[Towertalk] Rohn 25G rating

[Michael Keane, K1MK]

At 01:53 AM 5/22/02, Ford Peterson wrote:
>I lost a Rohn 25G (68') tiltover tower a few weeks ago to a monster storm
>that came through unexpectedly.  I need to understand the rating of the old
>tower for insurance purposes--I need to "replace" the tower.  Most
>advertising for antennas talk about Sq Ft at MPH ratings, typically 50, 70,
>90, 100MPH, etc.  The Rohn catalog talks about "20 PSF" and "30 PSF."
>
>My drawings that came with the tower talk about the following ratings:
>
>@ 20 PSF 11.7 Sq Ft
>@ 30 PSF 6.2 Sq Ft
>
>There is no reference to MPH anywhere.  How do I convert the notion of PSF
>to MPH and visa-versa?

With great, great care.

Those pressures probably represent the peak pressure on a flat surface and 
the tower was old enough that capacities are based upon EIA RS-222-C (or 
earlier); "Zone A" under RS-222-C which covered most of the country was 
nominally 30 PSF peak on a flat surface. In the context of EIA RS-222-C, 20 
PSF corresponds to a mean wind velocity of 71 mph and a peak velocity of 92 
mph and 30 PSF corresponds to a mean wind velocity of 87 mph and a peak 
velocity of 113 mph.

The methodology for computing pressure from mean velocity changed in 1986 
with EIA-222-D (and subsequent revisions). Under EIA-222-D and later, 20 
PSF peak pressure on a flat surface corresponds to a basic wind speed 
(fastest-mile) of 55 mph (at 10m height) and 30 PSF  corresponds to basic 
wind speed of 67 mph. The wind speed map was also revised in EIA-222-D so 
that locations that were characterized as RS-222-C "Zone A" may now be 
classified as having a basic speed of 70, 75, 80 or even 85 mph under 
TIA/EIA-222-F

Roughly speaking, and in terms of similar pressure on flat surfaces, the 
old EIA RS-222-C Zone A with an 87 mph average wind velocity is equivalent 
to what the present TIA/EIA-222-F calls a basic wind speed of 70 mph.

Now antennas are not flat surface, and part of the changes to the formalism 
for computing wind forces under EIA-222-D were changes to the values of the 
aerodynamic drag coefficients used when computing the forces on thin 
cylinders like antennas. The result is that a thin cylinder loading under 
TIA/EIA-222-F one must design for peak forces that roughly double the 
forces under EIA RS-222-C for the same peak stagnation (or flat surface) 
wind pressure. As a consequence, for the same structure the allowable 
antenna area (projected area) under TIA/EIA-222-F at a basic wind speed 70 
mph is roughly half what EIA RS-222-C would have allowed for Zone A.

So, bottom line, and assuming the original Rohn sq ft. ratings were 
projected areas and not something else sucha as effective area or 
equivalent square plate areas, then an equivalent tower today would be 
rated as having a capacity of approximately 3 sq. ft. projected area for a 
70 mph basic wind speed. And less area if TIA/EIA-222-F puts your county in 
a higher wind speed zone.

73,
Mike K1MK

Michael Keane, K1MK
k1mk@alum.mit.edu