The tubular structural system developed by Fazlur Khan

Many structural systems have been adopted in the design of tall buildings, one of the most effective being the tubular structural system. The advancement of technology and the development of the economy, all over the world, have led to the construction of higher and higher buildings.

The „father” of this system is the architect and structural engineer Fazlur Rahman Khan. In the 1960s – 1970s, he became known for his projects in Chicago: the John Hancock Center (a building with 100 floors) and the Willis Tower (Sears), the tallest building in the world at the time.

Fazlur Khan realized that the steel frame structure, dominant in the design and construction of tall buildings until then, is not the only system suitable for building skyscrapers. „His unique ability to bridge the gap between architectural design and structural engineering was what set Khan apart from other structural engineers” – John Zil. His projects have transformed the skyline of cities, most of the buildings over 40 floors, in the USA. They were built after the 1960s and used a tube design derived from his engineering principles.

The three types of tubes in the tubular structural system

1. The framed tube

Kahn developed three main variants of the tube design. These have been used to suit different structural requirements:

The frame consists of columns located at small distances, 2-4 m between the centers joined by foundation beams. We can imagine this system as a huge flexible backbone, made up of connected frames.

The first building to use this system was the DeWitt-Chestnut apartment building, designed by Khan and completed in 1963. This laid the foundation for the framed tube structure used in the construction of the World Trade Center.

„Framed tube” is the simplest system and can be adapted for square, circular and irregular shapes.

2. The braced tube (braced tube, trussed tube)

The purpose of this concept is to reduce the lateral load applied to the building by transferring it to the external columns. This leads to the achievement of a large space due to the reduced need for interior columns.

The John Hancock Center is the first building to use this system. Unlike previously used metal structures, such as the Empire State Building, the Hancock Center was much more efficient. Because it required only 145 kilograms of steel per square meter, compared to 206 kilograms used in the building in New York.

3. The modular tube (bundled tube)

The „bundled tube” system can be visualized as an assembly of individual tubes that form a tube of several cells. The system allows the highest height and the highest surface area. The modular tube was very efficient from an economic point of view, as well as innovative. Because the buildings no longer had to look like a box. The tubes can take various forms and be grouped in different ways, making buildings with structural aspects.

An emblematic building made with the help of „bundled tube” is the Willis Tower. Composed of nine tubes of different heights to achieve a distinctive appearance.

For the spectacular Burj Khalifa building, the „bundled tube” system and a composite of concrete and steel were used to reach the height of 828 m.

Advantages of the tubular structural system:

A clear advantage is from a material point of view. Well designed, tubular forms use approximately half the amount of material that would have been used for a conventionally made structure.

It allows greater flexibility in planning the interior space because all the columns are concentrated on the perimeter of the structure. The system allows creating a free space, without pillars inside.

Based on the concept that the external shell of the building can function as a structure in itself. Fazlur Rahman Khan designed a structural system that can be used for both steel and concrete buildings.