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How Has Skyscraper Design Changed Since 9/11?

September 6, 2011 at 12:00 AM EDT
The collapse of the Twin Towers served as a stark and stunning reminder to architects and engineers of the value of using concrete to protect skyscrapers from the ravages of fire. Miles O'Brien reports on how the design of skyscrapers has changed since 9/11. This report was produced in collaboration with the PBS program "NOVA."

JEFFREY BROWN: Next, the second in our series of reports on how the attacks of 9/11 have affected people in this country and around the world.

Tonight, we look at the design and engineering of skyscrapers then and now.

NewsHour science correspondent Miles O’Brien has our story, produced in collaboration with the PBS program “NOVA.”

MILES O’BRIEN: They don’t build them like they used to. And for some, the story of 90 West Street in Lower Manhattan is concrete proof they should. On 9/11, debris from the Twin Towers rained down on this historic building next door. It was completely engulfed in flames.

JOHN NORMAN, New York Fire Department: We thought that the building was too dangerous to approach. And the fact that we had been told it had been evacuated when the towers were struck made a decision very simple for us, that we weren’t going to fight that fire.

Retired FDNY Fire Chief John Norman is an expert on fighting high-rise fires. We met on the sidewalk in front of 90 West. Built in 1907, it is now an apartment building. Norman still marvels as how well it endured the conflagration 10 years ago.

JOHN NORMAN: This building had one steel beam that was affected by heat. It dropped a total of 1.5 inches. That was a horizontal column, so it was not a critical load-bearing member. The entire structural skeleton remained fine. And it’s, as you see, still in use today.

MILES O’BRIEN: I’m sorry, one beam sagged? That’s it?

JOHN NORMAN: One beam.

MILES O’BRIEN: Ninety West survived 9/11 because skyscrapers of that vintage were built like fortresses.

JOHN NORMAN: Everything was reinforced. You had a structural — a heavy structural steel skeleton much like the towers. But these were protected with poured concrete or terra-cotta block tile, which gives tremendous resistance to fire. It’s basically a redundant system of protection.

MILES O’BRIEN: Of course, skyscraper design and construction has evolved over the years. The newer glass skyscrapers are half as heavy per square foot as the old concrete fortresses.

Is it possible to do both, to build the gleaming glass towers and put enough technology in them to make them as safe as this building?

JOHN NORMAN: The glass towers are a problem. The glass skin is a weak point. But you can build buildings that are structurally stable and can withstand the fire long enough to allow the fire department to suppress it without giving in to the pressures of the economy. Yes, that’s possible.

MILES O’BRIEN: And that is precisely what they’re trying to prove with the design and construction of the $3 billion One World Trade Center.

David Childs is the architect.

DAVID CHILDS, One World Trade Center: It is very important that we did rebuild. It gave us a chance to rebuild better.

MILES O’BRIEN: Childs and his team have aimed to strike a balance between economics, aesthetics, safety and security. How those competing mandates are shaping this landmark building is the focus of a PBS “NOVA” program, “Engineering Ground Zero,” which chronicles the rise of One World Trade from the inside out, at the core of this story, the core of the building.

DAVID CHILDS: The outside of the core, this whole center spine of the building, is out of concrete.

MILES O’BRIEN: It is six feet thick. And it’s not just any concrete, a new formulation that makes it much stronger. The end result? Concrete that can withstand loads of 14,000 feet per square inch, three times the strength of the garden-variety.

Cas Bognacki is an engineer with the Port Authority of New York and New Jersey.

CAS BOGNACKI, Port Authority of New York and New Jersey: This is a concrete specimen that will be used for tower one at the World Trade Center. If you could set this cylinder and put a platform on it, you can accommodate 1,000 Americans standing on this cylinder, you know, normal people 175 pounds in weight; 1,000 people can be supported by this four-inch-diameter cylinder.

MILES O’BRIEN: The collapse of the Twin Towers served as a stark and stunning reminder to architects and engineers of the value of using concrete to protect skyscrapers from the ravages of fire.

CHARLIE THORNTON, structural engineer: So this is the reconstruction of the fires in the World Trade Center, aircraft impact, how the planes hit, what damage they did.

MILES O’BRIEN: Structural engineer Charlie Thornton is a towering presence in the world of towering buildings. He has engineered many of the world’s iconic buildings, including Petronas Towers in Malaysia, the tallest twin skyscrapers in the world.

He helped write the detailed engineering report of how and why the World Trade Center towers collapsed, something no one, including him, ever imagined.

CHARLIE THORNTON: The planes knocked out maybe 15 percent of the vertical capacity of the building. If you look at the pictures, they knocked out about 35 columns, let’s say, on the north side of the north wall, a couple of inside columns probably.

