[Top] [All Lists]

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

To: <>
Subject: [Towertalk] Wind pressure calculations (2/3 factor)
From: (Michael Keane, K1MK)
Date: 27 Feb 2002 10:43:03 -0800
On Wed, 27 February 2002, "Richard Karlquist" wrote: 
> I am trying to do a simple wind 
> pressure calculation on an antenna, 
> but I am confused about how to do 
> it.  
> ....  
> Can anyone boil this down to something 
> simple? 
Part of the reason is there is no really "simple" answer is that at least two 
"correct" ways exist for answering the question: a pure "physics" answer and an 
applied "engineering" answer. Either might be appropriate depending on how one 
chooses to interpret the term "wind speed". 
The physics answer assumes that when you say 80 mph you really do mean the 
instantaneous wind speed that's measured at the same height as the object of 
interest. This is how a lot of people would interpret wind speed when they 
encounter it. In this case: 
    F =        Qs           *        a         *      A 
Force = Stagnation Pressure * Drag Coefficient * Projected Area 
Qs(v) = 0.00256 * v^2  (Qs in lb/sq. ft.; v in mph)   
"a" depends on object's shape (how "streamlined" it is); a is 1.2 for a thin 
cylinder and 2 for a flat plate
"A" is the projected (shadow) area (length x width; no pi's); 1 square foot in 
your example.  
So the instantaneous pressure on a thin cylinder would be something like  19.7 
lb/sq. ft at 80 mph.  
On the other hand, the other answer assumes that the 80 mph in your question 
actually refers to a specifically defined averaging (fastest mile) of a time 
varying wind speed having defined statistical properties that is measured at a 
height  of 10m. This is the precise meaning of wind speed as it is used in the 
building codes and the EIA spec.  
In that case there are corrections that depend on height (because of drag the 
air feels near the ground). Using the methodology given ithe UBC/EIA, for a 
wind speed of 80 mph, the peak pressure that's exherted on a thin cylinder 
located at 10m height is 24.6 lb/sq. ft.
That 2/3 factor you're recalling comes from an older (1976) version of the EIA 
spec in which several factors, such as stagnation pressure and drag for a flat 
plate were "mixed together". It gives the same answer although perhaps obscures 
some of the underlying physics. The bottom line is that if one is consistent 
and always uses that 2/3 factor along  with the set constants that it was 
intended to be used, you end up with the same sansweras the current version of 
the spec, i.e. 20.5 lb/sq ft.  

Mike K1MK

PeoplePC:  It's for people. And it's just smart. 

<Prev in Thread] Current Thread [Next in Thread>