[TowerTalk] wind load vs Rohn specs

[Stan or Patricia Griffiths]

Pete Brunet/Austin/IBM wrote:

> >Pete, You are in the wrong code. There is no .67  To get the ROHN assumed
> EPA value you multiply their are by 1.2 for rounds and 2.0 for flats.
>
> According to the Force 12 brochure they start by knowing their antenna's
> projected area.  The generate their windload number by multiplying their
> projected area by .67 and they publish that number in their brochure.  In
> order to know what projected area they started with I would have to
> multiply their windload by 1.5.  Make sense?
>
> If I know their projected area, can I assume that since it is a round
> member antenna I can use the round member spec?
>
> The antenna is 30' of Rohn 25.  My zone is rated 70 mph.  The shortest
> tower I see in their manual is 40' so I will use those numbers.
>
> Concerning the coax, let's forget about it and say that our configuration
> will be close to the Rohn configuration.  Besides the antenna, what other
> top of tower items will a PE want to know about, e.g. mast and rotator?
>
> Thanks, Pete ws4g
>

Hi Pete,

My personal opinion is that you are correct in your
interpretation of Rohn's
loading specifications.

I found it strange that "Towers2sell" would say, " ROHN has
listed the actual
projected areas without shape factor" when you can plainly
see in any
relatively new Rohn catalog that Rohn lists TWO load
figures: one for flat
members and one for round members.  I would say Rohn HAS
listed load areas
with shape factors.

I further calculated the ratio of "round to flat" load areas
for many of the
Rohn guyed tower examples given and got a number very close
to 0.6 every
time.  So it appears that while the number "0.67" does not
appear to be the
Rohn choice, "0.60" does appear to be the Rohn choice.  This
apparently
compares to "0.67" which Pete says is Force 12's choice.  It
is really too bad
that the tower makers and the antenna makers can't seem to
settle on the same
number . . .  Frankly, I don't know enough about what the
current EIA specs
say to really know which one is more correct (today).

The guys that scare me are the ones who INCREASE their tower
load specs by
using the "round" load numbers given by Rohn and then use
the  DECREASED
antenna area numbers given by the antenna manufacturer. 
When you do that, you
have overloaded your tower so severely that it does not
matter whether the
number used was 0.67 or 0.60 . . .

Stan
w7ni@teleport.com



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