Tube structural system

[edit] Introduction

The tube is a structural engineering system that is used in high-rise buildings, enabling them to resist lateral loads from wind, seismic pressures and so on. It acts like a hollow cylinder, cantilevered perpendicular to the ground.

The system was developed in the 1960s by the engineer Fazlur Rahman Khan, and has been used to construct most high-rise buildings since then.

The tube system can be constructed using concrete, steel or a composite of both. In its simplest form, closely-spaced columns are tied together with deep spandrel beams through moment connections as part of the external perimeter of the building. The rigid frame that this assembly of columns and beams forms results in a dense and strong structural ‘tube’ around the exterior.

Since lateral loads can be resisted by this stiff exterior framing, interior columns can be located at the core and are fewer in number. The interior can be simply framed for gravity loads and floor space is left free from columns.

The first building designed by Khan using a tube frame was the DeWitt-Chestnut building, Chicago, in 1963. The first skyscraper to use the system was Chicago’s Willis Tower.

[edit] Types of system

The most common of variations are as follows:

[edit] Framed tube

This is the simplest form of the tube system and can be used on a variety of floor plan shapes, including square, rectangular, circular and freeform. This type is reasonably efficient from 38-300 m (125-1,000 ft) in height. It was the initial system type developed by Khan.

Khan defined a 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."

[edit] Trussed tube

Trussed (also known as braced) tube systems are similar to the framed tube but have fewer exterior columns space further apart. To compensate for the fewer columns, steel bracing or concrete shear walls are introduced to tie the columns together.

By interconnecting all the exterior columns, it forms a rigid box which is capable of resisting lateral shears by axial in its members rather than through flexure (bending or curving).

By having relatively broad column spacing, it is possible to have lots of clear space for windows.

The diagonals that are introduced on each façade should intersect at the same point on the corner column. These diagonals interact with the perpendicular face trusses to make the structure ‘tubular’ and equalise the gravity loads of the exterior columns.

[edit] Tube-in-tube

This system is also known as ‘hull and core’ and consists of a core tube inside the structure which holds services such as utilities and lifts, as well as the usual tube system on the exterior which takes the majority of the gravity and lateral loads.

The inner and outer tubes interact horizontally as the shear and flexural components of a wall-frame structure. They have the advantage of increased lateral stiffness.

[edit] Bundled tube

The bundled tube system involves, instead of one tube, several individual tubes interconnected to form a multi-cell tube. Together they work to resist the lateral loads and overturning moments. When the tubes fall within the building envelope, interior columns are positioned along their perimeters.

Not only is this system economically efficient but it also allows for more versatile building designs, adopting interesting shapes and bundled in dynamic groupings rather than being simply box-like towers.

The first type of building to use this system was the Willis Tower in Chicago.

[edit] Hybrid system

A hybrid system is often used where the building design has a slenderness such that a single system cannot provide adequate strength or stiffness. It combines two or more basic structural forms, either by direct combination or by adopting different forms in different parts of the structure. As high-rise building designs become more complex, hybrid systems are increasingly common.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

• Braced frame.
• Concept structural design of buildings.
• Concrete-steel composite structures.
• Shear wall.
• Skeleton frame.
• Skyscraper.
• Structural engineer.
• Structural principles.
• Structural steel.
• The development of structural membranes.
• Types of structural load.
• Willis Tower.

[edit] External resources

• The Constructor - High-rise structures