Tube (structure)

In structural engineering, the tube is the name given to the systems where in order to resist lateral loads (wind, seismic, etc.) a building is designed to act like a three-dimensional hollow tube, hence the name, cantilevered perpendicular to the ground. The system was introduced by Fazlur Rahman Khan while at Skidmore, Owings and Merrill's (SOM) Chicago office. The first example of the tube’s use is the 43-story Khan-designed DeWitt-Chestnut Apartment building in Chicago, Illinois.

The system can be constructed 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 in excess of 40 stories constructed in the United States from the period after World War II to the 1990s were of this structural type. It is slowly being overtaken by the use of core-supported type structures.

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 intentionally left 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 due to lateral loads.

History

After 1965, a new structural system of framed tubes appeared in skyscraper design and construction. The Bangladeshi engineer Fazlur Khan 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." [Cite web| title = Evolution of Concrete Skyscrapers| accessdate = 2007-05-14| url = http://www.civenv.unimelb.edu.au/ejse/Archives/Fulltext/200101/01/20010101.htm ] Closely spaced interconnected exterior columns form the tube. Horizontal loads, for example wind, 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. Where larger openings like garage doors are required, the tube frame must be interrupted, with transfer girders used to maintain structural integrity.

The first building to apply the tube-frame construction was in the DeWitt-Chestnut apartment building which Khan designed in Chicago. This laid the foundations for the tube structures of many other later skyscrapers, as can been seen in the construction of the World Trade Center.

Variations

From its inception, the tube has been varied to suit different structural requirements:

Framed tube

This is the simplest incarnation of the tube. It can take a variety of floor plan shapes from square and rectangular, circular, and freeform. The most notable examples are the Aon Center and the destroyed World Trade Center towers.

Trussed tube

Also known as the "braced tube" it is similar to the simple tube, but with comparatively fewer and farther-spaced exterior columns. Bracing is introduced along the exterior walls to compensate for the fewer columns by tying them together. The most notable examples are the John Hancock Center, the Citigroup Center, and the Bank of China Tower.

Bundled tube

Instead of one tube, a building consists of several tubes tied together to resist the lateral forces. Such buildings have interior columns along the perimeters of the tubes when they fall within the building envelop. Notable examples include Sears Tower and One Magnificent Mile.

Hybrid tube

A varied category of structures were the basic concept of the tube is used, but supplemented by other structural support methods. One example is 780 Third Avenue on Manhattan, a 50-story concrete frame office building. Its plan and height-to-width ratios as well as column spacing exceeded the criteria necessary to create an effective tube structure. A concrete braced tube (created by infilling window openings with concrete panels)supplemented with shear walls within an off-center core laterally stabilize the building.

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