Dr. Fazlur R. Khan - "Einstein of Structural Engineering" and "Greatest Structural Engineer of the 20th Century" khan.princeton.edu

Dr. Fazlur Rahman Khan, more than any other individual, ushered in a renaissance in skyscraper construction during the second half of the 20th century. He has been called the "Einstein of structural engineering" and the "Greatest Structural Engineer of the 20th Century" for his innovative use of structural systems that remain fundamental to modern skyscraper design and construction.


Dr. Fazlur Rahman Khan
BSc, Civil EngineeringBangladesh University of Engineering and Technology
MSc, Structural Engineering, University of Illinois at Urbana–Champaign, USA
MSc, Theoretical and Applied Mechanics, University of Illinois at Urbana–Champaign, USA
PhD, Structural Engineering, University of Illinois at Urbana–Champaign, USA

>> khan.princeton.edu/khanSears.html

Khan's bundled tube concept, initiated in the Sears Tower design (now the Willis Tower), offered an efficient structural system with unprecedented flexibility for architectural design (Timothy Hursley, courtesy of Skidmore, Owings & Merrill LLP) 

Dr. Fazlur R. Khan (1929–1982): Engineering Pioneer of Modern Architecture by Yasmin S. Khan

Fazlur R. Khan initiated structural systems that are fundamental to tall building design today. In a career marked by innovation in structural engineering and collaboration between engineering and architecture, Khan introduced design methods and concepts that set new standards for efficient use of material and suggested new possibilities for building architecture.

Fazlur Khan was born April 3, 1929 in Dhaka, Bangladesh (at the time, Dacca, East Bengal, in British India). With the encouragement of his father—a mathematics instructor, the author of mathematics textbooks, and, later, an Assistant Director of Public Instruction for Bengal—Khan discovered the pleasure of learning during his teen years. He became comfortable analyzing and discussing problems and considering them from various perspectives. At the same time, he learned to appreciate life; “and life,” Khan emphasized in 1971, on the occasion of his selection as Engineering News-Record’s Construction’s Man of the Year, “is art, drama, music, and most importantly, people.”

Fazlur Khan with his daughter Yasmin Khan, also a structural engineer (courtesy Yasmin Khan)

>> khan.princeton.edu/khanSears.html

Khan selected civil engineering for his undergraduate studies, excelled in his coursework, and received two scholarships for advanced studies in the United States, a Fulbright Scholarship and a Pakistan government scholarship. The Fulbright program placed him at the University of Illinois at Urbana-Champaign, a stimulating environment that suited him perfectly. The professors, themselves highly respected in their fields, were at once demanding and supportive. His coursework instilled the value of critical thinking and “broadening ones approach to new problems,” and, equally influential to his career, demonstrated the beauty of concepts that are, in his words, “elegantly simple.” www.lehigh.edu/~infrk/2011.08.article.html

When Fazlur Khan joined the Chicago office of Skidmore, Owings & Merrill (SOM) on a permanent basis in 1960, the options available to designers of multistory buildings were surprisingly limited. Beam–column “skeleton” frame construction, in which columns were spaced at 10 to 16 feet throughout the building, was the norm. Alternatively, engineers could design shear walls in concrete construction, or shear trusses in steel construction, to resist lateral loads. The frame, shear wall, and shear truss systems were appropriate for buildings up to 20 or 30 stories, but became inefficient and uneconomical in taller structures. It was generally accepted that unit area cost increased disproportionately to building height. - www.lehigh.edu/~infrk/2011.08.article.html

Dr. Fazlur Rahman Khan, more than any other individual, ushered in a renaissance in skyscraper construction during the second half of the 20th century. He has been called the "
Einstein of structural engineering" and the "Greatest Structural Engineer of the 20th Century" for his innovative use of structural systems that remain fundamental to modern skyscraper design and construction.

