Famous Engineers: Fazlur Rahman Khan

Fazlur Rahman Khan, a Bangladeshi-American structural engineer, influenced modern skyscrapers and high-rises with his ideas, and his influence can often be seen from the base of a modern skyscraper.

Before Khan’s work, many tall buildings relied on conventional beam-column frames, shear trusses, or shear wall systems that became increasingly inefficient as buildings grew taller. More height meant greater lateral forces from wind, greater stiffness requirements, more structural material, and often less usable floor area. Khan recognized that the challenge was not simply how to make a building taller, but how to make tall buildings structurally efficient and economically practical. His solution was a very important structural engineering idea of the 20th century: the tube system.

From Dhaka to Chicago

Fazlur Rahman Khan was born in 1929 in what is now Bangladesh. After undergraduate study in South Asia, including a bachelor’s degree from the University of Dacca in 1951, he came to the United States in 1952 for graduate study at the University of Illinois. In just three years, he earned two master’s degrees: one in civil engineering and one in theoretical and applied mechanics, as well as a Ph.D. in civil engineering.

Khan later joined Skidmore, Owings & Merrill (SOM) in Chicago, where he spent most of his professional career. In 1970, he became a general partner at SOM as the firm’s only engineer partner at the time. He worked closely with architect Bruce Graham, and their collaboration produced some of the most recognizable towers in the United States. Khan’s key contribution was the structural logic that made them possible.

The Problem with Going Taller

For low- and mid-rise buildings, traditional structural framing systems could perform well enough. But as buildings rose above 20, 30, or 40 stories, lateral forces began to control the design. Wind loads became a dominant concern. Buildings had to resist overturning, drift, and vibration while still providing usable space for occupants.

A conventional frame could be strengthened, but the penalty was significant. More material meant higher costs. More interior columns and bracing could reduce rentable floor space. The taller the building, the less efficient the older systems became. Khan recognized that a skyscraper should not behave like a stack of floor frames. It should behave more like a hollow vertical cantilever. In other words, the perimeter of the building could act like a tube.

The Tube Structural System

For low- and mid-rise buildings, traditional structural framing systems could perform well enough. But as buildings rose above 20, 30, or 40 stories, lateral forces began to control the design. Wind loads became a dominant concern. Buildings had to resist overturning, drift, and vibration while still providing usable space for occupants.

By using the full width and depth of the building to resist lateral loads, especially wind, the structure became more efficient. Interior space could be more open, and material could be used more strategically. Height became less economically punishing. From there, Khan continued developing variations of the concept, including framed tubes, trussed tubes, bundled tubes, tube-in-tube systems, and composite systems using both concrete and structural steel. These were structural systems matched to different building heights, forms, loads, and economic constraints.

The John Hancock Center

For low- and mid-rise buildings, traditional structural framing systems could perform well enough. But as buildings rose above 20, 30, or 40 stories, lateral forces began to control the design. Wind loads became a dominant concern. Buildings had to resist overturning, drift, and vibration while still providing usable space for occupants.

One of Khan’s most famous achievements was Chicago’s John Hancock Center, now 875 North Michigan Avenue. Completed in 1970, the 100-story tower used a trussed tube system. The building’s large exterior diagonal braces are not merely decorative. They are visible evidence of the load path. Those diagonals helped transfer lateral forces into the perimeter columns, improving stiffness and reducing the need for interior columns. The result was a tall, efficient, mixed-use building with offices, parking, retail, and apartments. At the time, the idea of living on the upper floors of such a structure helped redefine what a skyscraper could be.

The Sears Tower and the Bundled Tube

Khan’s most famous project may be the Sears Tower, now known as the Willis Tower, in Chicago. Completed in 1974, the 110-story tower became the tallest building in the world and held that distinction for more than two decades. The Sears Tower used Khan’s bundled tube system. Instead of one large tube, the building was composed of nine square tubes bundled together in a three-by-three arrangement. Some tubes stopped at different heights, creating the tower’s stepped profile. This system offered several advantages: tremendous stiffness, efficient use of structural material, and the ability to reach extraordinary heights. It also allowed the form of the building to respond to program needs rather than forcing every floor to be identical.

Khan had to satisfy gravity loads, wind loads, serviceability, constructability, economy, and the client’s space requirements. His solution addressed all of them at once. Although Khan is best known for high-rise design, his work was not limited to skyscrapers, but involvement in large-scale structures such as the Hajj Terminal at King Abdulaziz International Airport in Jeddah, Saudi Arabia. The terminal used a tensile fabric roof structure to shelter large numbers of pilgrims, demonstrating Khan’s interest in efficient structural form beyond steel and concrete towers. This breadth of work shows an important feature of Khan’s engineering philosophy. Rather than being committed to one material or one formula, he was committed to understanding a problem and developing the appropriate structural response.