[TowerTalk] wind load vs Rohn specs

[Kurt Andress]

Pete Smith wrote:

> At 09:58 PM 9/2/00 EDT, Tower2sell@aol.com wrote:
> >
> >Yes they are two different codes; but the projected area values listed in
> ROHN's catalog do not include shape factors. And for the math impaired I
> will give 3 examples.
> >
> >Take the ROHN guyed 25G 70 mph towers and look up the 40, 50 and 60 foot
> towers
> >
> >40'
> >Round 18.4x1.2=22.08
> >Flat 11.0x2.0=22.0
> >
> >50'
> >Round 17.5x1.2=21
> >Flat  10.5x2=21
> >
> >60'
> >Round 16.8x1.2=20.16
> >Flat 10.1x2=20.2
> >
> >As you can tell there is a pattern that you don't need to have the CIA
> decipher for you. As a matter of fact ROHN works in EPA and backfigured the
> flat or round for simplicity.
>
> 'Scuse me, teacher, but unless you explain it better than that I don't
> think you'll convince anyone.  It looks to me as if Rohn has downrated
> appropriately for flat member antennas.  Now what were you trying to prove?
>
> 73, Pete Smith N4ZR
>

Time out! There is some semantic confusion going on in this thread.

Take one example:

>40'
>Round 18.4x1.2=22.08
>Flat 11.0x2.0=22.0
>

This is correct!
It says that it takes 18.4 SqFt of Flat Projected Area (FPA of your antenna
made from cylindrical members) to create 22.08 SqFt of Effective Projected
Area. This is where the drag coefficient is applied. We take the EPA and apply
the wind pressure (and other factors) to it to get the load that the antenna
applies to the structure.

Now, if we make our antenna out of rectangular tubing, or angle stock, it takes
11.02 SqFt of Flat projected Area to create the same (~) 22.0 SqFt of Effective
Projected area.


Tower2sell is getting queried about saying that the drag coefficient is not in
the Flat projected Areas on the Rohn drawings. This is literally correct,
because it is not "IN" that number, it is applied as shown above.

Let's ask the question a different way.

"Do the Rohn Flat Projected antenna areas account for the differences between
the drag coefficients for cylindrical and rectangular antenna members?"

The answer is YES, that is exactly why the allowable antenna areas are
different.

I don't think there is a technical arguement in the later incarnations of this
thread, just a language and reference frame problem.



Now regarding comments about the ".67 or .60 or whatever" thing.
That is just the approximate relationship between the differences in drag
coefficients between cylindrical and flat'ish things in the wind.
Fine!!!!!
It IS NOT a valid technical reason for someone to reduce the area figure for
their antenna! The .67 drag coefficient only works inside EIA-222-C, not coming
into it from the outside..

Current EIA and UBC codes have been upgraded and use different methodologies
than the OLD 222-C.

What has NEVER changed is that everyone of them (even OLD 222-C) takes the Flat
Projected Area of the thing (antenna) and then grinds it thru its own
particular form of mathematics to arrive at the load to be applied to the
tower. Not one of then that I've seen has ever asked for an antenna area to be
arbitrarily derated by some other (or obsolete) spec! They all say "Give me the
FPA, and I'll do the rest."

So, why did we ever, and why do we still tolerate antenna area figures that are
anything but the Flat Projected Area that is required to consider the tower?


--
73, Kurt, K7NV

YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/



P.S. It's been getting pretty testy around here lately. Is the heat getting to
everyone? I hope the cold wet breeze we had here up on a tower today and the
snow on the peaks reaches everyone soon.


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