how much of a safety factor that Rohn uses.
You wouldn't believe at how much more windloading you can put on
these towers as compared to what the book says.
Hope this helped a little....
---
Chad Kurszewski, WE9V e-mail: Chad_Kurszewski@csg.mot.com
The Official "Sultans of Shwing" Web Site: http://www.QTH.com/sos
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