Perhaps I'm missing something here. You guys ought to know all this
better than me, a tower newcomer. The rough answer is that the rated
load at 70 mph is 50% of the rated load at 50 mph. Wind loading is
proportional to the square of the wind speed.
With this approximation, rated load at 80 mph is about 40% of 50 mph spec,
and rated load at 90 mph is 30% of the 50 mph spec.
This isn't exactly right because the tower loading ought to be computed
with a wind profile as specified in the applicable UBC (Uniform Building
Code) docs, but is reasonably close. If you read the UBC docs you'll
also find the spec is for flat open ground. Hills, trees, and structures
all work in our favor at reducing wind load on the tower - not that
you'd like to have your tower hidden at the bottom of the hill in the trees,
but it'll stay up better :-)
If you want to get the right numbers, ask the tower manufacturer for
the engineering docs and figure it out yourself for whatever wind
speed you like. I have a US Towers 89' crank-up and have been through
the engineering docs - I think the rating is actually better than
they advertise.
Nick KD4CPL
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Nick England nick@cs.unc.edu KD4CPL Chapel Hill NC
http://www.cs.unc.edu/~nick
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