"The tower crane is a modern form of balance crane. Fixed to the ground, tower cranes often give the best combination of height and lifting capacity and are used in the construction of tall buildings. To save space, the vertical part of the crane is often built in a well in the centre of the building, which is then converted to a lift (elevator) shaft when the crane is dismantled. 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 sits in a cabin at the top of the tower.

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 will sometimes be lifted to the roof of the completed tower to dismantle the tower crane afterward. A self-assembling tower crane has been demonstrated, which lifts itself off the ground using jacks, allowing the next section of the tower to be inserted at ground level."

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there you go sir, told you they could dismantle themselves.