Building on Ground Zero

PBS Airdate: September 5, 2006
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NARRATOR: In the heart of New York, one of the city's most valuable pieces of real estate has stood empty for years. There are plans to build anew, but the cranes stand idle, as if wary of the ghosts that will haunt this place forever.

More than 2,700 people perished on September 11, 2001, with the unthinkable destruction of New York's largest buildings. Once a towering icon of American economic might, the World Trade Center will now and forever symbolize that terrible day, when the nation awoke to a horrific attack by a ruthless enemy.

As Chief Engineer of the World Trade Center, Leslie Robertson was responsible for its strength and safety.

LESLIE ROBERTSON (Chief Engineer, World Trade Center): Ground Zero is a very disturbing place for me. And I cannot escape the people who died there or...even if I'm looking down into a pile of rubble, it's still, to me, somehow, up there in the air, burning. And I cannot make that go away.

NARRATOR: The fall of the twin towers would spark two federal investigations and produce 30 proposals to change the nation's building standards. But as other countries build soaring skyscrapers with vastly improved safety features, America still lacks national guidelines, five years after 9/11.

WILLIAM CONNOLLY (New Jersey Division of Codes and Standards): The thing that frustrates me the most about all of our reaction to the World Trade Center disaster is how long it has taken to take action. We need more staircases; we need fire resistant buildings and structures that are more resistant to collapse. Those things were obvious within the days after the event.

NARRATOR: Most experts agree we can make our buildings safer, but at what price? How safe is safe enough? And can the answers be found buried in the rubble of the nation's most shocking tragedy? Coming next on NOVA, Building on Ground Zero.

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NARRATOR: In the aftermath of 9/11, investigators from the American Society of Civil Engineers began studying what was left of the World Trade Center. Tasked by the federal government to try to determine what caused the collapse, the team was given six months to come up with some answers. The group's leader was Eugene Corley, whose experience with the Oklahoma City attack made him the logical choice for lead investigator.

On the morning of April 19, 1995, Timothy McVeigh, a home-grown terrorist, parked a rental truck in the loading zone of the Alfred A. Murrah Federal Building. The Murrah building contained regional offices of the Secret Service, D.E.A and A.T.F.—federal anti-crime agencies—and McVeigh and friends were angry at the federal government.

The truck was filled with ammonium nitrate and fuel oil.

WILLIAM EUGENE CORLEY (Forensic Structural Engineer): By coincidence, there was a hearing going on across the street, and they had a tape recorder going.

NARRATOR: There were no cameras to record the event, but this audiotape would prove central to Corley's investigation.

EUGENE CORLEY: And it starts out with the people talking about the water resources hearing.

FEMALE VOICE from the WATER HEARING RECORDING: Basically, there are four elements that I have to receive information regarding...

Sound of explosion and confusion.

NARRATOR: The tape recording would give Corley critical information for analyzing the attack.

EUGENE CORLEY: We needed to find out how long it took the building to collapse. And this shows the graphic output from the sound tape, and it starts out with the people just talking. Then we were able to analyze where the blast occurred, which is this section right here, this very small section. Then there's a slight pause, followed by very loud sounds of the building collapsing. That is a period of approximately three seconds.

NARRATOR: With only three seconds, no one had time to get out. Most died because the building collapsed on them, not from the bomb itself.

But why did the Murrah Building—a modern structure built to strict government standards—collapse so easily? This was the central question in Corley's investigation.

EUGENE CORLEY: The Murrah Building had been very well designed, and the records indicated it was well constructed.

NARRATOR: The Murrah building was nine stories tall, supported by 11 columns, spaced 20 feet apart, on the upper floors. And on the lower floors, the designers created an attractive two-story lobby, by reducing the number of columns to six, which opened up the space.

EUGENE CORLEY: To do this, it was necessary to put a transfer girder in. This is a very strong girder that supports every other column from above and takes the loads from those columns into the more widely spaced columns over the first two floors.

NARRATOR: McVeigh parked the truck in a service drive, just outside the lobby.

EUGENE CORLEY: McVeigh had left the truck right there at column G20. The explosion created a very large crater in the street and took out column G20. It also took out column G24 and G16.

NARRATOR: This left the large transfer girder with just three supporting columns, and it couldn't hold the weight.

EUGENE CORLEY: Once the transfer girder fails, there's no longer any support over most of its length, and almost half of the building collapses immediately. Whether McVeigh knew what he was doing or whether it was luck, he parked it in the place that it would do the most damage to the building.

