Author: Leslie E. Robertson

The lead structural engineer reflects on the rise and fall of the World Trade Center towers.



The journey toward the design and construction of the World Trade Center began prior to 1960 when Minoru Yamasaki Associates was selected to design the Federal Science Pavilion, a key element of the Seattle World’s Fair; NBBJ was selected as the local architect. Having accomplished many structural designs for NBBJ, it was only natural that we would obtain the commission for the structural design of the Pavilion. That structural design, reflecting the very highest attainments of our profession, was creatively conceived and executed by John V. Christiansen (NAE). Indeed, the Pavilion stands today as an example of the importance of fine structural engineering as it influences the overall architectural process. The entrepreneurship and skills of another of our partners, John B. Skilling (NAE), were instrumental in the development of the close relationship between our firm and that of Minoru Yamasaki and Associates; many wonderful projects were to follow.



When Yamasaki was commissioned to design the World Trade Center in New York, he proposed that we be retained as structural engineers. Although his recommendation was influential, we were in competition with many New York firms that had more experience in high-rise design than we had. Although we worked hard preparing for our interview with the Port Authority of New York and New Jersey, we wouldn’t have obtained the commission without the presence and the skills of John Skilling.



Once we had been awarded the commission, I moved from Seattle to New York with a team of expert engineers—Wayne A. Brewer (drawing production and coordination), Paul S.A. Foster (towers), Ernest T. Liu (plaza buildings and below-grade structures), Jostein Ness (detailing), Richard E. Taylor (computers), and E. James White (construction technology). Professor Alan G. Davenport (NAE), on sabbatical from the University of Western Ontario, joined us to head the wind-engineering research group. Although I was the titular leader, the energies and talents of the entire team led to our successes.



A list of the innovations incorporated into the World Trade Center would be very long. In the following pages, I describe just a few of the ideas and innovations conceived and developed by our team. Most, if not all, of this technology is now a part of the standard vocabulary of structural engineers.



The tubular framing system for the perimeter walls resisted all of the lateral forces imposed by wind and earthquake, as well as the impact loads imposed on September 11. Although we had used closely spaced columns in an earlier building, it was Minoru Yamasaki who proposed that we use narrow windows in the WTC towers to give people a sense of security as they looked down from on high. Our contribution was to make the closely spaced columns the fundamental lateral-force-resisting system for the two towers. The tubular framing system also precluded the need for the customary 30-foot column spacing in interior areas, making column-free, rentable space structurally desirable.



In support of Yamasaki’s design, during the construction, before the windows were installed, I noticed that people felt comfortable walking up to the outside wall, placing their hands on the columns to either side, and enjoying the wonderful view. If the wind was blowing toward them, they would walk right up to the outside wall; however, if they felt even a trace of pressure from a breeze from behind, they would at least hesitate before walking to within five feet of the wall . . . and many would not approach the wall at all.



Another structural innovation was the outrigger space frame, which structurally linked the outside wall to the services core. This system performed several functions. First, gravity-induced vertical deformations between the columns of the services core and the columns of the outside wall were made equal at the top of the building; at other levels, the differential deformations were ameliorated. Second, wind-induced overturning moments were resisted in part by the columns of the services core, thus providing additional lateral stiffness. Finally, the weight of, and the wind-induced overturning moment from the rooftop antenna (440 feet tall) was distributed to all columns in the building . . . adding additional redundancy and toughness to the design.



Prefabricated structural steel was used to an unprecedented degree. Two examples will give you an idea. Exterior wall panels three stories high and three columns wide were fabricated in Washington state. Floor panels 60 feet long and 20 feet wide, complete with profiled metal deck and electrical distribution cells, were assembled in New Jersey from components fabricated in Missouri and elsewhere.



We mounted a comprehensive program to determine the design-level gradient wind speed for New York City. Data were collected from all available sources and incorporated into an appropriate mathematical model. For the first time, we were able to obtain full-scale measurements of the turbulent structure of the wind and compare them with the turbulent structure obtained in a boundary-layer wind tunnel. This was done by mounting anemometers atop three high points in lower Manhattan and by making similar measurements on our wind tunnel model (Figure 1 see PDF version). The boundary-layer wind tunnel was further developed and used to predict the steady-state and dynamic forces on the structure and the glazing, as well as to develop the dynamic component of wind-induced motion of the structure. Jensen and Frank, two brilliant Danish engineers, had discovered that surface roughness in the wind tunnel allowed them to accurately predict wind pressures on farm structures. We expanded this technology upward to 110 stories by using a wind tunnel, constructed under the guidance of Dr. Jack E. Cermak, (NAE, Colorado State University), designed to study the dispersion of gases emitted from tall stacks. Thus, for the first time, we were able to analyze the steady-state and dynamic components of wind-induced structure deflections.



