The Achievement
For forty years, the Empire State Building stood as the tallest structure in the world. Not for lack of ambition. Engineers knew how to stack floors. The problem was economics: the taller you went, the more structural steel you had to bury inside the building to resist wind forces, and the more steel you buried, the more expensive each floor of rentable space became. Past roughly 30 to 40 stories, building tall simply stopped making financial sense.
Fazlur Rahman Khan solved that problem. The Bangladeshi-American structural engineer at Skidmore, Owings & Merrill in Chicago invented the tubular structural system, which moved the load-bearing work from a building's interior to its outer perimeter. The result was a fundamental shift in what tall buildings could be: taller, cheaper per square foot, and structurally more efficient than anything that had come before.
The Sears Tower, which Khan designed with architect Bruce Graham and which surpassed the Empire State Building's height record in 1973, required only 33 pounds of structural steel per square foot. The Empire State Building, completed 42 years earlier and 200 feet shorter, required more than 42 pounds per square foot. Khan had built higher while using less material. That is not just an engineering achievement. It is an economic revolution in the built environment.
Every supertall skyscraper built since 1963 uses structural principles Khan either invented or directly established. The tube concept is not historical curiosity. It is the engineering foundation of the modern skyline.
From Dhaka to Chicago
Fazlur Rahman Khan was born on April 3, 1929, in Dhaka, in what was then the Bengal Presidency of British India and is now Bangladesh. His father was a mathematics teacher, and Khan excelled with a distinctiveness that made his path forward clear. He graduated first in his class from the University of Dhaka in the early 1950s, then won a Fulbright Scholarship to the United States in 1952.
At the University of Illinois at Urbana-Champaign, he completed two master's degrees (one in structural engineering, one in theoretical and applied mechanics) and a PhD in structural engineering. All three degrees, in three years. He finished in 1955 at age 26.
What happened next says something about Khan's temperament. He walked into the Chicago offices of Skidmore, Owings & Merrill, one of the most prominent architectural and engineering firms in the country, and asked for an interview. He left with a job offer. He spent his entire American career at SOM, becoming a partner in 1966. He never needed a second employer.
The engineering landscape Khan entered was one of real constraint. The technology for very tall buildings had not advanced meaningfully since the 1930s. The structural systems in use, steel skeleton frames, concrete shear walls, shear trusses, functioned adequately up to a certain height and then became economically punishing. Khan's graduate work had given him the mathematical tools to understand why, and to think about what might replace those systems.
The Tube: What It Is and Why It Matters
The engineering challenge of tall buildings comes down to lateral forces, primarily wind. A building is not just fighting gravity. It is fighting horizontal loads that increase as you go higher and that must be transferred down to the foundation without shaking or collapsing the structure.
Before Khan, the conventional solution was to concentrate structural resistance in a building's interior core: thick shear walls, diagonal bracing systems, dense columns. The problem with this approach is that it consumes interior floor space that would otherwise be rentable, and as buildings get taller, the required structural mass grows faster than the floor area added. The economics worsen with every additional story.
Khan's reconceptualization was elegant in the way that genuine breakthroughs often are. Instead of treating a skyscraper as a column-and-beam frame scaled upward, he thought of it as a hollow tube, like a cardboard cylinder. A tube resists forces from any direction with extraordinary efficiency because its entire perimeter is doing the structural work. Move the load-bearing function from the building's interior to its exterior skin, and you free up the interior entirely while creating a structure that handles wind loads with far less material.
In practice, Khan achieved this by placing columns close together on a building's perimeter (typically five to ten feet apart) and connecting them with stiff horizontal beams. The resulting perimeter frame acts as a rigid shell. Interior columns could be minimized or eliminated, opening up floor plates for flexible use.
He developed three distinct variants of the system, each suited to different scales and design requirements.
