[TowerTalk] Tower windload with trees blocking

[Hank Lonberg]

     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 Lonberg, KR7X
     -  E-mail:  hlonberg@europa.com or hank.lonberg@harrisgrp.com 
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______________________________ Reply Separator
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Subject: [TowerTalk] Tower windload with trees blocking
Author:  "mcduffie@scottsbluff.net" [SMTP:mcduffie@scottsbluff.net] at
CORPORATE
Date:    4/12/99 5:10 AM


I'm curious about the good or bad effect of uneven wind load on a tower 
and its wind rating.  When I read a wind load rating, I assume that the 
manufacturer assumes the load will be applied nearly equally from top to 
bottom.  Now, what about reality?  What is the effect of protecting the 
bottom 3/4 of the tower with wind breaks, etc?  What happens when most 
of the load is applied to "only" the yagi and the top of the (HG-54HD) 
tower?
     
Gary - AGON
mcduffie@scottsbluff.net
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