Remembering the audacity of the twin towers
The soaring twin towers of the World Trade Center became an affirmation of the American value of dreaming big. To the engineer who designed them, their loss on 9/11 remains heartbreaking, but he's found the resilience to keep dreaming.
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The largest concern in engineering the World Trade Center was the wind, an even greater force than the downward load of the buildings, explained Robertson in a recent interview. Unlike traditional skyscrapers that relied on a dense grid of columns and beams, Robertson's and Skilling's solution thoroughly reconfigured the towers’ support. The two men designed a dense row of columns around the perimeter of each structure and another set of columns circumscribing the buildings' core. Long prefabricated floor trusses connected the two sets of columns.Skip to next paragraph
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In creating far more open floor space than in traditional skyscrapers, this structural configuration required the exterior walls to carry extraordinary weight. According to Robertson, this made them "even more robust than traditional skeletal walls to counter the lateral force of the wind."
To control the sense of movement inside (for the buildings needed to sway slightly to remain standing), his team, along with the 3M Corporation, devised and installed 11,000 unique viscoelastic dampers, i.e., structural shock absorbers.
The World Trade Center marked the first use of computer modeling to forecast how a structure would perform, leading to more precise design specifications of materials and construction. Robertson also used this technology to confirm the structures could withstand a hit by the largest plane of the time – a Boeing 707.
No one involved with the World Trade Center, however, was able to project what would happen to the jet fuel.
The National Institute of Standards and Technology’s investigation of the collapse of the World Trade Center – the official government study – found "computing resources and software necessary to conduct these analyses did not exist in the 1960s." Still, NIST concluded that if the structures’ fireproofing hadn’t been blown off by the impact of the jets, the towers would likely have remained standing, just as Robertson said.
Since 9/11, some studies, including one issued by the Federal Emergency Management Agency and the American Society of Structural Engineers, celebrated the buildings for having stood after impact for as long as they did, giving so many occupants time to exit. Engineers cited in the documentary series "NOVA" said, in the program "Why the Towers Fell," the search for efficiency may have produced structures that could have been more susceptible to progressive failure. Ten years after the disaster, the subject remains contentious and difficult to discuss dispassionately.
Prof. Robert Bea of the University of California, Berkeley – one of the country's leading forensic engineers who is heading the investigation of the Macondo oil well blowout in the Gulf of Mexico last year – describes Robertson's design as excellent. "One part of me as an engineer looks at that efficiency achieved and says, 'Well that's exactly what we should be doing.'