World Trade Centre engineer revisits the tragedy
By Bonnie Azab Powell
[Published Autumn 2002]

More than 100 civil engineering students and faculty filled the seats and lined the walls of Davis Hall last spring to hear Leslie Robertson discuss the 1993 and 2001 attacks on the World Trade Center.

Robertson showed several historical photos detailing the construction of the World Trade Center. Photo source: Port Authority of New York and New Jersey

A 1952 Berkeley civil engineering graduate, Robertson and his then-partner John Skilling were the original structural engineers for the Twin Towers. The offices of his firm, Leslie E. Robertson Associates, helped repair the structural damage caused by the February 1993 bombing. Robertson remains deeply affected by the responsibility he feels for the towers’ collapse.

Robertson took the audience on a visual journey through the birth of the World Trade Center. He detailed the many innovations his firm used, such as extensive prefabrication of column and spandrel wall panels and installation of viscoelastic damping units to reduce wind-induced motion. His firm also conducted the first studies of boundary layer wind tunnels and human sensitivity to building motion for the project, both now widely used.

Robertson praised the Port Authority of New York and New Jersey as the "best client an engineer could ask for," while showing archival photos of the construction process and recounting how a helicopter accidentally dropped a floor panel into the Hudson River.

Moving forward to the 1993 bomb set off in the Center’s lower parking levels, Robertson told how the blast left five floors of rubble sitting on the buildings’ cooling machines. He flicked through slides of twisted cars and mangled I-beams, then a diagram of the temporary bracing.

After the post-bombing repairs were complete, "we finished up and again went to sleep, not worrying about anyone," Robertson said quietly before advancing to the next slide—a September 11 photograph of flames billowing out of the two towers.

Leslie E. Robertson. Photo: Andre Souroujon

Overcome by emotion, he silently showed more of the now-familiar images from that day’s aftermath. In a soft voice, he began to talk about the comparative blast power of the two planes’ fuel loads. The Oklahoma City bomb that destroyed the federal building, for example, was the equivalent of 192 liters of jet fuel. The Boeing 767 that hit the first tower was estimated to be carrying 45,600 liters of fuel.

"A lot of people have told me, ‘You should have used more concrete in the structure,’" Robertson said. (A concrete-and-steel frame is believed to be more fire-resistant.) He showed a chart plotting the strength-versus-temperature-performance of steel and concrete. At the incendiary levels that raged in the towers, the two materials differ little in performance.

Taking questions from the audience, Robertson recounted his fear that Robertson & Associates would lose its commission to build the Shanghai World Financial Center. "That’s it, the building has collapsed—let’s get a new engineer." But the client opted to retain both the firm and the original design. When asked whether the design of skyscrapers should in fact change to protect them from attack by 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."