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What 9/11 Taught Us About Designing Skyscrapers

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.

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.

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-wearingfashionistas 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.

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Photo by Ofer Wolberger

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  • Victor Guarnera

    I think the article is spot on.  I would like to add some further background information.  The buildins were designed to withstand the impact of a 707 (I believe that would be the model 120) which at the time of the design process probably had max take-off weight of about 240,000 pounds.  I also believe that the thinking at that time was that if this plane were to impact the structure, the plane would be be in Instrument Conditions (Low cloud deck), at the end of its trip, looking for one of hte major airports, and would therefore be flying slowing as the approach speed would be about 150 MPH, and low on fuel.   This is a far cry from the actual event in which a 767 with a Max takeoff weight of 395,000, relatively full of fuel hit at 466 MPH for the North Tower, and 520 MPH for the South Tower.  I am sure that an engineer worth his/her salt would tell you that there is a huge difference in the amount of kinetic energy that was expended by the actual event and tha tthat was anticipated in the design consideration.  And as the NIST investigation has very clearly shown, that force not only went completely through the 80' wide core (in the North building) and the 140' long core in the South building, but also dislodged the fire redardent materials on the girders, all in addition to have a hige amount of fuel with which to ignite a huge BBQ with an extremely hot fire.  All-in-all, I think that the buildings structure held up remarably well considering the condition.  Of course none of us on the inside ever thought they would collapse, that is until the South did just that.       Thanks for letting me try to set the record straight.

  • Frederic Moraillon

    What about putting parachutes on each floors? Companies should be responsible for adding them (like life-jackets on a bot) and training their employees in base-jumping.

  • theeastwatch

    It doesn't really matter how you build a skyscraper, when someone brings them down with explosives.

  • Linda Tischler

    Thanks for the additional information, Barry. What a terrifying bit of commentary! So glad you managed to get out, despite the hurdles. 

  • Joe E

    Ms. Tischler, in your article you state "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.""

    There are many (and I do mean many) prominent structural engineers and architects and demolition experts, who have shown demonstrably that  the explanation you give above for the collapse of the WTC buildings (not to mention WTC 7 which was never hit by a plane) is simply not true, even though it the accepted explanation by many, including NIST. 

    The evidence overwhelmingly points to the Twin Towers being destroyed by explosives and not by fires resulting from the ignition of the jets' fuel.

    I invite you and other Fast Company readers to visit:   and find out why the official story doesn't add up.

    Here is a summary (there is much more in-depth info on the site):

    the Twin Towers' destruction exhibited all of the characteristics of destruction by explosives:
    Destruction proceeds through the path of greatest resistance at nearly free-fall acceleration

    Improbable symmetry of debris distribution
    Extremely rapid onset of destruction

    Over 100 first responders reported explosions and flashes

    Multi-ton steel sections ejected laterally

    Mid-air pulverization of 90,000 tons of concrete & metal decking

    Massive volume of expanding pyroclastic-like clouds

    1200-foot-diameter debris field: no "pancaked" floors found

    Isolated explosive ejections 20–40 stories below demolition front

    Total building destruction: dismemberment of steel frame

    Several tons of molten metal found under all 3 high-rises

    Evidence of thermite incendiaries found by FEMA in steel samples

    Evidence of explosives found in dust samples

    And exhibited none of the characteristics of destruction by fire:
    Slow onset with large visible deformationsAsymmetrical collapse which follows the path of least resistance
    (laws of conservation of momentum would cause a falling, intact, from
    the point of plane impact, to the side most damaged by the fires)Evidence of fire temperatures capable of softening steelHigh-rise buildings with much larger, hotter, and longer-lasting fires have never collapsed.

  • Barry Morse

    Thank you for spotlighting the progress of ground zero reconstruction.  I worked in the old South Tower during the 1990's, and I have one comment about the use of elevators in a fire.  I'm not a building expert but I do work with engineering problems. 

    Your article states "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."
    You can design floors with and ever-so-slight tilt away from the center, and design canalization to capture water that might enter an elevator shaft.  And besides, you don't really need electronics in an elevator except for the doors, which can still be pried open in an emergency. The real problem is that the elevator shaft is a chimney. Heat and smoke find the shaft and rise, pulling in even more heat and smoke, and it accelerates from there.  Opening the elevator doors would accelerate the fire by reducing resistance against the updraft.  I guess that is why the elevator shafts and the emergency stairwells in the original towers were staggered, so that they would not offer a continuous chimney from bottom to top.  The break was a the 76th floor.  During the first bombing, we took the stairwell from the 106th floor down to the 76th floor, then crawled across the lobby to a different stairwell.  While crawling across the lobby floor, you could feel the force of the hot chimney wind in your face.