The skylobby system is a type of elevator configuration used in supertall skyscrapers to reduce the amount of floorspace devoted to elevator shaftways.
In a conventional configuration, all floors in a building are accessible from elevators starting on the ground floor or lobby level, but in a supertall building (that typically greater than 80 floors), owing to the Elevator Conundrum the number of elevators required to adequately service the building would consume up ovaluable floor space to the point where the building would be uneconomic for its owner.
In a skylobby system, the building is divided up into two or more distinct zones, which are served typically by large express elevators which begin at the ground floor. At the lowest floor of each zone there is a skylobby, from where passengers change to smaller, local elevators to take them to the desired floor. The benefit is that the shaftways for the local elevators can be stacked on top of each other, thus saving rentable space from being used for elevators.
John Hancock Center, Chicago
The John Hancock Center in Chicago in 1969 was the first skyscraper to use the skylobby system, which used a single skylobby to serve the upper residential floors of the tower distinct from the lower office floors. The Twin Towers of the World Trade Center in New York used a dual system which had skylobbies on the 44 and 78 floors.
Some modern supertall buildings use a combination of skylobbies and destination dispatch systems to reduce the overall elevator requirement even more – examples being Burj Kalifa in Dubai, and the new One World Trade Center in New York.