deep. The piles were installed in sets of 5 to 20 and range from deep. Each set of pilings was then covered by a concrete cap. The building's structural loads are carried by an internal superstructure that includes about of steel. The columns of the superstructure are arranged in a 10×3 grid. The ten north–south bays are all wide, but the three west–east bays are all of different width. The westernmost bay is wide; the central bay is wide; and the easternmost bay is wide. The narrow central bay was used as an elevator core. The floors are generally made of mesh and reinforced concrete, which is covered by either terrazzo, cement, asphalt-tile, or carpeting. Electrical and air ducts are placed underneath each floor slab. The interior partition walls are made of rough masonry, marble, plaster, glass, aluminum, or pointed steel. Interior The Secretariat Building was constructed with of space and, at the time of its completion, could accommodate 4,000 workers. Floors 6, 16, and 28 are used as mechanical floors, and floor 39 serves as a mechanical penthouse, accessible only by stairs. In addition, the United Nations headquarters contained a pneumatic mail system, with tubes connecting to a central collection point in the Secretariat Building. Objects could be transported between floors via a conveyor belt system that traveled at . There was also a dumbwaiter that stopped at every floor except for the lobby and the mechanical stories. The pneumatic mail system only served two stories, while the conveyor belt and dumbwaiter systems were primarily used by the building's messenger stations, which occupied only eleven stories. The building is decorated with various pieces of art from the United Nations Art Collection. The Secretariat Building was built with 21 high-speed passenger elevators and eight bronze-and-glass escalators. The building has three banks of six passenger elevators, as well as two freight elevators serving all stories. The low-rise, mid-rise, and high-rise banks of elevators respectively serve floors 2–15, 16–27, and 28–39. The elevators were programmed so that, if a person on one of the office floors was waiting for a "down" elevator for more than 60 seconds, they would instead be able to enter the next "up" elevator. The elevators were initially staffed by elevator operators before being converted to manual operation in 1967. Lower stories Under the building is a three-story garage for UN employees, with 1,500 parking spaces. The first