Time laspe of the construction crane being erected.
Crane (machine). (2008, April 17). In Wikipedia, The Free Encyclopedia
The tower crane is a modern form of balance crane. Fixed to the ground (or “jacked up” and supported by the structure as the structure is being built), tower cranes often give the best combination of height and lifting capacity and are used in the construction of tall buildings. To save space and to provide stability the vertical part of the crane is often braced onto the completed structure which is normally the concrete lift shaft in the center of the building. A horizontal boom is balanced asymmetrically across the top of the tower. Its short arm carries a counterweight of concrete blocks, and its long arm carries the lifting gear. The crane operator either sits in a cabin at the top of the tower or controls the crane by radio remote control from the ground, usually standing near the load. In the first case the operator’s cabin is located at the top of the tower just below the horizontal boom. The boom is mounted on a slewing bearing and is rotated by means of a slewing motor. The lifting hook is operated by a system of sheaves.
A tower crane is usually assembled by a telescopic crane of smaller lifting capacity but greater height and in the case of tower cranes that have risen while constructing very tall skyscrapers, a smaller crane (or derrick) will sometimes be lifted to the roof of the completed tower to dismantle the tower crane afterwards.