Dr. Fazlur Rahman Khan
BSc, Civil EngineeringBangladesh University of Engineering and Technology, Dhaka, Bangladesh
MSc, Structural Engineering, University of Illinois at Urbana–Champaign, USA
MSc, Theoretical and Applied Mechanics, University of Illinois at Urbana–Champaign, USA
PhD, Structural Engineering, University of Illinois at Urbana–Champaign, USA

>> khan.princeton.edu
>> khan.princeton.edu/khan.html
>> khan.princeton.edu/works.html

List of buildings on which Khan was structural engineer include:

DeWitt-Chestnut Apartments, Chicago, 1963

Brunswick Building, Chicago, 1965

John Hancock Center, Chicago, 1965–1969

One Shell Square, New Orleans, Louisiana, 1972

140 William Street (formerly BHP House), Melbourne, 1972

Sears Tower, renamed Willis Tower, Chicago, 1970–1973

First Wisconsin Center, renamed U.S. Bank Center, Milwaukee, 1973

Hajj Terminal, King Abdulaziz International Airport, Jeddah, 1974–1980

King Abdulaziz University, Jeddah, 1977–1978

Hubert H. Humphrey Metrodome, Minneapolis, Minnesota, 1982

One Magnificent Mile, Chicago, completed 1983

Onterie Center, Chicago, completed 1986

United States Air Force Academy, Colorado Springs, Colorado




Written by Yasmin Sabina Khan, his daughter

When I was a young girl, around eight or nine, my father made up a math game for us to play in the car. He would think of a number (say, 6) and I would figure out how he had reached that number using a certain number of 4’s, for example. That is, how to get to 6, using four 4’s. (The answer is [4+4]/4 + 4 = 6.) These puzzles, as I remember them, were difficult enough to be challenging but suited to my abilities so that they could be solved fairly quickly. Most important, this car game was fun. So much so that we often played it with a girlfriend of mine—adding competition to the game, she and I would try to be the first to find a solution.

Fazlur Khan with his daughter Yasmin Khan, also a structural engineer (courtesy Yasmin Khan)

My father had a talent for making learning fun, and he seems to have been able to infuse work at SOM with interest and enjoyment in a similar fashion. When I was writing my book about him, Engineering Architecture: The Vision of Fazlur R. Khan, the stories I heard from his former colleagues consistently recalled this aspect of working with my father. When he explained the tubular system in the early 1960s, an architect involved in the design of Chestnut-DeWitt Apartments in Chicago, the concrete building that initiated the framed tube, told me, “it seemed really exciting. . . . He was full of creative ideas.” Engineers felt the same way. My father’s enthusiasm for each new system, they recalled, was irresistible.

I also heard from his former associates that he knew how to orchestrate community work and “get the best out of people.” One engineer who worked with my father on numerous projects told me that he made designers feel they were a part of the effort, sharing in the development of ideas. Despite long hours, at times, and considerable pressure, people were motivated to work together to achieve a goal. What was perhaps most amazing to me, was the wide range of people who remembered my father and his influence with affection; engineers, architects, material specialists, material fabricators, contractors, members of professional organizations in which he participated. “His easy way of dealing with people,” wrote the members of one group, combined with his “clarity of thought in technical matters,” drew them together.

Memories such as these strengthen my belief that my father’s character and values shaped his career. He must have felt the same way. Near the end of his life he had a chance to look back on his life and career for an oral history project. A recording was made over two days as he talked about his childhood, his university experience, and his years at SOM. He credited my grandfather, in particular, with encouraging him as a boy. My grandfather has a reputation in the family for his patience and gentleness, along with the guidance he provided for others. He valued education highly—himself teaching mathematics, writing math textbooks, and serving as an administrator for public education—and he knew how to inspire students. He must have recognized that my father, though not an eager student as a young child, was quite bright; in any case, he assumed responsibility for guiding his education. My father fondly remembered the time they spent together over the years. It seems that the schoolwork they spent the most time with was math; my grandfather would make up additional problems for my father to solve, or suggest nuances to a particular problem. Approaching homework exercises in this way, rather than just solving given problems, imbued them with greater interest. “I always had a feeling,” my father recalled, that “I was somehow enjoying it beyond the curriculum requirement.” Learning was a pleasurable experience; it also prepared him for critical thinking later on.

At the same time, my grandfather talked with him about life. “I was always philosophically inclined,” he remembered, “because my father used to sit down and talk to me.” They talked about helping people, about generosity, compassion, and humility, and about learning. “After all, learning for what, that eventually we should be able to help people.” These discussions, together with my grandfather’s example, must have contributed to the calm demeanor my father brought to his work.