The framed tube, first used in the DeWitt-Chestnut Apartment Building in Chicago in 1963, was the foundational form. It was a residential building, not a landmark, but it was the proof of concept that changed the field. The World Trade Center, designed by Minoru Yamasaki shortly after, drew directly on the framed tube principles Khan had demonstrated at DeWitt-Chestnut.
The trussed tube, applied at the John Hancock Center in 1969, added large X-bracing diagonals to the exterior. The diagonals carry lateral loads even more efficiently, allowing columns to be spaced further apart and interior structural elements to be reduced further. The X-bracing on the Hancock's exterior is immediately visible from street level and for miles around. It is a structural necessity that became the building's most recognizable visual feature. The Hancock required 145 kilograms of steel per square meter, compared to 206 kilograms for the Empire State Building. A 30% reduction, for a building taller and more slender.
The bundled tube, used at the Sears Tower in 1973, clustered nine square tubes together with shared walls. This solved a specific limitation of single-tube buildings (larger tubes suffer from uneven load distribution at corners, a phenomenon called shear lag), allowed the building's profile to step at different heights as outer tubes terminated, and pushed structural efficiency to its limit. The Sears Tower's 33 pounds per square foot of steel remains a benchmark engineers return to today.
Three Buildings That Defined a Skyline
Khan's career can be traced through three buildings, each of which was simultaneously the tallest or most efficient of its type when completed and the proof of a new structural principle.
The DeWitt-Chestnut Apartment Building (Chicago, 1963) is the least famous and arguably the most important. A concrete residential high-rise whose closely spaced perimeter columns formed the world's first framed tube structure. No photographs of it hang in architecture schools. But every major skyscraper built after it carries its structural DNA.
The John Hancock Center (Chicago, 1969) brought the principles to monumental scale. A hundred stories, 1,128 feet, mixed commercial and residential use in a single tower. Its trussed tube system required less steel than the Empire State Building despite being 122 feet taller. The diagonal bracing made the structural system legible from the street, an architecture that told you exactly how it worked. Engineers and architects from around the world came to Chicago to study it.
The Sears Tower (Chicago, 1973) was the culmination. At 110 stories and 1,450 feet, it surpassed every previous height record and held the title of world's tallest building for 25 years. Its bundled tube system stepped the building's profile as outer tubes terminated at different floors, creating the distinctive silhouette that defines the Chicago skyline. The steel efficiency figures from the Sears Tower have been cited in structural engineering papers, textbooks, and lectures ever since. No supertall building before or since has matched its combination of height and material economy.
Khan also demonstrated that tubular principles translated to concrete. One Shell Plaza in Houston (completed 1971), the first tube-in-tube structure, was the tallest reinforced concrete building in the world at completion and the tallest building west of the Mississippi River. The tube was not a steel trick. It was a general structural insight.
The Hajj Terminal and a Poignant Ending
The last major project of Khan's life was unlike anything else in his portfolio. The Hajj Terminal at King Abdulaziz International Airport in Jeddah, completed in 1981, is not a skyscraper. It is a vast open-air covered space, designed to shelter the millions of Muslim pilgrims who pass through Jeddah each year during the Hajj season.
The structure consists of 210 semi-conical fabric tents made from Teflon-coated fiberglass, suspended from steel pylons and organized into modules. The design is naturally ventilated, drawing air through the tent fabric in a climate where temperatures regularly exceed 100 degrees Fahrenheit. The capacity: 50,000 pilgrims at any time during arrival, 80,000 during departure. The Aga Khan Award for Architecture called it "an outstanding contribution to architecture for Muslims."
Khan was a devout Muslim. He had spent his career in Chicago building the commercial towers that defined American cities. At the end of his life, he was engineering a shelter specifically for the global Muslim pilgrimage. The project meant something to him beyond its structural interest.
On March 27, 1982, Fazlur Rahman Khan died of a heart attack in Jeddah. He was 52 years old, in the city where he had built the Hajj Terminal, on a business trip related to the project. He died, in other words, in the place where he had done something for his own people.