NARRATOR: In engineering terms, what happened in Oklahoma City is called a progressive collapse, the worst-case scenario for any structure.

EUGENE CORLEY: A progressive collapse is when a key member of a building is knocked out, for some reason, and the collapse of the building extends well beyond where that local damage occurs.

NARRATOR: The Murrah Building met all the building codes, but those codes did not require major structural features, like the transfer girder, to have a secondary support system. In this case, stronger base columns, or additional underpinning that tied the girder more firmly into the end columns, might have prevented the collapse.

In the wake of the 1995 disaster, Corley's team recommended that back-up systems to prevent progressive collapse become mandatory. But the codes were never changed.

The 767s slammed into the World Trade Center at 530 miles an hour and weighed well over a hundred tons.

Bill Forney was on the 47th floor of the north tower.

BILL FORNEY (World Trade Center Survivor): It lurched back and forth, and it was still standing. It was a scary situation. It was actually the first time that I had truly ever thought that I might die.

NARRATOR: Scores of people on the planes and in the buildings were killed instantly. But despite the immense force of the impacts, the buildings stood, for 1 hour, 42 minutes, in the case of the north tower, and 56 minutes for the south.

The Murrah Building had collapsed in three seconds.

The towers did not suffer an immediate progressive collapse, because their great mass helped them absorb some of the initial shock, and because the buildings had the kind of engineering features the Murrah Building lacked, beginning with the dense palisade of steel columns on the exterior walls, originally placed there by Leslie Robertson to resist the force of the wind.

LESLIE ROBERTSON: That whole issue of wind engineering is the most important part of the structural design of any very tall building.

NARRATOR: This wall of columns would prove very fortuitous. Although extensively damaged, the sheer number of columns gave the building added support.

EUGENE CORLEY: What happened was that the loads that were being carried by those columns arched across the opening, so that the columns adjacent to the hole now started picking up the loads that had been carried by those where the airplane went in.

NARRATOR: And in a prescient decision, Robertson placed an additional support system called a "hat truss" on the top floors of the buildings. By connecting the interior core columns to the stronger exterior columns, the hat truss helped the severely damaged core remain standing.

SHYAM SUNDER (National Institute of Standards and Technology): The hat truss prevented earlier failure of the core of the building, which contained all of the stairwells. Had the core failed earlier, there would have been much larger loss of life than we actually saw on 9/11.

NARRATOR: But in the end, the buildings did come down, and those who made it out will never forget.

BILL FORNEY: ...tidal wave of, of destruction just flowed.

ELIA ZEDENO (World Trade Center Survivor): And the man kept saying, "We are the lucky ones. Keep going. We're the lucky ones. We are the lucky ones."

BRIAN CLARK (World Trade Center Survivor): We lost 61 dear friends that we worked with and laughed with, for years. I'm deeply saddened that they aren't here.

NARRATOR: By not collapsing immediately, the towers saved many lives, but a key question remained: did their ultimate failure mean something was wrong with their design or construction?

After several months of intensive study, the American Society of Civil Engineers delivered its verdict.

EUGENE CORLEY: The buildings we found performed well, and there was no tradeoff of safety for economy in construction. It was the combination of the impact load doing great damage to the building, followed by the fire that caused collapse.

NARRATOR: But these conclusions did not satisfy some family members of the 9/11 victims. Upset at what they felt was a hasty and incomplete investigation, they began demanding a more thorough one.

VICTIM'S WIFE protesting the investigation results: We are asking for everyone to join us, and we're asking for God to bless us today, that we will have safe buildings and we can go on with the future of this city the way we want to. Thank you very much.

NARRATOR: In truth, Corley was given scant time and resources to come up with anything but a preliminary conclusion, and a tragedy of this magnitude certainly warranted more than that. So, in 2002, Congress charged the National Institute of Standards and Technology to conduct the most detailed building analysis ever undertaken.

The biggest hurdle, for the N.I.S.T. team, was the lack of physical evidence they had to work with.

SHYAM SUNDER: For most investigations, you usually have a building to investigate. So we had to recreate, in great detail, what happened on 9/11, and that's why the need for a painstaking reconstruction of the entire process.

