There IS one technique that has been used successfully
on the east coast (DC area, I believe) that has pulled up
towers of this size (and larger) with antennas on top.
It is an old technique that is called the "falling derrick"
method and it is shown in old Navy construction manuals.
It involves a number of key elements. The main idea
is that the 90 ft tower is on the ground horizontally and
a second small tower is attached to a hinged base
and guyed to the main tower at the normal guying
points. It is hinged in such a way that as the small tower
is pulled over it pulls the main tower up. This requires a
strong anchor point located the same distance from the base
as the short tower is tall for attachment of a pulley.
It also requires a pretty decent vehicle or winch
(NOT WENCH) to pull up the assembly.
To make things stable the main tower is usually guyed
with FOUR sets of guy wires so that the "side" two are
at right angles to the direction of pull and keep the main tower
from moving sideways. A fourth set is attached to the "back"
of the tower and pre-cut to the proper length so that the
main tower cannot go past center.
The "falling derrick" guys are moved (one by one) from
the derrick (now laying on the ground) to their final ground
anchor point. (So they must be longer than needed to attach
to the falling derrick so that when they are moved to their final
(and more distant) attachment point they will reach.
Actually for a 90 ft Rohn 25 the forces are not all that
bad once you do the math. But you need a 20 to 30 ft
pole or extra tower to do the pulling.
I have heard (and seen photos) of a number of Rohn 25 and
45 towers that are regularly put up and taken down in this
way for maintenance in a short time. But the technique ALSO
requires a solid base point and hinge.
73--John W0UN
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