Dick
Thank you for your very thoughtful, detailed and inspirational analysis and
information.
You guessed it...my tower is a hand crank model and at the age of almost 63
it has become an effort for me to crank it up and down (it was a lot easier
a few years ago :-) and your inputs and reasoning have motivated me to
seriously look into buying and installing the remote-controlled motorized
raising/lowering feature. I have also checked all the leveling nuts using
feeler gauges and as you surmised the leveling nut on the anchor bolt
diagonally opposite the snapped anchor bolt has a .135 inch gap which
supports the theory that the combination of an unusually high wind surge
plus a large antenna load plus a loose leveling nut all combined to put
excessive stress on the snapped anchor bolt.
The good news is that my XYL, Laine, K6XXN, has given me the OK to buying
and installing (or having installed) a remote-controlled motorized
raising/lowering feature from US Tower. I have learned over the years that
it is always important to get the XYL's *prior* concurrence before embarking
on a project that may cost a lot to implement....and the remote-controlled
motorized raising/lowering feature is expensive as you noted....apparently
ranging between approximately $1371 for the MD-75 Motor Drive to $2087 for
the MDP-750 not including sales tax, shipping charges, installation costs,
electrical contractor costs etc. Since I need to take everything down to
have the remote-controlled motorized raising/lowering feature installed it
would seem to be the perfect time to also replace all four anchor bolts and
all of the antennas at the same time. As a ball park figure I am probably
looking at a cumulative project cost of up to $5000 to do everything which
seems a lot...but not all that much when contemplating what the damage
repair costs could have been had the tower and antennas fallen through the
roof of our house...or into the power lines behind us. In any event I
foresee some serious tower and antenna improvements here at K6XN over the
next six months.
As an initial first step I plan to proceed with K7LXC's help next week to
weld the snapped anchor bolt and I will keep the tower cranked down to
approximately 35 feet except for contests and DX occurring during good
weather until we can install the remote-controlled motorized
raising/lowering feature. The second step....I am contacting US Tower today
to find out what is needed to convert (or replace) my hand crank MA550 to a
tubular tower having a remote-controlled motorized raising/lowering feature
later this year..probably late spring or early summer. The third step will
be to re-evaluate the antennas I am using (both of which perform very well)
to see if I can replace them if possible with equally performing newer
antennas having lower wind loads. Adding the remote-controlled motorized
raising/lowering feature will be expensive...but a lot less than the
potential damage which could result if I don't add it. MY XYL was out in the
storm with me watching my back and the tower with a flashlight, raingear and
a cellphone (to call 911 if necessary) as I slowly lowered the tower by hand
between wind gusts in the storm after the anchor bolt snapped and it was
pretty exciting. We actually moved our Siberian Husky to a different part of
the house so he wouldn't be hurt if the tower collapsed during the storm.
After this experience my XYL, Laine, seriously appreciates the merits of
adding a remote-controlled motorized raising/lowering feature :-) Its great
being loved by an XYL who also appreciates and sees the value in supporting
her ham radio husband and his hobby...(not to mention keeping him alive)!
Thanks again for the benefit of your insight and analysis!
73, Ted, K6XN
PS My XYL Laine is K6XXN, my daughter Ashley is N6MTJ and my son Andrew is
N6YDZ.
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Dick Green WC1M
Sent: Tuesday, January 17, 2006 10:07 AM
To: 'K6XN'; towertalk@contesting.com
Subject: Re: [TowerTalk] NEED ADVICE RE SNAPPED TOWER ANCHOR BOLT
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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