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analysis of multi-sided, tapered, steel

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I would tend to agree with the responses that you got suggesting that you use 
the more conservative drag coefficient.  However, if you want to be 
*precise*, you can draw a graph with the drag coefficient as the ordinate and 
the number of sides as the abscissa (with "round" being an asymptote) and 
see where the 18 side drag coefficient would probably be.

With wind calculations, it must always be remembered that they are highly 
approximate.  Channelization and shielding are generally ignored except for 
wind tunnel tests --- and those values are good only for the test setup.  If 
someone builds another building, tears down one, or trees become highly 
mature, the results of the tests go out the window.  I guess what I am saying 
is not to worry about being precise, but worry about being accurate.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Brian Vinson wrote:

. > I work for Valmont Industries and we have been recently been faced with 
. > the question of how to analyze 18-sided tapered poles. We have been 
. > designing 8,12, and 16 sided poles using recommendations in the EIA/TIA 
. > code for drag/force coefficients. The code does not however give a value 
. > for 18 sided poles.  Values for the different pole types are as follows.
. > 
. > 8 sided = 1.20
. > 12 sided = 1.03 ===> 16.5% lower than 8 sided
. > 16 sided = 0.72 ===> 43% lower than 12 sided
. > 18 sided = ?  =====> Does anyone have a recommendation of what to use or
. > where I can find some supporting documentation?
. > Round = 0.59 =====> only 22% lower than 16 sided
. > 
. > It seems to me that I could make some kind of correlation between the # of
. > sides for each type of pole and the force coefficients.  After all, the 
. > difference is not that much between 16 sided and round anyway. Would 
. > anyone say it to be unethical for me to use 0.59 for the 18 sided pole?
. > 
. > Brian Vinson, EIT
. > 

P.S.  Unethical, no;  Inappropriate IMO, yes.