A double-deck elevator or double-deck lift is an elevator with two cabs attached together, one on top of the other. This allows passengers on two consecutive floors to be able to use the elevator simultaneously, significantly increasing the passenger capacity of an elevator shaft. Such a scheme can improve efficiency in buildings where the volume of traffic would normally have a single elevator stopping at every floor. For example, a passenger may board the lower deck (which serves only odd-numbered floors) on the concourse level while another passenger may board the upper deck (which serves even-numbered floors) on the ground floor. The cab serving even floors is actually on top of the cab serving odd floors in the same elevator shaft. When a passenger disembarks from the even-floor serving cab at level 30, for instance, the passengers in the odd-floor serving cab beneath it (level 29) are kept waiting until the elevator doors above close.
Architecturally, this is important, as double-deck elevators occupy less building core space than traditional single-deck elevators do for the same level of traffic. In skyscrapers, this allows for much more efficient use of space, as the floor area required by elevators tends to be quite significant. The other main technique is shared-shaft elevators, where multiple elevators use different sections of the same shaft to serve different floors, with skylobbies separating the sections. The Thyssenkrupp TWIN elevator places 2 independent elevator cabs in 1 shaft.
Not all double-deck elevators are used to transport passengers simultaneously in both decks. Sometimes one or more elevators in a building has a double-deck car, where the second deck is used for transportation of goods, typically outside of peak traffic periods. This technique has the advantages of preventing damage to interior fixtures due to impact from trolleys, and does not require a dedicated shaft solely devoted to a goods-only elevator car. During peak periods, the car is switched back to passenger mode, where it can expedite passenger movement into or out of the building.
As of 2011, no triple-deck elevators have been built, although such a design had been considered for the 163-floor Burj Khalifa before the final design was scaled back to double-deck. Also, Frank Lloyd Wright had envisioned five-deck elevators in his 1956 proposed Mile High Illinois.