Hello to all,
A very hardy thanks to all that replied to my question.
I have summarized some of the messages and included below.
I hope this information will be as useful to others as is has been to me.
Rich AA2MF
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If you can get an answer from them, try the manufacturer. Years ago a Triex
engineer told me at what point above the tower the allowed wind load went to
zero (can't remember his name now).
He said to do a linear derating from the top of the tower to zero. Sounds
reasonable to me.
73 Don K5AQ
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Jim Lux
IF you assume identical mast and identical connection to the top of the
tower.
AND If you assume that the wind speed is the same at both places (not
necessarily valid.. average wind speed goes up with height, which is
factored into "real" wind loading calculations)
If the actual force due to wind remains the same, the bending force at the
mast/tower attachment will be 15 times greater, because the same force is
acting on a lever arm that is 15 times longer. Superficially, then, the
limit would be 1 square foot (based on keeping the bending load the same).
However, the mast's equivalent cross section will start to be a significant
fraction of the total cross section (a 2" pipe 12 feet long has a 2 square
foot cross section)
The real question, in a practical situation, will be what's the strength
limiting component? Is it the mast strength in flexure? Is it the load on
the bearings? In this case how much of the mast sticks into the tower
becomes quite important, although... if the moment is 15 times greater, the
loads are 15 times greater, assuming the geometry doesn't change.
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The first thing you need to know is your county windspeed rating. You can
look it up at <A HREF="http://www.championradio.com";>www.championradio.com
</A> under Tech Notes.
I offer a little program called MARC - the Mast, Antenna, and Rotator
Calculator. you can get it at the aforementioned website for ten bucks.
Steve K7LXC
TOWER TECH
CHAMPION RADIO PRODUCTS
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