Norm
Replying to your post is difficult. There may be others on this forum who
can reply more effectively than I can but let me pass on some thoughts.
First of all the Rohn guyed tower specification indicates different
allowable projected areas for round member antennas than it does for flat
plate antennas, depending on the height of the tower. The specification
provides details of the assumptions associated with the tower and the
antenna. Insofar as the antenna is concerned note 4 states the assumption
that the antenna and mounting hardware are place symmetrically at the apex
of the tower.
If you go above the apex of the tower with a vertical mast carrying the
antenna you not only add to the effective projected area, but you can assume
the wind loading figures will not be accurate. There will be a center of
pressure associated with the wind loading of such a design that will include
a moment at the top of the tower that is not present when the antenna is
mounted at the apex., where the wind loading presents only a horizontal load
on the top of the tower with very little moment. The implications of this
could be considerable, particularly when considering the resistance to
buckling presented by a tower structure with a vertical load and a moment
at the top. The inherent stability of any vertical column is greatly
reduced when the vertical loading is compounded by a moment at the top. At
the very least I would be very wary of using the antenna area allowables in
the Rohn catalog for such a design and would expect the allowable to be
significantly less. How much less would be a subject for a structural
engineer.
As a practical matter many hams do this, including me. But the choice is to
make the antenna much smaller than the limit stated in the catalog, in my
case with a 45G tower.
73
Bill W7VP.
----- Original Message -----
From: <NPAlex@aol.com>
To: <towertalk@contesting.com>; <k6xn@comcast.net>
Sent: Saturday, March 25, 2006 7:33 PM
Subject: [TowerTalk] Mast wind loading
> In a message dated 3/25/2006 7:16:21 PM Eastern Standard Time,
> towertalk-request@contesting.com writes:
> If for example the mast section extending above the Rohn 25G tower to be
> used is 2 inches in diameter and 10 feet long what is the value we
> should
> use for calculating the additional wind load "area" due to the 10 foot
> mast?
> Is it simply 2 times 10 times 12 for 240 square inches for 1.6 additional
> square feet or ??? Moving forward would the total effective wind load for
> the ensemble to be used in checking the Rohn 25G "Allowable Antenna Area"
> then be the wind load for the antennas plus the wind load for the mast?
>
> This is probably obvious to everyone but I wanted to double check before I
> put it up an additional tower. :-).
> ---------------------------------------
> You can apply the reduced drag factor for round surface of 2/3 in your
> calculations. Don't forget the pipe below the top adds area as well.
>
> Regards,
> Norm W4QN
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