Ted,
Your antenna load exceeds the maximum rating for the U.S. Tower MA-550 at 70
MPH, which is 9 sq ft and 65 lbs. The KT34XA has a windload rating of 9 sq
ft and weighs 75 lbs. If the tribander has been upgraded with the M2 kit,
it's 9.75 sq ft and 76 lbs. The KLM 40M-B1 dipole has a windload rating of 2
sq ft and weighs 15 lbs. That's a total of 11 sq ft and 90 lbs (11.75 sq ft
if you have the M2 upgrade.)
Windload ratings for U.S. Tower tubulars are a bit of a mystery. They used
to rate all four models (40', 50', 72', 89') at 10 sq ft for 50 MPH winds
(no weight limit specified.) Then, when they found out antenna manufacturers
specify windload area for round members, U.S. Tower changed the windspeed
rating to 10 sq ft at 70 MPH. Evidently, they had been using square members
in their calculations. Checking their website, the specs have changed yet
again and are now lower at 70 MPH:
Mast Height Windload/Weight Windload/Weight
@70MPH @50MPH
MA-40 40 ft 6.8/65 16.5/85
MA-550 55 ft 9.0/65 22.0/100
MA-770 71 ft 5.5/75 15.5/100
MA-850 85 ft 6.3/75 15.3/75
It appears that they've done a more thorough job of calculating windload and
weight limits, but I find the much-higher ratings for the MA-550 rather
suspicious. There's very little difference between these models in terms of
engineering -- they all use the same tubes, cables, pulleys, anchor bolts,
etc., with progressively larger bases and bottom sections as they increase
in height. I would hazard a not-so-wild guess that the MA-550 is by far
their most popular model (I'll bet more than 75% of their tubular sales),
which makes the windload ratings even more suspicious.
Back when I bought my MA-770 with MARB base, I relied on the old windload
ratings. We live in a 70 MPH zone, but in the 35 years I've lived here the
winds have never gotten anywhere near that high. Highest winds I've seen
were in the 40-50 MPH range. We once had a report of microbursts up to 60
MPH in the area. Hurricanes have hit NH in the distant past, however.
Anyway, I originally mounted a TH-7 on my MA-770. It's rated at 9.4 sq ft
and 75 lbs. But when the tower was fully extended to 71', it began to sway
when the windspeed got above about 15 MPH. The swaying became downright
sickening when the windspeed got over 25 MPH, so I vowed never to have the
tower fully extended with winds over 20 MPH. The structure appears to be
much more stable when cranked down even 10 feet, and quite strong when
cranked down to 50 feet. While I felt that a collapse was very unlikely with
the sections partly nested like this, I still didn't like the way the tower
swayed in higher winds. So, I kept it fully retracted except for contests
and always pulled it down to 50 feet if the wind got above 20 MPH during a
contest.
Later, I replaced the TH-7 with a Force 12 EF420 4-el 20m monobander (6.8 sq
ft and 61 lbs) and then a 40-2CD (6.38 sq ft and 44 lbs.) Those loads have
been much more friendly and the tower does not sway much in typical winds.
But I still keep it cranked all the way down when not in use.
The key to my strategy is the remote-controlled motorized raising/lowering
feature. While expensive, it ensures that I stick to the strategy of keeping
the tower retracted. You indicate that you should have retracted the tower.
If it's a hand-crank model, I can understand why you didn't. It takes
forever and a lot of elbow grease to raise and lower a crankup. It's a
common story I've heard about crankups over the years -- people get tired of
hand cranking these things and don't do it when they should.
It's not relevant that you've had the load up on this and other crankups in
the past. The tower is not designed for the load you have. Yes, their
catalog used to say that they have a built-in safety margin (don't know if
they say that now), but personally U.S. Tower engineering practices don't
give me any confidence in that statement (a good example is the unworkable
pulley system they put on the pull-down cable.) And I don't believe the
MA-550 specs.
Further, in my opinion the MARB base reduces the capacity of the structure.
Instead of using two thrust bearings, they have a single thrust bearing at
the bottom and a simple steel ring at the top of the base. The upper ring
just keeps the mast from tipping over. It doesn't hold the bottom section
securely -- there's a lot of clearance between the inside diameter of the
ring and the bottom tube. Consequently, the tower rocks back and forth in
the wind, slamming into the ring with each oscillation. Not being an
engineer, I don't know whether this is worse for the anchor bolts than the
forces exerted through the fixed base, but it certainly doesn't look good.
Note that although there's a lot less bending moment, with a max windload
the tower still bangs against the base when fully retracted and windspeed is
above about 15 MPH. I wonder if this constant banging had anything to do
with your bolt failure.
I'm pretty shocked that an anchor bolt snapped. The common failure mode on
these towers is a cable break. I suppose it could have been a defective
piece of steel, but that seems unlikely. Is it possible one or more of the
nuts was loose? I can see where that could put excessive force on one of the
anchor bolts.
Anyway, remember the LXC directive: obey the manufacturer's recommended
specs!
73, Dick WC1M
> -----Original Message-----
> From: K6XN [mailto:k6xn@comcast.net]
> Sent: Monday, January 16, 2006 10:13 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] NEED ADVICE RE SNAPPED TOWER ANCHOR BOLT
>
>
> Thanks to everyone who responded to my request for
> recommendations as to how to repair my snapped anchor bolt
> problem. I very much appreciate all your advice and also the
> sense of urgency and the helpful spirit in which it was
> given. What a great group this is!
>
> After considering all inputs and also a number of
> recommendations as to whom to turn to for professional help
> from members of the NCCC I intend to proceed with assistance
> from Steve, K7LXC aka "Tower Tech" to have the broken anchor
> bolt welded by Steve when he is in the Bay Area later this
> month and as extra (and hopefully completely unnecessary
> insurance)I intend to also have Steve install an additional
> four secondary anchor bolts by having additional holes
> drilled in the concrete and additional secondary anchor bolts
> epoxied in.
>
> Thanks again for all the helpful advice.
>
> FYI I have installed and used four self supporting tubular
> towers from US Towers over the years and I have never had a
> problem before. In this latest situation I believe I simply
> should have had the good sense to crank the tower down before
> this last unusually severe storm hit. In two of my other
> installations the self supporting towers were also each
> supporting a KLM KT-34XA 6 element yagi on a 34 foot boom and
> a KLM 40 meter dipole...the same antenna load as I have up
> now.....and all the others never had any difficulties with
> anchor bolts. This last storm a few weeks ago was a whopper
> though in my area and there were a number of large trees
> blown down in my area during the storm.
>
> In the future I will crank it down before serious storms hit :-)
>
> Best regards, Ted, K6XN
>
>
>
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