One of the personal strengths that influenced his career, I believe, was the confidence and self-assurance he acquired during the first thirty years of his life. By this I have in mind both his personal grounding and his educational training. As a youth he developed a perspective on life that would serve him well, and in his twenties he strengthened this personal footing by traveling and meeting people of different cultures and different backgrounds, listening to music, reading widely, from existentialism to writings about beauty, and learning about art (during a visit together to New York one summer, I was amazed by his familiarity with the paintings we saw at the Guggenheim). He built on his academic training in a similar manner. After earning his bachelor’s degree, he returned to the Engineering College in Dhaka to teach structures and applied mechanics. He found that he could communicate with the students in a way that sparked their interest—he, in turn, was rewarded by the “bright sparks in students’ eyes” when they grasped a new concept. Clearly he understood his subject, and yet he decided to pursue graduate studies. Two scholarships brought him to the United States for three years’ study at the University of Illinois at Urbana-Champaign. Making the most of his time there, he took enough classes to earn two master’s degrees, along with a PhD. Then, when he joined SOM in Chicago, he dedicated himself to an “intensive scrutiny,” in the words of another engineer, into structural behavior.

These years of focused study prepared him for a career characterized by innovation. By the time he started to design tall buildings in the early 1960s, he had a firm understanding of material and structural behavior, which enabled him to think creatively, venturing beyond the conventional methods of analysis and design, as well as an intuitive understanding of structural behavior and load flow. My father described it this way: “I had a visual feeling of what is going on . . . a kind of empathy to the structure.” In addition, his personal and professional grounding allowed him to be open to the probing of ideas and the give and take of working with others. His ability to participate in design as fully as he did owed much to his eagerness for communication.

My father felt strongly about people working together as a team, toward a common goal. This attitude, it seems to me, partly explains his comfortable way of working with others. Design is a process fostered by “natural communication,” he said. “If you start controlling design by hierarchy, it will never be done right; never natural.” I heard from his associates that he applied this approach to design meetings, preferring collegial dialogue to hierarchically structured exchange.

The year after my father died, the American Institute of Architects selected him for a 1983 Institute Honor. “Rarely has any engineer played as key a role in the shaping of architects’ ideas and the shaping of buildings themselves,” the nomination stated. “Fazlur Khan’s work and research had made him one of the most influential structural engineers of the century,” the AIA Jury on Institute Honors wrote. “Besides his innovations . . . he demonstrated a human awareness and commitment to structural and architectural design collaboration that has particular importance for architects today.”

My father was, undoubtedly, exceptionally gifted as an engineer and dedicated to the advancement of his field. But by complementing his technical insight with human awareness and collaboration, he not only made his work more enjoyable for himself and more meaningful for his profession, but also transformed the nature of his accomplishments.

-Yasmin Khan

To visit Yasmin Khan's Blog, click here.

© 2011 Department of Civil and Environmental Engineering - Princeton University



Chicago, Illinois, 1974, 1450ft, Steel

Social and Economic Context

Fig 1: The Willis Tower (formerly Sears Tower) dominates the Chicago skyline

Chicago natives now affectionately call the Willis Tower ‘Big Willie’, (Figure 1) to go with ‘Big John’, the John Hancock Center, and ‘Big Stan’, the former Standard Oil Building, a sign that the city may finally be accepting the building’s new name. In 2010, the building was renamed the Willis Tower after Willis Group Holdings, a London insurance broker and currently the largest tenant of the tower. The name change will stand for the duration of their ten year lease. Originally, the Willis Tower was called the Sears Tower, after Sears, Roebuck & Company. This company, who commissioned the building as their new headquarters in the early 1970s, played an integral role to the final form and size of the Tower.

