The tower crane is a self-erecting crane also commonly known as a Kangaroo crane. The name Kangaroo crane comes from Australia where they were developed for use in the construction of modern high rise buildings. Modern, tall buildings would not be possible without the use of these cranes. The World Trade Center’s twin towers, Taipei 101, and the Burj Dubai all used tower cranes for their construction. (Petroski, Henry. Tower Cranes, American Scientist, November 2008)
A crane that can lift itself seems an impossible concept; however, this is exactly how a tower crane works. These cranes virtually lift themselves up so a new section of its mast can be inserted. This allows them to grow taller as the height of the building increases.
Many people wonder how they are able to lift themselves. The answer is surprisingly simple. They have hydraulic jacks under the cabin towards the top of the mast which are able to raise the crane so a new section of its mast can be inserted and fixed in place. Once the new section is ready the jacks can then lower the crane down onto the new section. Thus the crane can grow with the building. This can be a very dangerous job for the workers who are bolting the new mast section in place. (Lyons, Richard D. A Skyline Stage for New York Cranes, The New York Times, December 7, 1986)
The tower crane has two jibs, the horizontal jib and the counter jib. The horizontal jib is the longer one, and it lifts the building materials while the counter jib is shorter and has a counter weight attached to it to balance the crane so it will not tip over when it is lifting heavy objects. The other main part is the operator’s cabin, which is usually at the top of the mast.
These cranes can be fixed to a concrete base next to the building being constructed, attached to the side of the structure, or placed on the top level of the building as it grows. When the building is finished, it can disassemble itself. In the case of a very high building it is necessary to lift a small crane to the roof of the new structure. This is then used to lower the larger crane to the ground. This small crane can then be dismantled and taken down in the elevators.
The characteristic T shape of the tower crane is now a common sight on the skylines of many modern developing cities around the world. Without them we would not have the tallest buildings in the world.
