I have taken down a 40 ft tower with legs on 14 ft centers. It is the
bottom 40 ft of a 100 ft tower that had a failure and crashed down. I took
it down, transported it to my ranch and reassembled it, raised it and
lowered it for modification. I am familiar with a "tipping point"
phenomenon. I used an A frame/gin pole in the falling derrick approach to
do my repetitive lower/raise evolutions. Because of the width of the base
(triangular tower with legs on 14 ft centers) you have to pull the tower
toward its lay down position to get the center of gravity past the two
hinged legs before the tower tries to fall and takes tension in the winch
line that passes over the A frame and on to the top of the tower.
Similarly, when raising the tower by tensioning the winch line passing over
the A-frame to the top of the tower, as the CG passes the "tipping point"
(CG passes an imaginary line running between the two hinges ) the tower will
try to fall forward to its final position. A "tag" line is used to hold the
tower from falling the last few feet into place. l attached the tag line to
my tractor and drover the tractor slowly forward to lower the tower into its
fully erect upright position.
With towers that have a narrow enough inter-leg dimension this "inflection"
in the cable tension is likely unnoticed. So long as there is a line or guy
or... to keep the tower from continuing past vertical, narrow towers should
not present much of a problem. With my 14 ft interleg spacing, ignoring the
phenomenon could lead to danger and damage.
I just this afternoon finished installing the last of 42 each 2x2x 1/8 angle
iron -braces on the tower mentioned above. Next to build a deck on the top
and a staircase leading to the top. I won't be climbing this tower or the
free standing tilt-over tower I will be installing on top of it. I will
walk up the stairs (with hand rails) and stand on the deck/observation
platform on top to raise/lower the crank-up tilt-over antenna atop the 40 ft
base tower.
73
Patrick NJ5G
-----Original Message-----
From: TexasRF--- via TowerTalk
Sent: Saturday, October 18, 2014 11:11 AM
To: richard@karlquist.com ; hanslg@aol.com ; towertalk@contesting.com
Subject: Re: [TowerTalk] Fwd: Guyed + self supporting /2 ??
Hi Rick, I have used the falling derrick system on towers up to 60ft. I am
not familiar with the tipping point phenomenon so you might elaborate on
that a bit.
73,
Gerald K5GW
In a message dated 10/18/2014 9:05:28 A.M. Pacific Daylight Time,
richard@karlquist.com writes:
An additional consideration in tilting up a
tower in one piece is how much guying is needed.
While Rohn 25 can be tilted up, it typically
requires multiple levels of guying just to be
tilted up. This is without any consideration
of wind. This in turn rules out a simple gin
pole or crane and requires a falling derrick, which
pulls on both the upper and lower guys. A
lighter weight tower with greater strength
such as an aluminum self supporting type can
be tilted with a simple gin pole using one
lift point. Additional levels of guying for
wind purposes can still be used after erection
is complete. Falling derricks have their place,
but are definitely more complicated, and are
harder to control because of the tipping point
phenomenon. You essentially need dueling winches
to get through the tipping point. If anyone
has solved this problem, please let us know.
Rick N6RK
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