How 9/11 Changed The Way Skyscrapers Are Designed

A new, safer model of skyscraper rises from the site of the attack.


The offices here at Fast Company enjoy a most remarkable view. From our aerie on the 29th floor of 7 World Trade Center, we look out over the New York harbor, Ellis Island, the Statue of Liberty, and the occasional jetliner floating down the Hudson. More breathtaking yet is the view directly below our windows: that 16-acre construction site known as Ground Zero.


When we moved to Lower Manhattan in April of 2007, the view was more dispiriting than inspiring. A muddy pit was shored up by concrete walls; every once in a while, a lone train from New Jersey snaked through a partially visible tunnel in the center. Without the World Trade Center’s office workers, the area had become a commercial wasteland, apart from tourist attractions like Century 21 and J&R Electronics. The city had to offer incentives for urban pioneers.

The turnaround started shortly after 9/11, when developer Larry Silverstein called the architects at Skidmore, Owings & Merrill (SOM) and insisted that he wanted to rebuild on the site. “At the time, the press was saying high-rises were the product of a bygone era,” says Nicholas Holt, SOM’s director of technical architecture. “Even Larry was asking, ‘If I build this, will anybody be willing to rent?’ ” The site, after all, was still a smoking pit.

SOM, which for 75 years has been the go-to architectural firm for companies wanting cutting-edge thinking in skyscraper innovation, knew the future of high-rises rested on one thing: making people feel safe working in tall towers. The first building to be erected adjacent to ground zero would be ours, 7 World Trade. Everyone involved knew it would become a test case for addressing the design failings of the ill-fated towers and forging a model for how skyscrapers should be built in the future.

A time lapse video created using the Canopy app

One of the mysteries that SOM had to address was why the towers collapsed in the first place. Holt, whose office was just blocks away from the World Trade Center, remembers looking out his window and thinking that the buildings’ sprinklers would eventually kick in. “I never imagined they would fall,” he says. “I had made it north to Chinatown when somebody stepped out of a bodega and said, ‘They’re both gone.’ “

As we now know, the impact of the planes alone was not enough to cause the towers’ collapse. The combination of the impacts and the fires in their aftermath were what proved fatal. Not only had the planes knocked out parts of the buildings’ structural frames, they also severed and disabled the sprinkler systems’ supply pipes. As the fires continued, the remaining structure weakened to the point where the failure of one critical structural element begat the failure of the next–what architects call “progressive collapse.”


Now, Holt says, top-tier buildings like ours have been redesigned to prevent that kind of collapse. These skyscrapers have steel connections capable of redirecting the path of the upper floors’ load downward through other structural members if one should fail. And sprinkler supply lines have been located within an impact-resistant core–a major difference from the Twin Towers. Both innovations are now part of New York City building codes. In addition, the newest SOM buildings have two interconnected standpipes, so that if one should fail, the other can compensate.

Another issue the Twin Towers’ disaster exposed was the difficulty of evacuating lots of people from very high floors. “The survival rate below the planes’ impact was very high,” Holt says. “Above it was very low. That was entirely linked to the damage to the core; the inability to navigate the stairs; and the heat, flames, and smoke not being mitigated by a sprinkler system.” Designers knew that if tenants were going to be attracted to the upper reaches of these buildings, they needed to feel that they could get out safely. Now, the best American high-rise designs borrow from existing international thinking on safety. In addition to designing wider staircases and building separate stairs for firefighters (a strategy borrowed from the British, who have long practiced this), SOM is pioneering an elevator-assisted exit system that would help people on the highest floors get out faster.

The view from our offices

Currently proposed for a 108-story tower in South Korea, the system reduces evacuation time by more than 20%. The Burj Kalifa, the 168-floor tower in Dubai that currently holds the record as the world’s tallest building, uses a similar system. The idea is that a building’s occupants can take the stairs to designated protected refuge areas on specific floors, at which point they can take elevators–called “lifeboats”–down to exits on the ground floor. The approach directly contradicts the conventional wisdom that you should never take an elevator in a burning building because its electronics could be compromised by the water used to fight the fire. The World Trade Center made clear that this approach needed modification, so some new skyscrapers, including 1 World Trade Center (originally known as the Freedom Tower), will allow for elevator use per instructions from a fire safety director or emergency responder.

Today, 1 World Trade is rising opposite the southwest corner of our building. The SOM-designed 102-story skyscraper, whose spire will reach 1,776 feet into the sky, will open in the first quarter of 2014. In a sign of how far we’ve come since the bleak days following the attack, the developer recently signed a 25-year, $2 billion lease with Conde Nast, the publisher of Vogue, The New Yorker, and Vanity Fair, for 1 million square feet of space, to house more than 3,000 employees. It’s a move that speaks to both New Yorkers’ resiliency and America’s traditional optimism. Holt says he was surprised at how quickly the post-9/11 conversation became so forward-looking. “It wasn’t about hunkering down,” he says. “It was more about quality of life and responsible development. It was like the phoenix rising from the ashes.”

Maybe it’s a sign that the big issue facing our future neighbors at 1 World Trade Center isn’t “how can a flock of stiletto-wearing fashionistas get down 102 stories safely?” Instead, they’re worrying, “Where, in a busy downtown streetscape, can we park our armada of town cars?” Believe it or not, that’s progress.


A version of this article appears in the September 2011 issue of Fast Company.

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[Body image by Ofer Wolberger for Fast Company]

About the author

Linda Tischler writes about the intersection of design and business for Fast Company.