The worst-case structural loads on one of these arrangements can occur when the
tower is being lowered. You can get what engineers call a "suddenly-applied
load" which can be as much as twice the static load associated with merely the
weight and distribution of the system components.
Buddy of mine had a setup like that, 70' aluminum self supporting tower with
one tribander, and no appreciable extending mast. His lowering technique was
to attach the winch cable and a rope at a point 2/3, or so, up the tower,
remove the third bolt in the base (permitting tilt pivot on the other two),
back off a slight bit of slack in the winch cable, and send his assistant (me)
out to pull on the rope to get it started tilting.
Well, one day he provided a bit excessive slack in the winch cable and when I
pulled on the rope, I observed the tower didn't immediately stop and I quickly
dove for cover out of the line of fire. When I looked up, the tower was
arrested at an angle of 50-60 degrees from the horizontal, and was oscillating
about the winch cable connection point, with an amplitude of a couple of feet
in the middle of the unsupported length, and at the top :
Boing-oing-oing-oing. Big-time loads on the system, but fortunately it all
held together with no damage. Buddy was LHFAO.
----- Original Message -----
From: Chuck Gerarden<mailto:email@example.com>
Sent: Tuesday, October 16, 2007 7:12 AM
Subject: [TowerTalk] falling derrick for Universal Tower raising
I recently bought and installed a 70' aluminum Universal tower to hold a
XM-240 40 meter beam. If you look at the installation drawings you see a
man walking up the tower. My contractor laughed pretty good at that
illustration, as did I. I decided to use the falling derrick method of
raising the tower. I bought a Rohn 25G tiltover base plate and 3 10'
sections of 25G for the derrick. I mounted the plate on the same
platform as the tower. I plan to use the winch on my 700lb ATV to pull
the derrick down and the tower up.
I have not done any calculations, as it is easier to ask you experts and
civil engineers on this reflector. If nobody has the answers I will
crack open the right books and figure it out. It looks like rather
simple math based on weight of the tower, beam weight, rotor weight,
mast, overhang past attachment point. Here are the facts.
70 foot alum. Universal tower HD21-70 322lbs
30' rohn 25 as derrick.
XM-240 beam 55lbs - mounted just above tower tube, little extension
beyond tower top
6' alum mast 5lbs
DX1000 Yaesu rotor 10 lbs, mounted 3 feet down in top section
steel line from 30' point on derrick to 40' point on Alum tower.
I am open to SWAG's as well as calculated answers. If I missed any
critical data, let me know. I know that calculating the sole tower
weight beyond the 40' foot attachment is not known, but taking 1/3 of
the tower weight as an estimate is close enough. based in the size of
the sections below this point, probably conservative.
any ideas from you geniuses out there?
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