when a skyscraper is done being built, how does the crane come down?
In pieces, section by section, and then the base is taken out by Helicopter. Or at least that is how I would do it.
Most probably you are referring to the Tower Crane. Interesting questions should have followed, like: How is it constructed while the construction is in progress? How is the placement of the Tower Crane determined? What happens to the space occupied by the Tower Craneafter work completion? How far should the 'boom' (the arm that has a moving pulley) go? Anyway, your question is a good one, because when I had my first construction work, the Tower Crane was already there and I was thinking of how to dismantle it afterwards. The dismantling process, is done part by part, set by set starting from the top going down. Another interesting question is: How would that excavating truck or bulldozer be able to get out of that deep matt excavation after all excavation works have been accomplished? It is interesting to note that an earth ramp is created while excavation progress, until the machine is down deep in the excavation and seems that it is impossible for it to get out after excavation work is done?
Skyscraper cranes (for construction) are usually dismantled into small sections and pieces after final stage of construction is done. Some of the smaller pieces are transported down by construction evaluators, while bigger pieces are lifted down by series of pulleys or cranes mounted at the side of the buildings edge.
