The Structural Design Of The Twin Towers
One of the structural engineers of the Twin Towers reflects on the destruction of the 9/11 terrorist attacks.
Published January 1, 2002
By Linda Hotchkiss Mehta
Academy Contributor

Although he lost many friends on September 11, Academy Member Leslie Robertson is thankful to be among the fortunate New Yorkers who did not lose family members or coworkers, as did thousands of others. Still, the shock and grief he felt during and after the attacks might be somewhat akin to the incomparable horror of suddenly losing two dear children.
For Robertson, now Director of Design at Leslie E. Robertson Associates, Consulting Structural Engineers, the World Trade Center has been a central part of his professional life –– the defining project that launched a distinguished career –– since the early 1960s. Together with then partner John Skilling and architect Minoru Yamasaki, Robertson and his team conceived, and helped develop the structural designs for five of the seven buildings in the WTC complex, including the 110-story Twin Towers.
An active member of the Academy’s Human Rights of Scientists Committee, Robertson was in Hong Kong on September 11 discussing a new skyscraper when he first received word that a plane had hit the WTC’s north tower. Everyone believed that it had been a helicopter or other small aircraft. He then was able to reach his wife, Saw-Teen See, an Academy Member and engineer in her own right, who reported the seriousness of the event and that the second tower had been struck. He rushed to his room to prepare for a return to New York.
The Structural Strength of the Towers
After turning on the TV and registering the shock of witnessing the dreaded images of death and destruction taking place, Robertson said his memory of the following hours are somewhat blurred. “You wanted to reach out and stop it,” he recalled, “but there was nothing you could do.”
Although he’s still plagued with thoughts about “what we might have done differently,” Robertson acknowledged in an interview that –– as many Members and other colleagues have told him –– the structural strength of the towers allowed them to stand long enough for perhaps 25,000 occupants to escape after each of the Boeing 767 aircraft crashed into them. The north tower was struck between the 94th and 99th floors at 8:45 a.m. and did not collapse until 10:28 a.m.; the south tower, which was impacted at a lower level, between the 78th and 84th floors, was the first to collapse, at 9:59 a.m., 53 minutes after the second aircraft struck.
“When I started work on this project, the tallest building I’d worked on had only 22 floors,” Robertson said. “The WTC engineering was a first of a new kind of high-rise building.” Aware of the military aircraft that hit the Empire State Building in a dense fog in 1945, Robertson said, “I thought we should consider the structural integrity that would be needed to sustain the impact of a (Boeing) 707 –– the largest aircraft at that time.”
Achieving Structural Strength

Robertson added, “We didn’t have suicidal terrorists in mind.” Rather, he was considering an accident, a 707 flying at low speed, most likely lost in a dense fog. To achieve the structural strength, Robertson and his team designed the Twin Towers as steel boxes around hollow steel cores. An unusually large number of rigid, load-bearing columns of hollow-tube steel –– each column being only 14 inches wide and set just 40 inches on center –– supported the Towers walls.
Because the 767s were traveling at high speeds, were somewhat larger than 707s and each carried about 80 tons of jet fuel, Robertson said, “the energy that was absorbed by the impact was not less than three-times, and probably as much as six-times greater than the impact we had considered.
“The idea that someone might plant a plastic explosive or the like somewhere in the structure was considered in the design. The structure was redundant –– two-thirds of the columns on one face of each of the two towers were removed (by the aircraft) and yet the buildings were able to stand. But it was the combination of the impact from the speeding aircraft and the burning jet fuel –– both the kinetic and petrochemical energy released –– that ultimately brought them down.”
Impact on Future Design
Robertson said he doubts that the attacks will have a major impact on the structural design of new tall structures. “If you design buildings as fortresses that can withstand anything, then the terrorists will just avoid the fortresses,” he said. “There are plenty of other, smaller buildings that could be targets, and the threat of chemical or biological weapons is an even greater concern.
“Structural engineering is applied science. If a ceiling sags or a lobby is too drafty, life goes on. But structural reliability has been high; building collapses are rare. When they do occur, they’re usually caused by natural events –– wind or water or the ground shaking. I don’t believe we should engineer against the kind of event that happened on September 11, much less the impact and fire that could be created by the much larger Boeing 747 or the new AirBus 380.”
Robertson concluded that the solution lies in confronting the root causes of hatred among mankind: “There’s no end to the number of ways that man can do harm to man.”