Framed tubes

Since 1963, the new structural system of framed tubes became very influential in the design and construction of skyscrapers. Khan defined the framed tube structure as "a three-dimensional spatial 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-shaped structural system capable of resisting lateral forces in any direction by cantilevering from the base."[14] Closely spaced interconnected exterior columns form the tube. Horizontal loads, for example wind and seismic, are supported by the structure as a whole. Approximately half of the exterior surface is available for windows. Framed tubes allow for fewer interior columns and therefore create more usable floor space. The bundled tube structure is more efficient for tall buildings, reducing the height penalty. The structural system also allows for smaller interior columns and the core of the building to be free of braced frames or shear walls that use up valuable floor space. Where larger openings are required, such as garage doors, the tube frame should be interrupted and transfer beams used to maintain structural integrity.

The first building to use framed tube construction was the DeWitt-Chestnut Apartment Building, which was renamed DeWitt Plaza, a building Bruce Graham designed and engineered by Khan, which was completed in Chicago in 1963. This laid the foundation for the framed tube structure used in the construction of the World Trade Center (1973-2001).

Tube with reinforcement and X bracing

Khan pioneered several other variants of the tube structure design. One of them was the concept of applying X-bracing to the outside of the tube to form a reinforced tube. X-bracing reduces lateral loading on a building by transferring the load to exterior columns, and the reduced need for interior columns provides greater usable floor space. Khan first employed exterior X bracing in his engineering of the John Hancock Center in 1965, and this can be clearly seen on the exterior of the building, making it an architectural icon.[16]​.

In contrast to earlier steel frame structures, such as the Empire State Building (1931), which required about 206 kilograms of steel per square meter, and One Chase Manhattan Plaza (1961), which required about 275 kilograms of steel per square meter, the John Hancock Center was much more efficient, requiring only 145 kilograms of steel per square meter.[17] The tube-frame concept was applied to many later skyscrapers, including the Onterie Center, the Citigroup Center, the Hotel Arts and the Bank of China Tower.[18]​.

Bundled tubes

One of Khan's most important variants of the tube structure concept was bundled tubes, which was used for the Willis Tower and the One Magnificent Mile. The bundled tube design was not only the most economically efficient, but was also "innovative in its potential for the versatile formulation of architectural space. Efficient towers no longer had to be box-like; tube units could take various shapes and could be grouped into different types of clusters."

tube in tube

The tube-in-tube system takes advantage of the core shear wall tubes in addition to the outer tubes. The inner tube and outer tube work together to resist gravity loads and lateral loads and to provide additional rigidity to the structure to prevent significant deflections at the top. This design was first used at One Shell Plaza.[19] Later buildings that used this structural system include the Petronas Towers.[20]

Stabilizer and belt structures

The beam and belt truss system is a lateral load resisting system in which the tube structure is connected to the central core wall with very rigid stabilizers and belt trusses at one or more levels. The BHP House was the first building to use this structural system followed by the First Wisconsin Center, since renamed U.S. Bank Center "U.S. Bank Center (Milwaukee)"), in Milwaukee. The center rises 601 feet, with three belt trusses at the bottom, middle and top of the building. The exposed belt trusses serve aesthetic and structural purposes.[21] Later buildings include the Shanghai World Financial Center.[22]

Concrete tube structures

The last major buildings designed by Khan were One Magnificent Mile and the Onterie Center in Chicago, which employed his bundled tube and trussed tube system designs respectively. In contrast to its previous buildings, which were primarily steel, its last two buildings were concrete. The DeWitt-Chestnut Apartments building, built in 1963 in Chicago, was also a tube-frame concrete building.[16]​ Trump Tower in New York is also another example that adopted this system.[23]​.

Shear Wall Frame Interaction System

Khan developed the shear wall frame interaction system for mid-rise buildings. This structural system uses combinations of shear walls and frames designed to resist lateral forces. The first building to use this structural system was the 35-story Brunswick Building,[21] which was completed in 1965 and became the tallest reinforced concrete structure of its time. The structural system of the Brunswick Building consists of a concrete shear wall core surrounded by an exterior concrete frame of columns.[24]​ Apartment buildings up to 70 stories high have successfully utilized this concept.[25]​.

