[Towertalk] Rohn 25G rating

[Michael Keane, K1MK]

At 06:39 AM 5/22/02, you wrote:

>I belive the figures given assume sea level.

Sea level at a nominal temperature and relative humidity.

>If you are at a high altitude, you can
>derate the conversion from Wind Velocity to Impact Pressure due
>to the air being less dense.  As I recall from my Colorado days,
>this is about 15% at 5000' evevation.

The density of the air is down by 15% (3% per 1000'), but the dynamic 
pressure goes like rho*v^2

>So, a wind of 100 MPH
>there is equivalent to ~85 MPH at sea level in terms of the
>pressure it generates.

A wind speed of 92 mph @ 5000' is the pressure equivalent of 85 mph at 0'.

>Those EIA ratings are
>meaningless anyway...at least they were for the front range of
>Colorado where we routinely got 100 MPH and occasional 140+ MPH
>Chinook winds...the EIA chart showed Zone A (lowest winds)!

The old EIA Zone A required a design to withstand a 113 mph peak gust. And 
wind zones are based on a 50 year mean recurrence, so there was always a 2% 
chance to exceed the peak in any given year and a 1 in 5 chance to exceed 
it over 10 years.

Nowadays, I believe EIA/TIA-222-F classifies most of that area as having an 
80 mph or 85 mph basic wind speed. This and other changes require that 
current designs be substantially stronger than did the previous Zone A 
requirements; so maybe experience does teach a lesson.

73,
Mike K1MK

Michael Keane, K1MK
k1mk@alum.mit.edu