The Council on Tall Buildings and Urban Habitat later created the Fazlur R. Khan Lifetime Achievement Medal in his honor. In 1983, the American Institute of Architects awarded him a posthumous Institute Honor for Distinguished Achievement. Lehigh University established both an endowed chair and a distinguished lecture series bearing his name. He had been elected to the National Academy of Engineering, the top recognition available to an American engineer, in 1973, the same year the Sears Tower was completed.
Why Khan's Name Is Not Better Known
Fazlur Rahman Khan is not a household name. This is worth addressing directly, because the scale of what he accomplished is difficult to overstate.
Part of the explanation is structural (in the social sense): structural engineers rarely receive the public recognition that architects do. The architect's name goes on the building. The engineer's name goes in the technical literature. Khan worked in collaboration with architect Bruce Graham, and for decades the Graham-Khan partnership was described primarily as Graham's buildings built on Khan's systems, rather than as the genuinely collaborative achievement it was.
Part of the explanation is timing. Khan died at 52, before the retrospective recognition that often arrives for major figures in their later decades. He did not live to see the Sears Tower celebrated on its twentieth or thirtieth anniversary, to receive honorary degrees from every major engineering school, to be the subject of the documentary projects that eventually followed.
And part is simply the nature of structural engineering: its greatest achievements are the things that do not happen. Buildings that do not sway, do not crack, do not fall. The public notices the exception, not the rule. Khan's tubes have been holding up the Chicago skyline for more than fifty years. Their quiet success is the measure of his work.
Bangladeshi engineers and the Bangladeshi-American community have claimed Khan's legacy with justified pride. The Council on Tall Buildings and Urban Habitat, the global authority on skyscraper research, has made his name permanent in its highest honor. Structural engineering as a field knows exactly what it owes him.
Frequently Asked Questions
Who is called the father of the modern skyscraper?
Fazlur Rahman Khan (1929-1982), a Bangladeshi-American structural engineer at Skidmore, Owings & Merrill in Chicago, is widely called the father of the modern skyscraper. His invention of the tubular structural system made it economically viable to build above 40 stories, enabling the supertall skylines that define cities today. He designed the Sears Tower (now Willis Tower) and the John Hancock Center.
What did Fazlur Rahman Khan invent?
Khan invented the tubular structural system for skyscrapers, which uses a building's outer perimeter as the primary load-bearing structure rather than interior columns and cores. He developed three variants: the framed tube (1963, DeWitt-Chestnut Apartments), the trussed tube (1969, John Hancock Center), and the bundled tube (1973, Sears Tower). These systems reduced structural steel requirements by up to 30% compared to earlier buildings while enabling far greater heights.
Why was the tubular system such a dramatic improvement over previous skyscraper engineering?
Before Khan's innovations, traditional structural systems became uneconomical above roughly 30-40 stories. The more floors you added, the more steel you had to place in the building's interior core to resist wind forces, consuming floor space and raising costs per square foot. Khan's tube system moved the structural work to the building's perimeter, freeing the interior entirely and dramatically cutting material use. The Sears Tower required only 33 lb/sq ft of steel versus 42+ lb/sq ft for the Empire State Building, which is 200 feet shorter. Khan made supertall buildings both structurally feasible and financially rational.
Where did Fazlur Rahman Khan study?
Khan graduated first in his class from the University of Dhaka. In 1952, he won a Fulbright Scholarship to the University of Illinois at Urbana-Champaign, where he earned two master's degrees (structural engineering and theoretical and applied mechanics) plus a PhD in structural engineering, all within three years. He joined Skidmore, Owings & Merrill in Chicago in 1955 and became a partner in 1966.
How did Fazlur Rahman Khan die?
Khan died on March 27, 1982, in Jeddah, Saudi Arabia, from a heart attack. He was 52 years old and was in Jeddah on a business trip related to the Hajj Terminal at King Abdulaziz International Airport, which he had designed to shelter millions of Muslim pilgrims during Hajj. He died in the city where he had built that shelter.