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Consideration of the effect of lateral forces is one of the significant design criteria, especially for tall buildings. So, different lateral force resisting systems have been established over the years. Of these systems, the three main kinds are- braced wall, shear wall and moment resisting frames. There are both advantages and disadvantages of these systems. Studies showed that when you want to build very tall structure (like 400 feet tall) these systems are not enough to fulfill all the design criteria. Then tubular structure system came to the rescue.
Why Tubular Structure System?
The tubular designs resist lateral forces (such as wind forces, seismic forces and forces caused by earthquakes) for tall buildings. Tube structures are stiff and have significant advantages over other framing systems. They not only make the buildings structurally stronger and more efficient, but also significantly reduce the structural material requirements. The reduction of material makes the buildings economically more efficient and reduces environmental impact. Tubular systems allow greater interior space and further enable buildings to take on various shapes, offering added freedom to architects. Another important feature of the tubular systems is that buildings can be constructed using steel or reinforced concrete, or a composite of the two, to reach greater heights.
The “Tube Concept”
The tubular structure system was introduced by Fazlur Rahman Khan. He is considered as the “father of tubular designs”. He has been called the "Einstein of structural engineering" and the "Greatest Structural Engineer of the 20th Century" for his innovative use of structural systems that remain fundamental to modern skyscraper design and construction. He was a Bangladeshi-American Engineer.
The “tube concept “involves using all the exterior wall perimeter structure of a building to simulate a thin-walled tube. In traditional buildings, columns are placed in a grid format at regular intervals. If a tall structure is to be built keeping in mind that lateral stability, reduced material requirement and cost efficiency have to be ensured, there was no way but coming out of traditional grid systems of columns. Basically what tubular structure system does is placing most of the columns along the periphery and some columns in the middle as a core to resist lateral effects. This concept decreases material requirement and cost ensuring lateral stability. Moreover this system allows more free spaces in tall buildings than the traditional grid of columns.
The “tube concept” includes framed tube, trussed tube, bundled tube and tube in tube.
Framed tube structure is 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. Horizontal loads, for example from wind and earthquakes, are supported by the structure as a whole. About half the exterior surface is available for windows. Framed tubes allow fewer interior columns, and so create more usable floor space
Trussed tube is formed by applying X-bracing to the exterior of the tube. X-bracing reduces the lateral load on a building by transferring the load into the exterior columns, and the reduced need for interior columns provides a greater usable floor space. Exterior X-bracing was employed on the design of the John Hancock Center in 1965, and this can be clearly seen on the building's exterior, making it an architectural icon. 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 Chase Manhattan Bank Building (1961), which required around 275 kilograms of steel per square meter, the John Hancock Center was far more efficient, requiring only 145 kilograms of steel per square meter. The trussed tube concept was applied to many later skyscrapers, including the Onterie Center, Citigroup Center and Bank of China Tower.
Bundled tube structure is more efficient for tall buildings, lessening the penalty for height. The structural system also allows the interior columns to be smaller and the core of the building to be free of braced frames or shear walls that use valuable floor space. Where larger openings like garage doors are required, the tube frame must be interrupted, with transfer girders used to maintain structural integrity.
Tube-in-tube system takes advantage of core shear wall tubes in addition to exterior 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 in One Shell Plaza.
We feel proud of Fazlur Rahman Khan without whom, we cannot imagine the modern skyscrapers. His invention has taken structural engineering to a new height.
"The technical man must not be lost in his own technology; he must be able to appreciate life, and life is art, drama, music, and most importantly, people.”
--Fazlur Rahman Khan