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Remembering the audacity of the twin towers

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In 1962, Minoru Yamasaki was selected as the architect. Few outside the project, however, knew at the time that the steel structure that made his vision possible was conceived by a relatively unknown Seattle-based engineering firm, Worthington, Skilling, Helle and Jackson. The partner-in-charge was John Skilling, and the lead structural engineer was 34-year-old Leslie Robertson.

To Mr. Robertson, “the Trade Center was a matter of expanding the basic ideas of structure." It was to be much more.

Based on accepted engineering practice, skyscrapers had topped out in 1931 with the Empire State Building's 102 floors of sequentially diminishing floor space. Robertson was going to rewrite that limitation, with each tower providing more than twice the area contained in the Empire State Building.

The young engineer had already demonstrated an aptitude for solving challenging problems. He designed uniquely fortified foundations for a 5-mile-long bridge. He was among the first to use cross-bracing to build more efficiently. And he designed liquid-filled structural columns to fireproof exposed steel.

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.

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."

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