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[TowerTalk] Wind Ratings

To: <towertalk@contesting.com>
Subject: [TowerTalk] Wind Ratings
From: "Wilson Lamb" <infomet@embarqmail.com>
Reply-to: Wilson Lamb <infomet@embarqmail.com>
Date: Sun, 11 Jun 2017 15:34:56 -0400
List-post: <mailto:towertalk@contesting.com>

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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