When two planes crashed into the Twin Towers on Sept. 11, 2001, architects knew skyscraper design would never be the same.
One of those was Dan Kaplan, a senior partner at the firm FXFOWLE. He watched the South Tower being hit from the roof of his office in Chelsea.
He said 9/11 revealed a major flaw in skyscraper design.
"One of the things that happened was that the explosion ripped through the spine of the building," he said.
The core of the original Twin Towers was made up steel x-bracing and sheetrock. It allowed the buildings to be tall, and even sway in the wind, but it also meant that the towers collapsed while people were still evacuating. Also, essential services, like power and water were cut off, which meant the towers were dark for evacuees and first responders, and firefighters couldn't access water to put the fire out.
Kaplan was part of a community of architects that began thinking about how to design a radically different building. One that could withstand a massive impact.
Standing on the 35th floor of Eleven Times Square, a skyscraper he helped design that opened in 2011, he noted that open floor spaces haven't gone away, but now when you go into a new skyscraper, what's behind the walls is different.
In the recently opened One World Trade Center, and in Kaplan's building at Eleven Times Square, the core or spine of the building is now reinforced with a beefy, 3-foot wall of concrete. All essential utilities, as well as the elevators and the stairs, are built into what is essentially a concrete bunker.
And unlike the old World Trade Center, the city's newest skyscrapers are designed for quick and efficient evacuations.
The narrow stairs in the old World Trade Center helped force a slow evacuation that ultimately cost lives. In Kaplan's new building, the stairs are more than 6 feet wide.
"This is a stair that deals with the population coming down and then if need be, fire fighters or first responders going in the opposite direction," he said.
Kaplan pointed out another subtle but important changes to buildings since 9/11. Historically, city architects had designed buildings that came right up to the property line. But that's not ideal from a security perspective. Embassies and sensitive buildings are usually behind walls and fences, something not really feasible in a city.
So now, many new buildings are set back from the street. "Even a modest pull-back is a good thing," he said.
At Eleven Times Square, there's a wide open lobby and outside on the street there's a row of black granite bollards that light up at night. They're to meant to prevent a vehicle from crashing into the building.
"We took it as design opportunity to make something gracious and welcoming, but it also has a defensive function as well," Kaplan said.
Another thing that 9/11 did was expose how antiquated our building codes have become.
"New York City 100 years ago was the cat's meow, it had one of the biggest and best building codes out there," said Glenn Corbett, a professor of fire science at John Jay College. He helped design new building codes after September 11th. He said at the end turn of the 20th century, New York was a leader when it came to skyscraper innovations. But "by 2000, the last building code had been adopted in 1968."
Corbett and families of September 11th victims urged the city to adopt the international safety standards for tall buildings and by 2008, the city had done so. In the latest code revision from 2014, the Department of Buildings requires buildings taller than 120 feet to have one elevator that is protected for firefighters to use in an emergency, and requires buildings taller than 420 feet to have an additional stairway or an elevator for evacuations.
Other recent requirements include a sprinkler system for new residential buildings taller than three stories, interconnected hard-wired smoke alarms in all new buildings, and standpipes that would provide more water for firefighters in an emergency.
Now the the Department of Buildings updates building codes every three years; experts say the city's newest buildings are among some of the safest skyscrapers in the world.