NARRATOR: They looked at a thousand hours of videotape, meticulously measuring the visible damage. They tracked temperatures inside the buildings. They staged model fires to measure their effect on everything from office furniture to steel columns. And they created computer models to help them assess the extent and location of the damage, all in an effort to understand exactly how and why the towers fell.

Their prime suspect was the building's floor trusses. When construction began on the towers in the 1960s, Leslie Robertson stabilized the tall steel columns with the building's floors. And the large, heavy floors were supported by thin steel members called trusses.

By connecting the exterior and interior columns, the floor trusses were critical to the building's support system. If the trusses or their connections failed, floors could collapse, and the buildings could fall.

SHYAM SUNDER: Soon after 9/11, there was a lot of concern about the role of the floor trusses from experts who had looked at the buildings as well as from the lay public. And so, we wanted to make sure we looked at this carefully and with full detail, so that we could then make a definitive finding.

NARRATOR: N.I.S.T. computer simulations show the mostly-aluminum planes shredding on impact. But the steel engines tore through multiple floor trusses, and the fast spreading fires subjected them to intense heat.

So, in the immediate aftermath of 9/11, many experts wondered if these trusses were too weak to handle a building under stress.

CHARLES THORNTON (Structural Engineer): Had the floor system been more robust, with much stronger connections between the exterior and the inside, I think the buildings probably would have lasted longer. Would they ultimately have collapsed? Maybe not.

NARRATOR: Another suspect was the fireproofing that covered the trusses and other steel members. It was clearly blown off by the airplane impacts, leaving the steel exposed to the fires.

CHARLES THORNTON: Once you lose the "spray on" fireproofing, you have bare steel. And once you have bare steel, you don't have a fire rating anymore.

EUGENE CORLEY: Prior to 9/11, the public, I believe, felt that steel was fireproof and was not affected by fire. In fact, all building materials are affected by fire. When the temperature goes up, each building material that is used loses strength.

NARRATOR: This was particularly true for the lattice-like floor trusses. They were thin and difficult to fireproof evenly. And after extensive testing, N.I.S.T. found that some of the longer trusses did not meet the two-hour fire rating mandated by the building code.

But, as they studied the minute details of the collapse, they made a remarkable discovery, one that would not only provide a final verdict on the trusses, but would completely revise previous theories of how the towers fell.

In 2002, NOVA depicted a scenario envisioned by many experts at the time, that the truss connections failed in the extreme heat, causing the floors to fall onto one another, precipitating the collapse.

SHYAM SUNDER: When you did it previously, you showed that the floors actually pancaked, and we did not see any evidence of pancaking in the videos or photographs we have.

NARRATOR: By creating computer-enhanced images of the exterior walls, N.I.S.T. discovered that the truss connections did not fail. In fact, the trusses stayed connected to the columns even as they sagged from the heat. They pulled on the columns, bowing them inward, nearly five feet in some areas, until the columns reached the breaking point.

SHYAM SUNDER: Suddenly the columns snapped, and, as a result, the entire top of the building came down, pretty much in freefall, because kinetic energy that was unleashed was just huge.

NARRATOR: After months of analysis, N.I.S.T. concluded that the World Trade Center had no structural flaws that could account for its collapse. It was the interplay of impact damage and fire that brought the towers down.

SHYAM SUNDER: It was the combination of the impact, the fireproofing that was dislodged, and the jet fuel fires that caused the buildings to collapse. These buildings were sound, well designed, highly innovative, and there was nothing that could have changed the outcome on 9/11.

NARRATOR: This was the same conclusion Corley had reached four years earlier and would finally close the books on the engineering of the World Trade Center.

For Leslie Robertson, so anguished by the tragedy, the criticisms of his building had hurt him deeply.

LESLIE ROBERTSON: Initially, many architects and engineers stepped to the cameras and spoke out about the project, without having very much information. And that has done us, as a company and, I would say, the relatives of the survivors a disservice that was totally unnecessary, that this structure was very robust and that's what all structures should be.

NARRATOR: But, on 9/11, there were problems that went far beyond the physical strength of the towers. Fire control, communications, rescue procedures, and the entire evacuation process were all severely tested that terrible day. And N.I.S.T.'s final report addresses these problems with 30 recommendations for improving safety standards in American building codes.

One of the first, centers on the type of spray-on fireproofing that is still the industry standard.

SHYAM SUNDER: Fireproofing should be improved by enhancing the bond strength of the fireproofing and by increasing the quality control when it's applied in the field.

