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World Trade Center engineer revisits the
tragedy
By Bonnie Azab Powell
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.
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| 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.
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| 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."
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