We designed motion simulators to determine acceptable levels of wind-induced structure motion. The simulators measured the response of human subjects to lateral motions similar to those anticipated for the two towers. The accumulated data were used to establish the criteria for an acceptable level of the swaying motion of the two towers.



A viscoelastic damping system was invented and patented to ameliorate the wind-induced dynamic component of building motion by dissipating much of the energy of that motion . . . acting more or less like shock absorbers in an automobile. With these dampers, we could control the swaying motion without having to use large quantities of structural steel. This was the first time engineered dampers were used to resist the wind-induced swaying motion of a building.



A theory was developed for integrating the statistical strength of glass with the dynamic forces of the wind to predict the breakage rate of the glass of the exterior wall. Coupled with a testing program of actual glass samples, we were able to determine rationally the necessary thickness and grade of the glass. Another theory was developed to predict stack action and temperature-induced and wind-induced airflow within a high-rise building; an understanding of these airflows is crucial to controlling fire-generated smoke and reducing the energy consumption of the building. A theory to predict appropriate “parking floors” for elevators was developed to minimize the oscillation of elevator cables, which oscillation is stimulated by the wind-induced, swaying motion of a building. Figure 2 is a comparison of the wind-induced dynamic components of the structure response of the two towers and of the Empire State Building.



The two towers were the first structures outside of the military and nuclear industries designed to resist the impact of a jet airliner, the Boeing 707. It was assumed that the jetliner would be lost in the fog, seeking to land at JFK or at Newark. To the best of our knowledge, little was known about the effects of a fire from such an aircraft, and no designs were prepared for that circumstance. Indeed, at that time, no fireproofing systems were available to control the effects of such fires.



We developed the concept of and made use of the fire-rated shaft-wall partition system, which is now widely used in place of masonry and plaster walls. At that time, masonry was the standard enclosure for elevators, stairs, duct shafts, and other internal structures. The partition system eliminates the need for within-the-shaft scaffolding, which was the common practice, provides more smoke-proof stairs and shafts, and improves safety on the job site. The shaft-wall completely changed the nature of the structural system for the two towers, making them the first of a new kind of high-rise building.



A computerized system was conceived and developed for ordering structural steel and producing shop drawings for structural steel, as well as the operation of digitally directed tools, all directly from digital information developed as a part of our design.



When the two towers were finished, the World Trade Center stood proud, strong, and tall. Indeed, with little effort, the towers shrugged off the efforts of terrorist bombers in 1993 to bring them down. The events of September 11, however, are not well understood by me . . . and perhaps cannot really be understood by anyone. So I will simply state matters of fact:



The events of September 11 ended the lives of almost 2,900 people, many of them snuffed out by the collapse of structures designed by me. The damage created by the impact of the aircraft was followed by raging fires, which were enormously enhanced by the fuel aboard the aircraft. The temperatures above the impact zones must have been unimaginable; none of us will ever forget the sight of those who took destiny into their own hands by leaping into space.



It appears that about 25,000 people safely exited the buildings, almost all of them from below the impact floors; almost everyone above the impact floors perished, either from the impact and fire or from the subsequent collapse. The structures of the buildings were heroic in some ways but less so in others. The buildings survived the impact of the Boeing 767 aircraft, an impact very much greater than had been contemplated in our design (a slow-flying Boeing 707 lost in the fog and seeking a landing field). Therefore, the robustness of the towers was exemplary. At the same time, the fires raging in the inner reaches of the buildings undermined their strength. In time, the unimaginable happened . . . wounded by the impact of the aircraft and bleeding from the fires, both of the towers of the World Trade Center collapsed.



