[TowerTalk] Wind loading comment misdirected? W7VP

NPAlex at aol.com NPAlex at aol.com
Sun Mar 26 17:44:17 EST 2006

I think there is some confusion as the question asked about calculating the 
effect of a mast was sent by k6xn and a copy follows.

I responded to him suggesting that he could us the common value for drag 
coefficient on round surfaces of 2/3.  I know the elements in calculating the drag 
factor is more complicated, but 0.67 is often used.  I also pointed out the 
mast within the tower contributes to wind loading and must be considered.

I think reading your response to me sounds like it should have been directed 
to K6XN giving him even more insight into the calculations.
Norman W4QN
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 
>> 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 
>> 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.  :-).
W7VP email-to W4QN
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 
make the antenna much smaller than the limit stated in the catalog, in my case 
with a 45G tower.
> 73
> Bill W7VP.
I don't think I said a thing that contridicted your comments and Rohn's 
requirements.  I believe it is IO (intuitively Obvious) that a Flat surface 
drag/profile factor would be larger then a round surface.

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