MILES O’BRIEN: It was fire that brought the buildings down, but why? Skyscrapers have become real-life towering infernos on several occasions over the years. And none of them came even close to crumbling. The steel cores of the Twin Towers were covered in drywall, rather than concrete, no match for the impact of the airliners.

DAVID CHILDS: Where the planes hit, that Sheetrock wasn’t strong enough to take that punching shear, which, of course, the concrete is.

MILES O’BRIEN: But there is more to Childs’ core design than a lot of super-strong concrete. The stairs are much wider, and the air inside will be kept at a higher pressure than the rest of the building to keep smoke out in a fire.

The first 19 stories of this building are built like a bunker to protect this structure from the force of a car bomb. Eventually, they will be covered over with glass, bomb-resistant glass.

DAVID CHILDS: Because of the nature of the destruction that was here before, you don’t want to give the impression that this is a place of refuge and of fear. You want it to be a sense of openness to it. This building needs to appear open.

MILES O’BRIEN: But are all the safety features enough?

Former FDNY Fire Chief Dan Daly is leery. He offers a very personal endorsement of the old-style buildings.

DAN DALY, New York Fire Department: And when we got here, we found out there was a fire on the 51st floor.

MILES O’BRIEN: In 1990, he fought a fire at the Empire State Building.

DAN DALY: The fire took several hours to go out. And it probably, to a large extent, burned itself out. I mean, this type of building, you have thick concrete walls and floors. And it was like a — really like a pizza oven. All the heat was contained within that floor. And it wasn’t passing up to the other floors and spreading around the building.

But what happens in a fire like that, it’s superheated inside and everything becomes incinerated in that area.

MILES O’BRIEN: But it didn’t spread beyond the 51st floor.

Why don’t they build them like this anymore?

DAN DALY: Economics — economics and aesthetics. The amount of money it would cost to build one of these today is probably twice as much as other construction. And people don’t want to build a fortress. They want something with light and glass and airy.

MILES O’BRIEN: Lots of glass and wide-open floor plans are what developers want, so architects and builders deliver.

Architecture professor Garth Rockcastle says there is no turning back to the pre-modern era, but he says One World Trade is really a concrete fortress in glass clothing.

GARTH ROCKCASTLE, University of Maryland: When events like 9/11 occur, they accelerate and intensify that ongoing innovation. So I think that context needs to be appreciated.

So what did it awaken in us? What did we start? What were we — what became mission-critical to avoid, frankly, the professional embarrassment and the catastrophe that could happen in other buildings like the World Trade Center around the world?

MILES O’BRIEN: Charlie Thornton is pushing an over-the-top skyscraper building technique that takes the notion of a sturdy inner fortress with open floor plans and a glass skin to a new level.

CHARLIE THORNTON: But these cores are concrete cores, so they’re like bunkers. And they go up at the rate of a foot an hour 24 hours a day.

MILES O’BRIEN: Using a technique called slip forming, the concrete cores are poured first. Then the floors are built entirely on the ground, hoisted up and bolted into place. The cores eliminate columns and thus potential points of failure.

You would suggest this is clearly a better way to build a building?

CHARLIE THORNTON: I believe it’s faster. I believe the quality is better. I believe the lateral load resisting for earthquakes and wind is better, because on — with no columns, all of the weight of the building is compressing the core.

MILES O’BRIEN: Thornton says his technique costs up to 30 percent less. He has about 15 customers in the design phase. He believe it is just a matter of time before someone builds a mile-high tower three times higher than One World Trade will be.

CHARLIE THORNTON: It’s like being the Jetsons.


CHARLIE THORNTON: Yes. Yes. All that stuff from “The Jetsons” will happen.

MILES O’BRIEN: You think?

CHARLIE THORNTON: Oh, absolutely.


MILES O’BRIEN: Can that possibly be safe?

Firefighters say, once buildings got taller than their ladders, about 10 stories, the onus of safety shifted to the architects, engineers and developers.

John Norman wonders if they really listen to the people who climb up the steps when the rest of us are headed down.

JOHN NORMAN: There was a line in the movie “The Towering Inferno” where Steve McQueen comes out of the burning building and the architect is sitting there on the curb.

STEVE MCQUEEN, actor: You know, one of these days, they’re going to kill 10,000 in one of these firetraps, and I’m going to keep eating smoke and bringing out bodies until somebody asks us how to build them.

JOHN NORMAN: And it’s very true. I mean, we know the answers. The question is, will they want to hear the answers?

MILES O’BRIEN: Ten years after 9/11, One World Trade Center seems to prove someone is. Anything less would be an insult to the 3,000 people who died here. And if it is a success, it may be a model for how tall buildings are built everywhere.

JEFFREY BROWN: “NOVA” has had behind-the-scenes access to the construction of One World Trade Center and to the new 9/11 Memorial.

Its documentary “Engineering Ground Zero” airs tomorrow night on most PBS stations. You can check your local listings for the time.