Stan wrote:
> Second, the tower itself has windload. Just for discussion purposes,
let's
>assume the tower equals 20 square feet of load by itself. I have never
seen
>a crankup tower manufacturer ever mention this point.
I don't think this is correct. I believe that the tower cross section is
included in the standard engineering calculations to determine maximum
antenna wind load. I'm no PE, but it looks that way in the engineering specs
for my U.S. Tower MA-770. A lot of data about the tube sections, including
O.D., wall thickness, I.D., area, and so forth are included in the
calculations, so I think the antenna wind rating includes the tower
windload. The engineer calculates the force of the wind on every part of the
tower, then analyzes the resulting bending moment at representative heights.
Also, I don't think you can use a simple number for the windload presented
by the tower. It's more complicated than that. While the windload of the
entire tower might affect the bending moment applied to the concrete base
and anchor bolts, remember that at any given point on the tower, only the
windload above it matters. For example, the bending moment applied to the
second section from the top is not affected by the windload of the bottom
section. I'm not sure, but I think the typical failure mode is at one of the
sections, not the base. If one of the upper sections happens to be the
weakest link, it will fail from the bending moment of the windload of itself
and everything above it. In this case, the windload of lower sections is
irrelevant. That's probably why the manufacturer's don't specify it: it's
not a simple number.
>You get the idea. This is precisely WHY your tower was rated at 50 mph.
At
>70 mph, I would expect it to fold with nothing on it . . .
I don't think it's this bad, but Stan is generally correct. The effect of
derating is very severe. For example, while my tower is rated at 10 sq. ft.
at 50 MPH, the rating drops to 1.6 sq. ft. at 70 MPH. Actually, the engineer
does some calculations that show an approximate maximum of 1.85 sq. ft. Then
he decides to lower it to 1.6 sq. ft. He does a bunch of calculations with
an antenna weighing 50 lbs at one foot above the tower with a windload of
1.6 sq. ft. His calculations show that such a windload at 70 MPH will not
exceed the stress limits of each section. I think there's a pretty healthy
margin built into the computations, but they are so complex, and so many
abbreviations are used, that I can't deteremine just how much. But I think
2.0 - 2.5 sq. ft. of windload is in the ballpark. In any event, I don't
think the tower will fail at 70 MPH with nothing on it.
>Get the PE that someone else suggested. Don't you find it strange that
>derating information of this type is not published for crankups? Certainly
>you are not the first guy who asked this very question. The reason is
>simple. Publishing that data would "unsell" a LOT of crankups . . .
I absolutely agree, and the amateur community really ought to be putting
pressure on these manufacturers to practice "truth in advertising". From the
stories I've heard on this reflector about crankups with big stacked HF
yagis on long masts, I'd say there are a bunch of accidents out there
waiting to happen. No doubt, the innocent owners are relying on specs like
"30 sq ft at 50 MPH".
There is no question in my mind that my 70' tower, with its 9.5 sq. ft. of
TH7, could not survive a blow of over 50 MPH. It sways enough at 15-20 MPH
that I'm not even comfortable leaving it fully extended when the wind is
blowing that hard (OK, I'm a chicken.) It seems to be much sturdier when
cranked down to 50 feet (maybe proving Stan's point that the tower cross
section is important.) I'd love to have the calculations for just how
sturdy, but my guess is that it could take 70 MPH. Still, to be
conservative, I'll lower it below 50 ft. if the wind gets above 25 MPH.
When it's down at 40 ft., I'm comfortable up to about 35 MPH. Although I'm
sure it could take anything short of a hurricane at 30 ft., I still crank it
all the way down if gusts are predicted to go higher than 35 MPH. Why take
the chance? Actually, it's amazing how well a TH7 will perform at 25 ft. I
suppose if it was CQWW CW, and the wind was gusting to 40 MPH, I'd crank it
up to 30 ft. If A5 was on just for the day, maybe I'd sneak it up to 40 ft.
If that didn't get me through, I'd probably go up to 50 ft., but I'd be
sweating (after all, A5 has only been on once in my 15 years as a ham --
I'll take some risks to nab it.) Maybe I'm being too conservative, but I
don't want to lose that tower.
If weather conditions are good and calm, I generally leave the tower up at
50 ft. It can be up there for days or even weeks at a time. But I check the
weather reports throughout the day and keep an eye on the wind speed gauge
(installed just for the tower.) If the wind is predicted to go higher than
15 MPH, I'll lower it some more before leaving the house. I suppose I'm
still vulnerable to a freak unpredicted big gust when I'm out of the house,
but I'm pretty confident in the tower's ability to withstand a lot of wind
at 50 ft. However, I never leave the tower fully extended unless I'm present
and operating. Actually, it's up that high only for rare DX and big
contests. I always crank it all the way down if I'm gone overnight.
In case you're wondering, I work at home and the tower is motorized. Given
the derating problem, the only practical solution is a tower that can be
raised and lowered quickly and easily. If I had to run out and do it by hand
all the time, I'd get lazy and leave it up all the time (with the inevitable
result that it would come crashing down in a big wind someday.) Yes, the
cables have to be replaced more frequently, but that's the price we pay for
the convenience of a crankup. Last week, when the great ice storm was
bringing down hundreds of towers to the north and south of us here in
western NH, I was glad that I could crank my tower all the way down. The TH7
might have bent, but the tower would have been safe.
73, Dick, WC1M
P.S. Yes, I'm thinking about a circuit to automatically lower the tower when
the wind speed gets above a certain level (others have built these
successfully.) But since the tower is 250 feet away behind a screen of
trees, I have to come up with a counter circuit to tell me how high the
tower is so I'll know if the automatic circuit lowered it (when I lower it
now, I compute the height by how long the motor is on.)
--
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