eet, remained the world's tallest and largest buildings until they were surpassed by the Sears Tower in 1974.
             The twin towers were the tallest buildings when they were completed in 1972. The design, created by architect Minoru Yamasaki, was innovative in several ways, including its elevator system, and its structural system. These innovations would be widely adopted in later skyscrapers. A conventional elevator system would have taken up half the space of the lower floors. A novel system employing express and local elevators was developed by Otis Elevators. The express elevators took people to "sky lobbies" on the 44th and 78th floors, where they could board local elevators. This system halved the number of elevator shafts that would have been required in a conventional system, and resulted in shorter average transit times for tenants.
             A March 2001 article in Elevator World describes contemporary work on the elevator system of the Twin Towers by Ace Elevator as "one of the largest, most sophisticated elevator modernization programs in the industry's history." (Drive to the Top, Elevator World, March 2001) 1 and 2 World Trade Center used the so-called tube within a tube structure, in which closely-spaced external columns from the building's perimeter walls and dense bundle of columns form its core. Tall buildings have to resist primarily two kinds of forces: lateral loading (horizontal force) due mainly to the wind, and gravity loading (downward force) due to the building's weight. The tube within the tube design uses a specially reinforced perimeter wall to resist all lateral loading and some of the gravity loading, and a heavily reinforced central core to resist the bulk of the gravity loading.
             The tube within the tube architecture was relatively new at the time the Twin Towers were built, but has since been widely employed in the design of new skyscrapers. In fact most of the world&ap...