Tube (structure)

John Hancock Center in Chicago, designed in 1965 and completed in 1969, is an example of the trussed tube structural design

In structural engineering, the tube is the system where in order to resist lateral loads (wind, seismic, etc.) 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 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, completed in 1963.

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 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 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

By 1963, a new structural system of framed tubes had appeared in skyscraper design and construction. Fazlur Rahman 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." Closely spaced interconnected exterior columns form the tube. Horizontal loads, like wind for example, 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 the DeWitt-Chestnut apartment building which Khan designed and which was completed 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, Petronas Towers, Jin Mao Building, and most other supertall skyscrapers since the 1960s, including the world's current tallest building, the Burj Khalifa.

Variations

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

Framed tube

The World Trade Center's Twin Towers were some of the first structures to use a framed tube design. The many columns of the framed tube can be seen around the exterior of this horizontal cross section. The towers had a core for services, seen in the center. The design was not tube-in-tube since the core had 47 columns spaced relatively evenly, rather than around the edge of the core.

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 completed in 1965, but the most notable examples are the Aon Center and the original World Trade Center towers.

Trussed/Braced Tube

Also known as the braced tube, the trussed tube is similar to the simple tube but with comparatively fewer and farther-spaced exterior columns. Steel bracings or concrete shear walls are introduced along the exterior walls to compensate for the fewer columns by tying them together. The most notable examples incorporating steel bracing are the John Hancock Center, the Citigroup Center and the Bank of China Tower.

Tube in tube

Also known as hull and core, these structures have a core tube inside the structure, holding the elevator and other services, and another tube around the exterior. The majority of the gravity and lateral loads are normally taken by the outer tube because of its greater strength. 780 Third Avenue, a 50-story concrete frame office building in Manhattan, uses concrete shear walls for bracing and an off-center core to allow column-free interiors.

Bundled tube

Breakdown of the bundled tube structure of the Willis Tower with simplified floor plans.

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 envelope. Notable examples include Willis Tower and One Magnificent Mile.

Willis Tower, completed in 1973, introduced the bundled tube structural design and was the world's tallest building until 1998

The bundle tube design was not only highly efficient in economic terms, but it was also "innovative in its potential for versatile formulation of architectural space. Efficient towers no longer had to be box-like; the tube-units could take on various shapes and could be bundled together in different sorts of groupings." The bundled tube structure meant that "buildings no longer need be boxlike in appearance: they could become sculpture."

Hybrid System

A varied category of structures where the basic concept of tube is used, but has been supplemented by other structural support.This method is used where the slenderness of the building is such that a single system cannot provide adequate strength or stiffness.

Diagram