NARRATOR: Protecting a building's structural elements from fire has always been a key provision in American building codes. But now, the kinds of materials used and their ability to stay more firmly in place have come under intense scrutiny.

WILLIAM CONNOLLY: What happened on 9/11 really focused our attention on issues that were always there but were a little less important when we weren't dealing with the possibility of deliberate damage to buildings.

NARRATOR: Bill Connolly is the Director of Codes and Standards for the State of New Jersey. He believes building codes should be changed as a result of 9/11, starting with tougher requirements for fireproofing.

WILLIAM CONNOLLY: There are lots of ways that spray-on fireproofing could be dislodged and knocked off. It could be a blast, a much smaller blast. It could be the elevators going up and down the building would shake the steel. It could be maintenance, ongoing maintenance activities, where people get into the suspended ceilings, and they're working with the wiring and the ducts, and they knock it off, if it's not particularly strong. But it's fundamental that it stay in place, because if it's not there, the steel simply will not resist a fire in any building, and collapse is almost inevitable.

NARRATOR: Here's an example of fireproofing that has greater bond strength and density. It's mixed with cement, so it's far more difficult to dislodge than the standard spray-on foam, but it costs more.

Another type of fireproofing is applied like paint and bonds tightly with the steel. In a fire, it bubbles up into a thick insulating layer. But it's expensive, so most builders still apply the kind of spray-on foam used in the World Trade Center.

N.I.S.T. also focused on the wallboard used to enclose the core and stairwells. It was extremely fire-resistant and easily met code requirements. But there were no requirements for impact resistance. Wallboard is weak and brittle; on 9/11, this weakness had tragic consequences.

In the areas of direct impact, the planes severely damaged the interior cores, destroying emergency stairs, elevators and sprinkler systems. With the stairs and elevators gone, those above the impact zones were trapped in a mounting inferno that would reach 1800 degrees.

LESLIE ROBERTSON: The people above, obviously they're suffering terribly, the people who elected to take their own destiny in their hands by jumping. I mean, it must have been an incredibly awful place above the impact.

NARRATOR: Above the impact zone, in the south tower, one damaged staircase remained passable. Brian Clark and a few colleagues somehow managed to find it.

BRIAN CLARK: So we started down that stairway, and we only went three floors. There was a group of seven of us, myself and six others. We met two people that had come up from the 80th floor, a heavy-set woman and a, by comparison, a rather frail male. She said, "You've got to go, you've got to go up. You can't go down, there's too much smoke and flame below."

NARRATOR: The woman and her companion never made it out. Brian Clark and 17 others from above the impact zone barely escaped before the south tower collapsed.

Nearly 1,000 people trapped in the north tower died. Since stronger walls may have prevented complete destruction of the stairs, another key N.I.S.T. recommendation is to harden stairwells with more concrete and steel. And the agency is also calling for improvements to evacuation planning and the design of emergency stairs.

FRANK MASSERO (World Trade Center Survivor): We got out to the staircase, and it was congested. It was very congested. It was moving, but moving slowly.

JOHN D'ANTONIO (World Trade Center Survivor): There were just so many people, and the stairwells really, literally, couldn't hold more than two people abreast.

TOM CANAVAN (World Trade Center Survivor): The fire department started coming up—I would say down in the lower floors, about 30, 31—and we were forced, then, to merge into single file.

ELIA ZEDENO: The way I see it is, if it takes me more than five minutes to evacuate, I do not belong in there.

NARRATOR: Although most people were able evacuate safely, many experts agreed we were lucky the towers were only half full when the first plane struck at 8:46 that morning.

JAKE PAULS (Building Safety Analyst): If the buildings were full, the evacuation time would have been longer. So, I mean, people would be standing there, waiting for others to clear the bottom of the stair. And so some people would be still be waiting when the towers collapsed, and there would have been many more deaths.

NARRATOR: Jake Pauls is one of the world's leading authorities on building evacuation. A passionate advocate for occupant safety, his views often put him at odds with the owners and developers of big buildings.

JAKE PAULS: There might be a bit of a perception, out there in the building industry, that I tend to be a crank. But I have a fairly good understanding how people use stairs, what their safety is, how much width they need for passing and for crowding and so on. And my views on this are based on work going back 40 years.

NARRATOR: His major concerns center on time. He's convinced that designers and code writers do not have a clear understanding of how long it takes to evacuate a big building in an emergency.

