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[Towertalk] Wind pressure calculations (2/3 factor)

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Subject: [Towertalk] Wind pressure calculations (2/3 factor)
From: (Richard Karlquist)
Date: Wed, 27 Feb 2002 08:32:32 -0800
I am trying to do a simple wind
pressure calculation on an antenna,
but I am confused about how to do
it.  I see conflicting information
on towertalk archives and various
ham vendor sites.  Let me see if
I can get a consensus on a very
basic example.

If I have a 1 inch diameter round
tube 12 feet long in an 80 MPH wind,
what is the wind pressure.

The projected area is 1 square foot
(ie the shadow the tubing would cast
on a plane surface)

The total frontal area is pi/2 times this
or 1.59 square feet

Some references say to multiply by 2/3
to get "effective area" whatever that
means.  Other references talk about 1.5,
which is the reciprocal of 2/3 and close
to pi/2.

Anyway, some sort of area needs to be
multiplied by some fudge factor and then needs to
be multiplied by wind stagnation pressure.
Am I correct in assuming this is 30 lbs
per square foot for 80 MPH?  I am thinking
this is an ideal value in free space
(ie pushed by an airplane traveling at
80 MPH).

I see references to a "UBC basic wind speed
of 80 MPH" where 80 MPH doesn't mean 80 MPH,
rather it refers to a table of pressure vs
height.  I see at 70 feet this is something
like 24 lbs/square foot.  This seems to imply
a ground effect.

I'm not trying to apply for a building permit,
I'm just trying to see if an antenna will
survive 80 MPH winds.

Can anyone boil this down to something

Rick Karlquist    N6RK 

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