Framed tubes

Since 1963, the new structural system of framed tubes became very influential in the design and construction of skyscrapers. Khan defined the framed tube structure as "a three-dimensional spatial 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-shaped structural system capable of resisting lateral forces in any direction by cantilevering from the base."[14] Closely spaced interconnected exterior columns form the tube. Horizontal loads, for example wind and seismic, are supported by the structure as a whole. Approximately half of the exterior surface is available for windows. Framed tubes allow for fewer interior columns and therefore create more usable floor space. The bundled tube structure is more efficient for tall buildings, reducing the height penalty. The structural system also allows for smaller interior columns and the core of the building to be free of braced frames or shear walls that use up valuable floor space. Where larger openings are required, such as garage doors, the tube frame should be interrupted and transfer beams used to maintain structural integrity.

The first building to use framed tube construction was the DeWitt-Chestnut Apartment Building, which was renamed DeWitt Plaza, a building Bruce Graham designed and engineered by Khan, which was completed in Chicago in 1963. This laid the foundation for the framed tube structure used in the construction of the World Trade Center (1973-2001).

Tube with reinforcement and X bracing

Khan pioneered several other variants of the tube structure design. One of them was the concept of applying X-bracing to the outside of the tube to form a reinforced tube. X-bracing reduces lateral loading on a building by transferring the load to exterior columns, and the reduced need for interior columns provides greater usable floor space. Khan first employed exterior X bracing in his engineering of the John Hancock Center in 1965, and this can be clearly seen on the exterior of the building, making it an architectural icon.[16]​.

In contrast to earlier steel frame structures, such as the Empire State Building (1931), which required about 206 kilograms of steel per square meter, and One Chase Manhattan Plaza (1961), which required about 275 kilograms of steel per square meter, the John Hancock Center was much more efficient, requiring only 145 kilograms of steel per square meter.[17] The tube-frame concept was applied to many later skyscrapers, including the Onterie Center, the Citigroup Center, the Hotel Arts and the Bank of China Tower.[18]​.

Bundled tubes

One of Khan's most important variants of the tube structure concept was bundled tubes, which was used for the Willis Tower and the One Magnificent Mile. The bundled tube design was not only the most economically efficient, but was also "innovative in its potential for the versatile formulation of architectural space. Efficient towers no longer had to be box-like; tube units could take various shapes and could be grouped into different types of clusters."

tube in tube

The tube-in-tube system takes advantage of the core shear wall tubes in addition to the outer tubes. The inner tube and outer tube work together to resist gravity loads and lateral loads and to provide additional rigidity to the structure to prevent significant deflections at the top. This design was first used at One Shell Plaza.[19] Later buildings that used this structural system include the Petronas Towers.[20]

Stabilizer and belt structures

The beam and belt truss system is a lateral load resisting system in which the tube structure is connected to the central core wall with very rigid stabilizers and belt trusses at one or more levels. The BHP House was the first building to use this structural system followed by the First Wisconsin Center, since renamed U.S. Bank Center "U.S. Bank Center (Milwaukee)"), in Milwaukee. The center rises 601 feet, with three belt trusses at the bottom, middle and top of the building. The exposed belt trusses serve aesthetic and structural purposes.[21] Later buildings include the Shanghai World Financial Center.[22]

Concrete tube structures

The last major buildings designed by Khan were One Magnificent Mile and the Onterie Center in Chicago, which employed his bundled tube and trussed tube system designs respectively. In contrast to its previous buildings, which were primarily steel, its last two buildings were concrete. The DeWitt-Chestnut Apartments building, built in 1963 in Chicago, was also a tube-frame concrete building.[16]​ Trump Tower in New York is also another example that adopted this system.[23]​.

Shear Wall Frame Interaction System

Khan developed the shear wall frame interaction system for mid-rise buildings. This structural system uses combinations of shear walls and frames designed to resist lateral forces. The first building to use this structural system was the 35-story Brunswick Building,[21] which was completed in 1965 and became the tallest reinforced concrete structure of its time. The structural system of the Brunswick Building consists of a concrete shear wall core surrounded by an exterior concrete frame of columns.[24]​ Apartment buildings up to 70 stories high have successfully utilized this concept.[25]​.