Sears, Roebuck and Company was incorporated in 1893 by Richard Warren Sears and Alvah Curtis Roebuck. In the years that followed, thesmall company grew tremendously, becoming the largest mail order business in the world by 1906.1 In the mid-1920s, Sears transitioned from the mail order business into regional stores. By 1969, Sears was the largest retailer in the nation, and looking to update their outdated headquarters.2 With a net income of $441 million that year, the company could financially support the development of an impressive modern building.3 However, Sears needed Chicago’s support in order to make their move to the city center at South Wacker Drive. The company found support from Chicago’s mayor, Richard Daley. He was keen on the commerce that Sears’ headquarters would bring to the Chicago Loop area, which was partly undeveloped land.4 Daley also lifted the height restriction on buildings through a zoning ordinance revision in 1955; the ordinance changed the maximum building height to sixteen times the area of the lot.5 The only remaining obstacle to build higher was the Federal Aviation Administration, which set the maximum building height for Chicago at 2000 ft. above sea level, or 1450 ft above ground.6However, height did not initially concernSears, as the original Tower design differed greatly from the final design.

It was only after Skidmore, Owings and Merrill (SOM), a structural engineering and architecture firm based in Chicago, got involved that the building took its final form. Sears performed several studies about the company’s project growth and current business practices and concluded that their current and future space requirements were 2 and 4 million square ft, respectively7, with a floor area of 110,000 square ft per department. Also looking for a cost-effective solution, Sears pictured a large 40-story cube. Instead, SOM determined by performing its own studies that departments could save significant amounts of time by stacking two 55,000 square ft stories on top of one another.8 This would change the building into an 80-story tower, but only if SOM could make it cost-effective.

Fazlur Khan, the engineer selected to work on the project, came up with the structural system that defined the Sears Tower. This bundled tube system gave the building its physical strength while saving Sears $10 million as compared to pre-existing structural systems.9 Khan had already worked on another Chicago project, the John Hancock Center, with architect Bruce Graham. In that building, he had used another innovative structural system, the braced tube, which tapered towards the top to provide for various tenant requirements. As the building contained an excess 2 million square ft compared to the initial space demand, Sears needed to consider various tenant requirements, and the shape of the John Hancock Center was similar to what they needed. However, Sears and Graham both wanted something distinctive, not another ‘Big John’. This required a new structural system.

Forces and Form

Fig. 2: Seven of the nine ‘tubes’ discontinue at certain levels creating various floor plans. The dark bands represent the truss levels. Sketch by Razvan I. Ghilic-Micu.

The Willis Tower draws its strength, both visual and physical, from its structural form, the bundled tube. The building plan consists of 9 squares, each 75 feet across, placed in a three-by-three grid arrangement(Figure 2). Each square has 5 columns per side spaced 15 feet on centers, with adjacent squares sharing columns. As the columns rise up the building, each square in the plan forms a tube, which can be seen on the exterior of the building. These tubes are independently strong but are further strengthened by the interactions between each other through truss connections. While the tubes connectat each floor level with beams and floors trusses, several large trussed levels act as the main horizontal connectors in the buildings. These trussed levels, which also contain the mechanical systems for the building, appear as black horizontal bands on the façade (Figure 2). While the louvres covering the trussed levels mask the structural details, the purpose of these levels remains abundantly clear visually.

One of the greatest concerns for the bundled tube system was achieving sufficient lateral stiffness with an economic use of steel. The choice of steel likely stemmed from the building practices of the time, which used steel for tall buildings rather than concrete in Chicago. Though the Sears Tower is significantly taller than the John Hancock Center, the structural system uses a comparable quantity of steel per unit area. The closely spaced interior and exterior columns are tied at each floor with deep spandrel beams. At the truss levels, these tubes are tied together.10 These ties resulted in a stiffer structure, as the building acts as a unified system of stiffened tubes. The interaction between the individual tubes and the belt trusses at mechanical levels allows the building to attain its extreme height.

These trusses serve an additional purpose beyond stiffening the tube structure against winds. Due to the drop offs, the gravity loading on the system is not evenly distributed along the height of the building. These trusses take the gravity loads from above and redistribute them evenly onto the tubes below. This is particularly important for the uppermost section of the tower, due to its asymmetry about the central axis of the building. Because the section is offset, its weight causes columns on one side of the building to experience a greater load than those on the other side. The presence of the belt trusses help to mitigate these effects of differential settlement, which cause the building to tilt.


