Anecdote follows:
Frank Clement, W6KPC (SK), who owned Tri-Ex for many years, told me
in personal conversation that their crankups were designed so that if
a catastrophic failure were to occur with the tower extended, it
would be in the top section and not the bottom (biggest)
section. His logic, paraphrasing here, was that "you can deal with a
bent, or folded, top section (bring in a crane, cut things loose and
take it down safely), BUT if you had a failure in the bottom section,
you likely would have a REAL disaster on your hands (destroyed your
roof, or your neighbor's roof-- or car-- or worse, personal injury)."
I always thought Frank was a smart guy.
Glenn K6NA
6,000 hours on towers...
and yes TIA standards have changed, so most crankups designed years
ago have been derated -- or redesigned-- today.
At 05:29 PM 2/8/2013, you wrote:
It's an interesting question about the strength of the top section
in a crank up, so I looked at the UST calcs for the HDX589, 85mph 3
sec gust, 71mph fastest mile (EIA-222-F). I'm not a PE so this not a
definitive look and mostly an educated guesstimate.
A number of factors limit the crankup tower capacity - lift cable
strength, leg compressive strength, section max bending moment, etc.
There are five sections in an HDX589, UST numbers 9 (base) to 5 (top).
cable safety factor: top section 14.3 next to bottom section 2.7
(max antenna + rotator + coax = 277lbs)
leg compressive strength safety factor: top section 7.8; bottom
section 1.58 (max antenna 12.1 sq ft)
outside overlap strength: top 11; bottom 5 (max antenna 74 sq ft)
overlap web strength: top 13; bottom 1.5 (max antenna 12.3 sq ft)
foundation moment safety: 1.4 (although the calculation appears to
be very conservative).
When multiple loading factors are calculated I used the worst case
ratio safety factor. The lowest safety factor governs the maximum load.
This tower is rated 13.7 sq ft round members at 1 ft above the top
plate for the specified EIA conditions.
So, it appears that the limiting factors are in order: the
foundation (1.4), then the bottom section web strength in the
overlap (1.5) and the bottom section leg compressive strength
(1.58). Since the top section safety factors are so large, it seems
unlikely that it would fail first in an overload situation. i.e
it's unlikely the rotator and mast will rip out first.
Since I'm out on a limb, I thought I keep sawing re a conventional
wisdom re towers. It has been said "freestanding towers are
designed to fail somewhere near the middle". Logic seems to
contradict that since unguyed towers are made at different heights
and capacities out of a family of identical sections. UST uses the
same series of sections for all HDX crankup heights and loads.
Trylon does the same for the fixed T200/300/400 etc series of
tapered free standing. So it would take some magic to make a
section #6 too strong at the top and too weak in the middle and then
too strong at as the base section of a tower. The calcs show the
bottom section is the most likely to fail for an HDX589 and I think
that is most likely for many free standing towers. There are
probably many contradictions given the uncertainties of winds and
maintenance history etc etc.
However, falling towers and other objects often fail (buckle) at a
distance up their height AS THEY FALL. This leads the observer of
the failed (or those observing the fall) structure to conclude the
failure started at that point. google "falling chimney problem" or
go to http://myweb.lmu.edu/gvarieschi/chimney/chimney.html
Grant KZ1W
On 2/8/2013 3:23 PM, kr2q@optimum.net wrote:
Gosh...just go to the US Tower site.
Seehttp://www.ustower.com/#!__product-pages/ham-towers
and click on the DOWNLOAD FAQ
Here are some excerpts from their FAQ
QUESTION : Why do I need a thrust bearing?
ANSWER : The thrust bearing is designed to support the weight of
your antenna(s) and mast
off of your rotor and extending its life.
Translation: As Ward said, "you don't need one with the
OR2800...save your money"
QUESTION : How tall of a mast is recommend for a tower?
ANSWER : This is dependent on the tower. TMM SERIES: up to a 15'
mast. MA SERIES: up to a 15' mast.
TX SERIES: up to a 20' mast. HDX SERIES: up to a 20' mast.
Translation: "We say 20 feet, but we don't say how much wind load
- so YMMV. IE, BE CAREFUL."
I'm no engineer, but one calculation I would be concerned about is
not the bending moment at the base,
but rather at the top of the tower (via the sleeve) and down to the
rotor plate. Yes, it is giant fulcrum
and you could possibly rip the hell out of the top 4 feet of the
top section...if the wind is high enough.
If you keep the tower cranked down in a big wind, well, now you
have all those other sections helping
to "support" the inner section (and the winds are usually less at
lower levels above ground). YMMV
Finally, in the real world, thrust bearings can serve two purposes.
1. As used on a ham tower, they redistribute the vertical weight
at the level of the bearing
2. More traditionally, the are used to hold "an axle." Think of
the old time printing presses:
big, heavy steel drums spinning horizontally at high speed.
Way back when, when I designed a rotating tower (in the late 1970's),
See
https://picasaweb.google.com/dougzzz/K2GLFullHistory#5246737104987133522
https://picasaweb.google.com/dougzzz/K2GLFullHistory#5246736666452767074
BTW, that is 10 over 10 on 10m (Telrex)
I didn't use a collar that wrapped around the tower; I used a big,
fat beacon mount (Rohn) and
put a bearing on that (inverted). The bearing had four places to
anchor it - I used those as the tie points
for the 4 guy wires. When I went to Bobker Bearing to buy one (I
was clueless), I asked "how much
weight can it hold." It was in the multi-ton range. They were
surprised that I asked that question.
They asked me, "No, you want about how many rpms." They said the
one I was looking at was good
for 3000 rpm. When they asked me, "How many RPM," we all laughed
when I said ONE!
de Doug KR2Q
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