[TowerTalk] Mast wind loading

Bill w7vp at comcast.net
Sun Mar 26 02:38:14 EST 2006

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 

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.

Bill W7VP.
----- Original Message ----- 
From: <NPAlex at aol.com>
To: <towertalk at contesting.com>; <k6xn at 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 at 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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