It's a small statistical sample, but I find it interesting that both crankup
towers failed immediately after some antenna work was completed. No doubt,
while rusted cables were surely to blame in both cases, and both towers were
destined to fail anyway, it seems to me that the maintenance work itself may
have contributed to the failure.
>From the description of the work done on the 90' U.S. Tower, it sounds like
>the weight of two men was added to a tower that was already supporting a
>considerable amount of weight (I'm deducing this from the phrase "one of the
>monobanders" and the fact that an 18' mast is pretty heavy) As I recall from
>the SkyNeedle incident, the rotor had just been replaced. I wouldn't be
>surprised if this procedure at times put some additional weight on the tower
>(i.e., as the men leaned on the top of the tower or as the antenna and mast
>were raised and lowered.)
My question is: At the time the antenna work was done, was the cable supporting
the weight of the tower and antennas (and the two men)? I'm assuming that the
tower was retracted in both cases, but does that necessarily mean that the
weight was off the cables? I have a tubular crank-up, so I can't see inside to
determine whether the sections rest against stops when the tower is in the
full-down position, or whether the sections come to rest just above the safety
stops and are still supported by the cable(s). Even if the towers are supposed
to come to rest against safety stops, it's certainly possible that the towers
in question were not lowered completely. All it takes is a fraction of an inch
to put all the weight on the cables.
I think this is an important subject for a number of reasons. First, it
suggests that cables should be thoroghly inspected before and after doing any
work on the tower or antennas, regardless of whether the tower is fully
retracted or not. Second, care should be taken not to overload the vertical
weight bearing capacity of the cables, both during maintenance and normal
operation. Third, the extra expense of a tiltover fixture might be further
justified if it minimizes the risk of overloading the cables during maintenance.
Speaking of the load bearing capacity of cables, why don't crankup
manufacturers publish this spec? We are all concerned about the windload
capacity of these towers, but what about a spec for how much vertical weight
the cables and pulleys can take? After all, they support the entire weight of
the tower, rotor, mast, antennas, coax, cables, and so forth. How can we be
sure we're not overloading them?
One more point. The description says that while the two workers were routing
the coax through the arms, the owner started raising the tower. Does this mean
that the workers were on the tower as it was being raised? If so, that's a big
no-no. One should never climb any part of a crankup while it is extended or
being extended, even if it's only the bottom section. If there's a failure, the
tower will become a guillotine. If the men were on the tower, they should be
thankful that they didn't lose life or limb. They were lucky.
73, Dick, WC1M
-----Original Message-----
From: J.P. Kleinhaus . <w2xx@cloud9.net>
To: Roderick M. Fitz-Randolph . <w5hvv@aeneas.net>
Cc: towertalk@contesting.com <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Thursday, January 08, 1998 11:13 AM
Subject: Re: [TowerTalk] Tower Lubrication
>> I would tend to use stainless cable after seeing
>> what happened to a 105 foot SkyNeedle in Florida 30 seconds after I
>> descended from the platform (and the owner raised it to the top while I
>> was climbing down)
>
>The same thing happened to me a few short weeks ago. Performing
>antenna replacement of some VHF/UHF sticks on a friend's 90 foot
>US Tower. A buddy and I were up at the top of the mast (~18 ft over
>the top of the tower) for about an hour. When we climbed down, the
>owner started raising the tower and we were routing the new coax's
>through the arms when the thing made a funny noise and then came
>crashing down. It had been raised about 10-15 feet at the time.
>The new antennas were toast, as was one of the large monobanders (a
>telrex 6 el. 20).
>
>Upon doing a post mortem (and changing my underwear :-) ) we found
>that the cable snapped between the 3rd, 4th and 5th sections, causing
>the top two sections to collapse into the 3rd. The bottom of the
>3rd section is mangled from the impact as well. Luckily, the other
>cables held, and the antennas didn't drive themselves through the roof
>or ourselves! What a mess....
>
>Of course, the broken cable was rusted clear through the inner strands
>and had never been replaced (the tower was 10 years old).
>
>I thought that stainless steel was not as strong for this application
>as the galvanized type of cable. Although, if the cable doesn't rust,
>this type of accident would not happen...yes, of course, if the
>proper maintenance was done, it wouldn't happen either.
>
>Personally, I'm glad my tower is guyed, not collapsing.
>
>73, J.P. W2XX
>--
>======================================================================
>J.P. Kleinhaus, W2XX (fdba AA2DU)
>E-mail: w2xx@cloud9.net
>
>As we say in the software business: "You are hosed."
>======================================================================
>
>
>
>--
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