[TowerTalk] QUESTION RE FIGURING ADDITIONAL WINDLOAD DUE TO MAST

[ersmar at comcast.net]

Ted:

     Your method of calculation would give you the outer bound for the area of the mast.   That is, it's the worst case number, as if your mast were actually a flat plate 2 inches wide and ten feet long.

     In reality there is some reduction in this effective area figure because the wind blows over a cylindrical surface, resulting in the wind force acting on the mast at shallower and shallower angles as it blows around the sides of the tubing.  How much this reduces the worst case figure of 1.6 sqft would be a guess on my part.  (A few years ago we had a heated discussion of antenna tubing shape factor on this reflector and I don't intend to restart it now, but I might have already.)

     Also, remember that the wind force or antenna size limitation on any tower is based on that force being applied at a single point on the tower/mast, usually within a couple of feet of the top.  If you mount antennas more than a foot or two above the tower top, i.e., stack 'em, you have to further derate this maximum wind force or allowable antenna area due to the longer moment arm against which this wind force pushes.   

73 de
Gene Smar  AD3F


 -------------- Original message ----------------------
From: "K6XN" <k6xn at comcast.net>
> Team
> 
> When using the allowable antenna area charts for Rohn 25G towers, after we
> calculate the wind area for the antennas to go on the tower how does one
> allow for the additional surface area of the mast section above the Rohn 25G
> that the antennas will be mounted on?
> 
> 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.  :-).
> 
> Thanks, Ted, K6XN 
> 
> Ps FYI I am putting up an additional Rohn 25G tower to handle a pair of
> vertically stacked 5 element long boom M2 horizontally polarized yagis for 6
> meters spaced approximately 13 feet apart.
> 
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