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[TowerTalk] Tower windload with trees blocking

To: <towertalk@contesting.com>
Subject: [TowerTalk] Tower windload with trees blocking
From: km1h@juno.com (km1h@juno.com)
Date: Tue, 13 Apr 1999 10:27:38 -0400


On Mon, 12 Apr 1999 09:03:00 +0100 Hank Lonberg
<Hank.Lonberg@Harrisgrp.com> writes:
>
>     Gary:
>     
>     Actually the wind pressure varies from the ground up due to the 
>     "boundary layer effect" of the fluid (air) and the friction of 
>the 
>     ground surface. The building code wind pressure design factors 
>account 
>     for this and the higher you go the larger the wind pressure for 
>the 
>     same design wind speed. In most cases the largest individual area 
>the 
>     wind sees is the antenna and this is typically at the top of the 
>
>     tower. The tower effective area is distributed over its 
>height(length) 
>     and resulting force is at the mid-height.
>     
>     As you can deduce the largest point load is at the top; the 
>antenna. I 
>     would be very reluctant to eliminate the wind on the tower itself 
>due 
>     to "tree line wind breaks" as your local building official may 
>not see 
>     it the same as you.
>     
>     The manufacturers typically rate their towers for the published 
>wind 
>     loading area at the top to the tower and included the uniformly 
>     distributed load of the wind on the tower also.
>     
>     I would not worry about the uneven loading of the wind it is 
>included 
>     in the assumptions of the actual design process. I would worry 
>more 
>     about the assumption that the trees block the wind.
>     
>     73
>     
>     Hank Lonberg P.E.

Hank, I'm a bit confused with that last paragraph.

As a for instance I have 70-80' trees at my QTH. Just about anytime I
climb the 180' tower I can start with almost dead calm at ground level
all the way up to the treetops. About 20' above the trees the wind starts
to become noticable and at 180' it is a definite factor. So much in fact
that I have to add a few pounds of weight to a drop line.
Once the line hits the treeline it goes straight down with the upper part
in full sail effect.

Tnx for any assistance.

Carl  KM1H


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