Figure 3 shows the comparative energy of impact for the Mitchell bomber that hit the Empire State Building during World War II, a 707, and a 767. The energy contained in the fuel is shown in Figure 4. Considerations of larger aircraft are shown in Figures 5 and 6. The physical sizes of these aircraft are compared with the size of the floor plate of one of the towers in Figure 7. These charts demonstrate conclusively that we should not and cannot design buildings and structures to resist the impact of these aircraft. Instead, we must concentrate our efforts on keeping aircraft away from our tall buildings, sports stadiums, symbolic buildings, atomic plants, and other potential targets.



The extent of damage to the World Trade Center is almost beyond comprehension. Figure 8 shows an overview of the site and the location of the various buildings. We did not design the superstructures of Building 3 (Marriott Hotel) or of Building 7. Towers 1 and 2, which were totally destroyed, left behind utter chaos surrounded by towers of naked structural steel. The remaining steel towers were in some ways painful beyond belief, in other ways strangely beautiful. Building 3 collapsed down to a structural transfer level designed by us. Fortunately, the people who sought refuge in the lobby of the hotel, which was located immediately below the transfer level, survived. Buildings 4, 5, and 6 remained standing but were partially collapsed by falling debris; all three burned for about 24 hours. Although there was nothing special about the structural design of these buildings, the remaining structures stalwartly resisted the impacts of the wrecking ball. Building 7, after burning for nearly 10 hours, collapsed down to a structural transfer level designed by us. The below-grade areas under Towers 1 and 2 were almost totally collapsed; in areas outside of the towers they were partially damaged or collapsed.



In my mind, the loss of life and the loss of the buildings are somehow separated. Thoughts of the thousands who lost their lives as my structures crashed down upon them come to me at night, rousing me from sleep, and interrupting my thoughts at unexpected times throughout the day. Those who were trapped above the impact floors, those who endured the intense heat only to be crushed by falling structure, are merged with those who chose to take control of their own destinies by leaping from the towers.



The loss of the buildings is more abstract. The buildings represented about 10 years of concerted effort both in design and in construction on the part of talented men and women from many disciplines. It just isn’t possible for me to take the posture that the towers were only buildings . . . that these material things are not worthy of grieving.



It would be good to conclude this journey in a positive mode. We have received almost a thousand letters, e-pistles, and telephone calls in support of our designs. The poignant letters from those who survived the event and from the families of those who both did and did not survive cannot help but bring tears to one’s eyes. They have taught me how little I know of my own skills and how fragile are the emotions that lie within me. Yes, I can laugh, I can compose a little story . . . but I cannot escape.



Do those communications help? In some ways they do; in others, they are constant reminders of my own limitations. In essence, the overly laudatory comments only heighten my sense that, if I were as farseeing and talented as the letters would have me be, the buildings would surely have been even more stalwart, would have stood even longer . . . would have allowed even more people to escape.



Yes, no doubt I could have made the towers braver, more stalwart. Indeed, the power to do so rested almost solely with me. The fine line between needless conservatism and appropriate increases in structural integrity can only be defined after careful thought and consideration of all of the alternatives. But these decisions are made in the heat of battle and in the quiet of one’s dreams. Perhaps, if there had been more time for the dreaming . . .



Recognition must be given to the Port Authority of New York and New Jersey, who provided unparalleled support and guidance throughout the design and construction of the World Trade Center. Their understanding of the need to explore new avenues and break new ground reflected their sound professional and technical posture. We could not have asked for a more competent, more responsible, or more involved client. The men and women of our company who participated in the design and construction are without parallel. Their talents, energies, and good humor carried us through a most arduous journey. Dr. Alan G. Davenport (NAE) provided invaluable knowledge, insight, and support; his willingness to join us on this journey made many facets of the design possible. Minoru Yamasaki and his team, particularly Aaron Schreier, and the office of Emery Roth and Sons produced a wonderful architecture while making the entire process both fun and exciting. Richard T. Baum (NAE), of Jaros, Baum & Bolles, headed the HVAC (heating, ventilation, air conditioning) team and taught me much about these systems. Joseph R. Loring provided full professional services as the electrical engineer for the project.



In conclusion, the events of September 11 have profoundly affected the lives of countless millions of people. To the extent that the structural design of the World Trade Center contributed to the loss of life, the responsibility must surely rest with me. At the same time, the fact that the structures stood long enough for tens of thousands to escape is a tribute to the many talented men and women who spent endless hours toiling over the design and construction of the project . . . making us very proud of our profession. Surely, we have all learned the most important lesson—that the sanctity of human life rises far above all other values.



