Tower crane arrives at the construction site on 10 to
12 tractor-trailer rigs. The crew uses a mobile tower crane to assemble the jib and the machinery section, and
places these horizontal members on a 40-foot (12-m) mast that consists of two
mast sections. The mobile tower crane
then adds thecounterweights.
The mast rises from this firm foundation.
The mast is a large, triangulated lattice structure, typically 10
feet (3.2 meters) square. The triangulated structure gives the mast the
strength to remain upright.
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To rise to its maximum height, the tower crane grows itself one
mast section at a time! The crew uses a top climber or climbing
frame that fits between the slewing unit and the top of the mast. Here's
the process:
1.
The
crew hangs a weight on the jib to balance the counterweight.
2.
The
crew detaches the slewing unit from the top of the mast. Large hydraulic
rams in the top climber push the slewing unit up 20 feet (6 m).
3.
The
crane operator uses the crane to lift another 20-foot mast section into the gap
opened by the climbing frame. Once bolted in place, the crane is 20 feet
taller!
Once the building is finished and it is
time for the tower crane to come
down, the process is reversed -- the tower
crane disassembles its own mast and then smaller cranes disassemble the
rest.
