[TowerTalk] Windload at 90 mph (was: Plumbing a tower)

[W7CE]

Thanks for the info.  I don't know as much about this as I'd like, but I'm 
learning.  From your comments I assume that 20-26 lbs would apply to round 
tower legs (Rohn 65G here) and typical yagi booms and elements.  Since I've 
been using 36 lbs for my rough calculations, I should be sufficiently 
over-designed :)  I live in a 90 mph area and don't want any mistakes.

Would a 2" diameter tower leg have more or less drag than a 1/2" element?  I 
remember reading some info on this and seem to recall that the smaller 
elements actually have more drag than the larger elements.

BTW, I'm sorry about the double post.  I realized that I sent the original 
question using the wrong email account, so I resent it this morning from the 
correct account figuring the other one would bounce.

73,
Clay  W7CE

----- Original Message ----- 
From: "Chris Wendling" <cpwendling at yahoo.com>
To: <towertalk at contesting.com>
Sent: Wednesday, January 23, 2008 7:09 AM
Subject: Re: [TowerTalk] Windload at 90 mph (was: Plumbing a tower)


> Jim,
>
>  Yes, I figured you used a Cd of 1.0, which is a reasonable starting 
> place.
>  Nice to know there is another "aero" guy on the towertalk list.
>
>  Below is a link to some very good information on communication 
> structures and wind loads under various conditions of icing, etc.   I 
> think you'll find it very interesting.
>
>  73,
>  CW-AI4MI
>
>
> 
> <http://books.google.com/books?id=m-Uhn_O7O08C&pg=PA144&lpg=PA144&dq=coefficient+of+drag+for+wire+cable&source=web&ots=em_0WoRPLK&sig=ZeJ8s2lWzdEklgFL3Xu1JnQ2Af8#PPA133,M1>
>
> Jim Lux <jimlux at earthlink.net> wrote:  Chris Wendling wrote:
>> Clay,
>>
>>     Yes, you're in the ballpark.
>>
>>     The formula for drag is 1/2* 
>> rho*velocity-squared*frontal-area*coefficient-of-drag.
>>
>>     I calculate about 26 lbs force for 90 mph for 1 sq-ft frontal 
>> projection area.
>>
>>     This assumes a coefficient of drag of 1.3 - typical for tubing or 
>> wire profiles.
>>     Also, the density of the atmosphere was assumed to be at STP 
>> (standard temp and pressure at sea level.)
>>
>>  The largest error contributor to these calculations is most oftem the 
>> estimate of the drag coefficient- Surface roughness, Reynolds number, 
>> interconnections on the tower, etc., may yeild different results.
>
>
> Indeed.. I just used 1.0 for Cd for rough and ready order of magnitude.
> I was thinking in terms of a tubular free standing tower (as opposed to,
> say, a lattice HDBX style)    1.3 might be a reasonable number of a
> smallish tube in that sort of wind, it would be near the peak in the
> Cd/Reynolds number curve.  Somewhere around I have a spreadsheet that
> figures this stuff out, with handy data taken from Blevins, Fluid
> Dynamics Handbook.
>
>
>
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