When they’re finished building a skyscraper, how do they get the giant crane off the roof?—Natasha Sanjabi, Regal Heights
In a word: slowly. The towering cranes busy filling our skyline with condos are trucked to construction sites in pieces and jigsawed together in a few short hours. At first, they’re bolted to a concrete pad at ground level, but as the edifice rises, the cranes are raised with it. It’s getting them down that’s the real challenge. That process requires a Russian doll approach. The main crane has to haul a smaller crane-like contraption, called a derrick, up to the rooftop, where it’s bolted to the building. The crane is then disassembled into chunks small enough for the derrick to painstakingly lower, piece by piece. The catch is, the derrick itself is a big sucker, and now it’s stuck on the roof. The solution, of course, is to bring up an even smaller derrick. The big derrick is, in turn, broken up and lowered piece by piece by the smaller derrick. And, at last, the small derrick is disassembled into pieces little enough to get down to the ground in a more civilized manner: via the elevator.