Hi all,
I was recently given a 150 foot Rohn 25 tower. It was a old surplus Loran
transmitter tower. The tower is actually the transmit antenna and sets on a
big Lapp insulator. The lower set of guys were Phyllistran (old potted
style) and the upper guys were EHS steel connected to the tower and part of
the antenna. The last 21 feet of the upper guys were Phyllistran. The
whole tower formed a large vertical with the four of the guys serving a top
loading. There were five sets of guys and nine other guy wires off the top
servings as additional top hat loading.
When I got there, I inspected the guy anchors and everything looked fine.
One problem noted was that all of the tower sections were tack welded
together. That was a bit of a issue as I would have to come back with a
generator and surface grinder to grind off the tack welds. I suspect the
tack welded legs were probably a effort to improve the conductivity of the
vertical.
I started up the tower inspecting sections as I climbed. Some light surface
rust was present but everything looked fine. The lower set of guys also
looked fine. The fifth tower section had some suspicious rust. I took the
hook on the pulley and stated poking around and pretty soon had a hole in
the tower leg big enough to put my thumb into. There were other areas in
that section that also failed when poked at with the hook on the pulley.
Either time for a crane or time to let her fall. See picture at:
http://www.ab5k.net/images/LoranTowerLeg.jpg
The contact point who gave me the tower came up a bit later. I told him I
would do a "controlled fall". He was just happy that the tower would soon
be gone and did not really care. So now I had to determine what a
controlled fall was it there was such a thing. The tower was in a fenced
area and if it fell flat, it would take out the fence which was not
desirable. I decided to take off the second set of guys and pull the tower
over at that point. My reasoning was that the pull point would be close to
the rusted section which would be a failure point. The tower would bend at
that point and the rest would accordion under the influence of the guys.
Besides safety, the two goals were try and save the Lapp insulator and keep
the tower with in the fenced area. The second set of guys were removed and
one of the guys was extended and ran over to the truck where it was attached
to a come-a-long. After a lot of pulling there was a significant dog-leg in
the tower, but it was still up. I was surprised by the amount of side force
it took to bring it down. The tower broke up in four sections the longest
being the top section. Most of the sections are ruined but the Lap
insulator was saved. A few of the sections can be cut off and be used a
concrete base sections. The Phyllistran survived as well as the turnbuckles
and equalizer plates.
Bob, NX5M, took video of the tower coming down. The DVD was converted to
MPG and can be downloaded from either of the two sites listed below. Thanks
to Matt, W5LL and Jim, K5LAD for hosting the file.
Jim - K5LAD:
http://www.hayseed.net/~jpk5lad/download/Rohn25fall.mpg (77.9 meg)
or a slightly smaller zipped version
http://www.hayseed.net/~jpk5lad/download/Rohn25fall.zip (71.7 meg)
Matt, W5LL:
http://w5ll.com/mpg/Rohn25Fall.mpg
I'm sure there will be comments especially after seeing the video. It was a
really interesting project. By the way, the bad rusted section did not
buckle. If I had it to do over again I would probably drop the first and
second set of guys.
Thoughts?
73 Terry - AB5K
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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