In structural engineering, the tube is a system where, to resist lateral loads (wind, seismic, impact), a building is designed to act like a hollow cylinder, cantilevered perpendicular to the ground. This system was introduced by Fazlur Rahman Khan while at the architectural firm Skidmore, Owings & Merrill (SOM), in their Chicago office. The first example of the tube's use is the 43-story Khan-designed DeWitt-Chestnut Apartment Building, since renamed Plaza on DeWitt, in Chicago, Illinois, finished in 1966.
The system can be built using steel, concrete, or composite construction (the discrete use of both steel and concrete). It can be used for office, apartment, and mixed-use buildings. Most buildings of over 40 stories built since the 1960s are of this structural type.
Concept
The tube system concept is based on the idea that a building can be designed to resist lateral loads by designing it as a hollow cantilever perpendicular to the ground. In the simplest incarnation of the tube, the perimeter of the exterior consists of closely spaced columns that are tied together with deep spandrel beams through moment connections. This assembly of columns and beams forms a rigid frame that amounts to a dense and strong structural wall along the exterior of the building.
This exterior framing is designed sufficiently strong to resist all lateral loads on the building, thereby allowing the interior of the building to be simply framed for gravity loads. Interior columns are comparatively few and located at the core. The distance between the exterior and the core frames is spanned with beams or trusses and can be column-free. This maximizes the effectiveness of the perimeter tube by transferring some of the gravity loads within the structure to it, and increases its ability to resist overturning via lateral loads.
History
By 1963, a new structural system of framed tubes had appeared in skyscraper design and construction. Fazlur Rahman Khan, a structural engineer from Bangladesh (then called East Pakistan) who worked at Skidmore, Owings & Merrill, defined the framed tube structure as "a three dimensional space structure composed of three, four, or possibly more frames, braced frames, or shear walls, joined at or near their edges to form a vertical tube-like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation." Closely spaced interconnected exterior columns form the tube. Lateral or horizontal loads (wind, seismic, impact) are supported by the structure as a whole. About half the exterior surface is available for windows. Framed tubes require fewer interior columns, and so allow more usable floor space. Where larger openings like garage doors are needed, the tube frame must be interrupted, with transfer girders used to maintain structural integrity.
Khan's tube concept was inspired by his hometown in Dhaka, Bangladesh. His hometown did not have any buildings taller than three stories. He also did not see his first skyscraper in person until the age of 21 years old, and he had not stepped inside a mid-rise building until he moved to the United States for graduate school. Despite this, the environment of his hometown in Dhaka later influenced his tube building concept, which was inspired by the bamboo that sprouted around Dhaka. He found that a hollow tube, like the bamboo in Dhaka, lent a high-rise vertical durability.
The first building to apply the tube-frame construction was the DeWitt-Chestnut Apartment Building which Khan designed and which was finished in Chicago by 1963. This laid the foundations for the tube structural design of many later skyscrapers, including his own John Hancock Center and Willis Tower, and the construction of the World Trade Center, the Petronas Towers, the Jin Mao Building, and most other tall skyscrapers since the 1960s, including the world's tallest building , the Burj Khalifa.
Variants
From its conception, the tube has been varied to suit different structural needs.
Framed
This is the simplest incarnation of the tube. It can appear in a variety of floor plan shapes, including square, rectangular, circular, and freeform. This design was first used in Chicago's DeWitt-Chestnut Apartment Building, designed by Khan and finished in 1965, but the most notable examples are the Aon Center and the original World Trade Center towers.
