I don't have expertise in this area, but I know people who do.
ALL such ratings are based upon a long string of assumptions, some pretty
loosey goosey.
The drag on a nice clean cylinder, in straight line wind, with a surface
finish of known characteristics, is fairly well known and agreed.
When you group it with other members, make it much shorter than the original
specimen, add some bracing at various angles and add 'em up, you are fairly
far out on a limb.
What's the wind profile? Where are the trees, buildings, hills?
How does the environment change with azimuth? Hurricane winds can come
from ANY direction and, if the eye goes over your location, you can see up
to 180 deg of change.
Is there small scale turbulence involved, to modify the profile?
What about downdrafts? Could they make wind that's not horizontal at the
tower?
Is the drag of your antenna a function of the relative wind direction?
What were the "safety factors" used for CYA? There's no point in comparing
an analysis with a SF of two to one with a SF of 1.5, unless you know the
SFs
If the engineer published the SF, lots of people would just get comfortable
with the unfudged numbers, pre SF, and uprate their plans accordingly.
It would be neat to strain gage the legs of a free standing tower and
measure the forces actually seen at the base, while measuring the wind
profile with an anemometer that can go up and down to get a profile.
Of course you'd likely have to wait years for wind strong enough to make the
data relevant to the design max spec's.
Precision analysis/design is no better than the basic assumptions. Adding
decimal places doesn't make it better.
OK, this was a rant, because it pains me to see local government people who
know nothing, or less, about towers accepting design criteria put out by
committees of people who are paid to write codes and imposing them in
circumstances where it doesn't matter what happens. If MY tower can't fall
on my house or anyone else's property, it should not be subject to
regulation. Am I sensitive? Yes. A friend on 52 acres was just told that
a 40X40 foot shop was too much impermeable area and would disturb the
watershed! What the disturbance might be was not disclosed, but I'm pretty
sure the water is going to go into the river no matter what! Said water
will be a lot cleaner if his dirty stuff is inside, not out in the rain.
Does anyone have records of failed ham towers, pictures? The fact that
something survives says very little about the analysis by which it was
approved, except that the analysis plus SF produced a design that survived
the load experience. There is no bound on how much overdesign there may
have been.
How about tower sections tested in a wind tunnel? Full size, of course.
We had a massive failure here a few years ago, of a 1500 ft TV tower,
brought down by ICE. Not too much ice, but a dynamic overload caused by
rapid ice shedding from guywires. At least that's the theory published.
www.oldradio.com/archives/warstories/WRAL.htm
I feel better, target up.
Wilson
W4BOH
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