World Trade Center remembered
Structural engineer Leslie Robertson, ’52, reflects on the towers he helped build

By Bonnie Azab Powell, College of Engineering

This aerial view of the World Trade Center in the late 1960s shows how the “skin” of exterior steel columns followed the floor-by-floor construction of the core. Photo courtesy of Port Authority of New York and New Jersey

17 April 2002 | More than 100 civil engineering students and faculty filled the seats and lined the walls of 502 Davis Hall on April 8 to hear Leslie Robertson discuss the 1993 and 2001 attacks on the World Trade Center.

A 1952 Berkeley civil engineering graduate, Robertson and his then-business partner John Skilling were the original structural engineers for the Twin Towers. His firm, Leslie E. Robertson Associates — which helped repair the structural damage caused by the February 1993 bombing — is located in lower Manhattan overlooking Ground Zero.

Robertson remains deeply affected by the towers’ collapse. “The World Trade Center was a team effort, but the collapse of the World Trade Center is my responsibility, and that’s the way I feel about it,” he told a New Yorker reporter in a Nov. 19 article, “The Tower Builder.”

Design and construction
The Cal alum, after confessing to feeling “a little hinky” following knee-replacement surgery, took his Berkeley audience on a visual journey through the life of the World Trade Center.

Robertson worked for the firm selected in 1963 to oversee structural aspects of the construction of the World Trade Center. In his talk, he detailed many innovations employed on the project. Prefabri-cated columns and wall panels were used extensively, and the team designed mechanical units to help dampen the swaying that plagues tall buildings in windy locations. Robertson also tested models of the towers in a wind tunnel to see how they would perform and conducted the first studies of human sensitivity to building motion by observing the reactions of people in motion simulators.
While praising the Port Authority of New York and New Jersey as the “best client an engineer could ask for,” Robertson poked gentle fun at the Twin Towers’ mercurial architect, the late Minoru Yamasaki. “I learned how to think like another person so I could initiate changes in the project smoothly, without creating antagonism,” he said.

Documenting the construction process with archival photos, he recounted how a helicopter accidentally dropped a floor panel into the Hudson River. On another occasion, a camera crew arrived to film him conducting a structural inspection. More embarrassed than afraid, he felt compelled to venture nervously out to the edge of the top floor, which was slick with ice.

Towers under seige
Moving forward in time to 1993, when terrorists set off a bomb in the center’s lower parking levels, Robertson showed how five floors of the building collapsed onto the towers’ cooling machines. Projecting slides of mangled I-beams and autos flattened by falling debris, he recalled how he asked steelworkers to set aside one of the twisted columns — now installed in his garden as a piece of outdoor art.

“Then we finished up and again we went to sleep, not worrying about anyone,” he said quietly, before advancing to a Sept. 11 photo of flames billowing from one of the two towers.

Overcome by emotion, Robertson was silent as he showed haunting, now-familiar images taken in the aftermath of that terrible event. In a soft voice, he began to talk about the blast power of the jet fuel in the two hijacked planes. As a point of comparison, the bomb that destroyed the Oklahoma City federal building was the equivalent of 192 liters of jet fuel; the Boeing 767 that hit the first tower carried an estimated 45,600 liters.

Like many high-rises built in the 1960s, the Twin Towers were constructed with their weight distributed between a hollow steel core (containing services like elevators) and steel columns around the perimeter, maximizing open floor space. Many believe the older high-rise design, in which steel columns are often encased in concrete, is more fire resistant.

“A lot of people have told me, ‘You should have used more concrete in the structure,’” said Robertson. However, his chart plotting the strength of steel vs. concrete at various temperatures showed that at the incendiary levels that raged in the towers, the two materials become similarly weak.

In response to questions from the audience, Robertson recounted his anxiety, following the collapse of the Twin Towers, that his firm would lose its commission for its current high-rise project, the Shanghai World Financial Center: “That’s it, the building has collapsed — let’s get a new engineer,” he imagined his clients saying. Instead, they opted to retain both the New York firm and the original design.

Asked whether the design of high-rises should change to protect them from the impact of large airplanes, he reflected for a moment. “I don’t think we can solve the problem that way,” he said. “The problem is with us, not our buildings, and it will be with us for a very long time.”