The elevator conundrum is a term used by architects and building design engineers to describe the paradox that whilst elevators enable tall buildings, they also limit the practical height of tall buildings owing to the need to provide more elevators as the occupancy of the building increases proportionately with the number of floors in a building.
More elevators in a building therefore require more area to be devoted to shaftways instead of revenue producing, rentable (or sellable) useable floor area. Overcoming the elevator conundrum has given rise to several innovative solutions to the problem.
Following the construction of the Empire State Building in New York in the 1930s (which has 85 usable floors), it was generally considered that around 80 floors was the maximum practical height of a skyscraper. The first solution to the problem of going higher was the skylobby system in the Twin Towers of the original World Trade Center, which divded the building into three zones, whereby local elevator shafts were stacked on top of each other, with high speed express cars from the ground floor lobby linking the three zones together, whereby passengers transferred from express to local cars in skylobbies one third, and two thirds of the way up the tower.
Modern destination dispatch systems attempt to overcome the problem by using a smaller number elevators more efficiently by directing passengers going to the same floor in one car, by making them select their destination before entering an elevator. This therefore reduces the number of unneccessary stops, and allows cars to run to full capacity more often. It is thought that destination control elevator systems can increase the capacity by between 30-50% over a traditional, collective call elevator system.