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[TowerTalk] Wind Loading Question

To: <towertalk@contesting.com>
Subject: [TowerTalk] Wind Loading Question
From: Tower2sell@aol.com (Tower2sell@aol.com)
Date: Fri, 15 Oct 1999 21:34:51 EDT
                                      EPA DEFINED!

The last answer was only partially correct (75% passing) on projected area, 
et al. So here is my "sort of correct way" of finding "EPA". I mean "sort of" 
because there are a number of variations and  interpretations that can be 
used to arrive at slightly different answers. Calculating EPA is an art form 
and is subject to one's own style and taste. The end result should be within 
10% to be called "close enough."

Let's define some terms:

PA - Projected Area - length x width. 

FPA - Flat Plate Area: converts round areas to flat areas by multiplying  PA 
times .67

EPA - Effective projected area: The projected area (PA) times the shape 
factor. (PA x Ca). The term Ca varies with the shape (round, flat, other) and 
the length to width ratio. 

Ca - shape factor of appurtenance. for rounds, varies between .8 and 1.2  for 
L/W = 7 to 25 respectively. For flats, varies from 1.4 to 2.0 for L/W = 7 to 
25 respectively.

Where did the confusion on EPA, FPA and PA come from -- The EIA-222 and its 
various revisions. For Rev C the world used FPA and when Rev D came out the 
procedures were changed to EPA. Many antenna manufacturers have still not 
updated their catalogs. I suspect that they do not like the larger EPA 
numbers or they have limited manpower. 

There are a lot of antenna manufacturers out there and many of them use 
different values and don't say which method they are using.  The EIA-222-F 
(current version) say that if the antenna is made up of rounds you can 
conservatively multiply FPA by 1.8 to get EPA. One of the best methods that 
some antenna manufacturers publish is the thrust value (T) and the wind speed 
(V). To get EPA = T/(.00256 x V x V) (I can't do the squared thing in plain 
text)

Now the problem gets worse, because almost none of the antenna manufacturers 
publish information on their antennas with "ICE" and ice controls the design 
of guyed towers (but it is "optional" whether or not to design for ice and 
how much ice). The majority of commercial towers are designed for 1/2" radial 
ice with 75% of the basic wind speed wind pressure.

So the trick is to figure out which is the value given is PA, FPA, or EPA. 
This is mainly done by looking at the antenna and determining the length and 
width of each antenna element and find its Ca x PA. The total of all elements 
is the EPA. For ice you can repeat the process by adding 1" (for 1/2" radial 
ice) to all lengths & widths. Or for a quick approximation if the antenna is 
made up of many different diameters, take the (EPA/1.2) and divided it by the 
total length of all the round elements. This will give the average diameter. 

Then EPA(1/2") = (D+1") x (L +1") x 1.2

Life can never be simple can it.

Tower2sell@aol.com

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