Though Khan had already estimated that the structure itself would save Sears a great deal of money, SOM continued to reduce costs for its client, as interest rates on the $175 million dollar project became higher and higher. In order to complete the project in a timely and economical manner, several new construction techniques were employed in order to fast track the construction.

The initial stages of design and construction went smoothly for the Sears Tower. As the design moved forward, Fazlur Khan was unable to spend his full time on the project, as he was a partner at SOM. Instead, the project team was composed of six to eight people at any time, with Hal Iyengar as the team leader. The structural drawings were fast tracked along with the construction schedule, taking only 3 months to complete, rather than 8 months. The project team worked overtime, using computer modeling in order to meet the deadline.11Once the final design was completed, construction began with the foundations, and proceeded according to plan.

Fig. 3: Prefabricated beam-column modules

Due to the rapid construction process, prefabrication was one of the most important principles employed during the building of the Sears Tower. Structural units called ‘Christmas trees’ allowed for a 95% reduction in welding on the job site.12 The units were welded offsite and consisted of a two-story column with half-length beams welded to either side of the column (Figure 3). On site, the units only needed bolted splice plate connections between beams and web bolted connections for the column splices.13 As welding on site is one of the most costly and time consuming aspects of steel construction, this process saved a great deal of money for Sears. This use of prefabrication of the Sears Tower project significantly streamlined the construction process.

The assembly process was also streamlined through the use of four standard S2-type stiff-legged derricks. These derricks were used to lift modular units of up to 45 tons up to the 90th floor construction. A final guy derrick was added on the 90th floor for construction up to the roof (Figure 10). For the entire construction process, the derricks were moved after four stories had been completed (two tiers of the prefabricated steel units). With this efficient use of machinery, the erection speed was typically eight stories a month. The steel construction assembly finished on time after 15 months.14 Overall, the construction of the Tower was mostly without incident, as the designers and contractors obviated most issues associated with super tall steel building design.


Viewed from afar, the Willis Tower integrates into the Chicago skyline, standing tall, proud, and respectful. The overall style of the building, with its simple black facade stressing verticality, reflects the prevalence of Miesian architecture in Downtown Chicago. From a distance, the simplicity helps demonstrate the structural system, as the lack of decoration instead features the tubular construction. The intersecting column lines are also abundantly clear, even from several miles away, adding to the visual separation of individual tubes. Black louvres cover the trussed levels of the building, making these levels apparent and visually clean.

The Willis Tower has been a part of the Chicago skyline for nearly 40 years. The exterior building material shows very little aging, a sign of its durable construction.Not only has the skyscraper become an integral part of Chicago’s skyline, many of its details have allowed it to remain an effective and modern symbol of downtown Chicago.


1. Langmead, Donald. Icons of American architecture : from the Alamo to the World Trade Center / Donald Langmead. Series Title: Greenwood icons, page 364.
2. Pridmore, Jay C. A. F. Sears Tower: a building book from the Chicago Architecture Foundation / JayPridmore ; photographs by Hedrich Blessing, pp. 10-14.
3. Khan, Yasmin Sabina.Engineering Architecture. New York: W.W. Norton & Company, Inc. 2004, page 208.
4. Khan, Y. S. pp. 209-210.
5. Willis, C. 1995, Form follows finance : skyscrapers and skylines in New York and Chicago, 1st edn, Princeton Architectural Press, New York, page 138.
6. [Anonymous] 1973.“Bigger may be better: Chicago’s Sears Tower is nine skyscrapers in one.”Architecture Plus. 1(7): 56-59.
7. Pridmore, page 16.
8. “Bigger may be better”.
9. Adams, N. & Skidmore, O.&.M. 2007, Skidmore, Owings & Merrill : SOM since 1936, Electa Architecture; distributed by Phaidon Press, Milan; London, page 252.
10. Khan, Fazlur.Sears Tower: Special Structural Design and Construction Considerations. Report for Skidmore, Owings, & Merrill LLP. 1976, page 5.
11. Ali, M.M. 2001, Art of the skyscraper : the genius of Fazlur Khan, Rizzoli International Publications, Inc., New York, page 130.
12. Ali, page 126.
13. Ali, page 126.
14. Ali, page 130.
© 2011 Department of Civil and Environmental Engineering - Princeton University