On Thursday, 8 January 1998 at 16:37:57, Dick Green
<dick.green@valley.net> wrote:
>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tower was retracted in both cases, but does that necessarily mean that the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.
___________________________________________________________________________
Au contraire, Dick. The SkyNeedle had a platform at about the 20+ foot level
on which we were standing during the rotor chanegout. The rotor is offset
and, as the SkyNeedle is lowered, comes to rest about 3 feet above the
platform. All our weight was on the platform itself, which is permanently
attached to the tubular bottom section of the Skyneedle; therefore, no weight
was placed on the cable at that time. The SkyNeedle had been nested for
months prior to the work on the rotor. At no time was any weight placed on
the upper sections of the tower.
The problem apparently occurred due to the non-stainless cable being in that
position for a period of months (years?) and as the east coast of Florida is
wont to do, daily moisture coming off the Atlantic (bringing microscopic
amounts of salt) had permeated the atmosphere inside the tubular structure
where the pulleys and cable were located (as best my memory of the situation
recalls). This moisture collected on the cable during the morning hours while
the cable was still cool from the previous nights and the warmer salt-
containing moisture collected on the cable, dripped down the cable to the
lowest point (at the bottom of the lowest pulley) and it was at that point,
the post-mortem revealed, that the cable was rusted almost all the way through.
The cable on either side of where the cable was nearly rusted through was
still covered with galvanizing. Only a few strands of the galvanized cable
(sold to the owner of the tower as stainless!!!!) were still in place when the
tower started to be raised. The fact that those few strands were able to
resist the weight of the tower and Christmas tree of monobanders is truly
amazing, even if for the few moments it took to raise the SkyNeedle to full
height.
All of this suggests that regular maintenance and thorough examination of the
tower-raising cable be carried out on a regular basis.... especially where
the cable is subject to continuously recurring heavy moisture and salt laden
atmosphere, as is the case on the east coast.
I, personally, will not use other than stainless in any future project in
which I include a weight-bearing cable. Others may disagree but I've learned
a profound lesson: "Galvanizing is not forever!"
Rod, N5HV
w5hvv@aeneas.net
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