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
To: <towertalk@contesting.com>
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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