In 1993, when the World Trade Center was attacked with a car bomb, Pauls learned that it took more than five hours for some people to get down the emergency stairs. This was nearly three times as long as the fire protection rating for the steel.

The stairwells were smoky and the lighting had failed. Those problems had been fixed prior to 9/11, but to Pauls, the real cause of congestion was the width of the stairs.

JAKE PAULS: The current minimum width for stairs is 44 inches. Now 44 inches is not very wide—for example, it's about that...44 inches...very narrow—and came from work done about 100 years ago, with somewhat smaller people. Today people are less fit. They're larger, they're heavier.

People walk in what's called a staggered file. They walk a step or two ahead of the person closest to them. And this is to allow for body sway. And there's a lot of stop and go movement, but this is very frustrating for people. They don't understand what's happening. They like to keep moving and they can't.

NARRATOR: This was especially true on 9/11, as people started bunching up on the lower floors.

JOHN D'ANTONIO: We found it frustrating that we weren't moving continuously. I mean, I could have moved a lot faster without all of these slow people in front of me.

NARRATOR: And all of these problems are intensely magnified when people going down have to share the stairs with firemen going up.

JOHN D'ANTONIO: We would all have to move off to the side, up against the wall, to let the firemen through, because here these guys were carrying these incredible packs. And we knew they were going to save people, and everybody just got out of the way, but of course, that slowed progress for us on our way out, in a big way.

JAKE PAULS: In perfect hindsight, it would have been better if the firefighters hadn't gone up the stairs, certainly in the numbers they did. It caused congestion for people coming down, and the firefighters couldn't really do very much up on those upper floors.

NARRATOR: Pauls believes that to help eliminate this kind of congestion, emergency stairs should be at least 56 inches wide. But even this increased width would not have helped the disabled on 9/11, many of whom were told to wait on a lower floor until the others had exited.

JAKE PAULS: And there were a couple of dozen, maybe three dozen people who were held there, and they were there or trying to come down as tower one collapsed. Those people, because of their physical disabilities, and because of the procedure used with them, may have died needlessly.

NARRATOR: When exit speed is critical, as it was on 9/11, any delay could mean the difference between life and death. So why do tall buildings have so few escape routes? The reason may come as a surprise.

WILLIAM CONNOLLY: Building codes traditionally have not been designed to accommodate a full evacuation of a building. Most people think that when you're in a building and the fire alarm goes off, you all go out. We all have that experience from elementary school. We've all been through fire drills, and we understand the fire alarm goes off and everybody leaves the building.

In a building that's 80 stories tall, where it's going to take two or three hours for everybody to leave the building, we don't take the same approach. The conventional wisdom, at the present time, has been that people will be defended in place. They'll be encouraged to stay where they are. People, obviously, from the fire floor will move, maybe the floor just above and just below, but other people will remain in place.

NARRATOR: This protocol is based on two critical assumptions: that fires will start small and spread at a rate that sprinkler systems or firemen can manage, and if the fire does get out of control, people will still be able to escape through fire-protected stairways.

9/11 proved that neither of these assumptions could be counted on and prompted N.I.S.T. to make its most controversial recommendation.

SHYAM SUNDER: Based on these findings, we also have made recommendations for a tall building to be able to evacuate fully in an emergency.

NARRATOR: Designing big buildings so everyone can exit at the same time will increase building costs. More and wider stairwells, better fireproofing to protect against collapse, and specially-designed elevators for firemen or evacuees will not come cheap. And many experts believe these recommendations are an expensive over-reaction to one very bad day.

At a research lab in Massachusetts, fire protection engineer Jonathan Barnett contends that it's "ordinary" fires, not jet fuel fires or airplane attacks, we need to worry about. And protecting high-rise buildings from most fires is something we already do pretty well.

JONATHAN BARNETT (Fire Safety Engineer): Prior to 9/11, we've never had a collapse of a protected steel building. We put fireproofing on our buildings so that they don't collapse in case of a fire, so that there's time to get people out, so there's time for a fire department to go in and put out the fire.

NARRATOR: A key member of Gene Corley's original investigative team, Barnett has studied all aspects of fire protection, from clothing to sprinklers. He argues that the costs inherent in N.I.S.T.'s recommendations are not justified, given the history of fires in tall buildings.

JONATHAN BARNETT: The risk of dying by fire at home is far greater than the risk of dying in a high rise office fire. High rise buildings in general are safer than other buildings.

NARRATOR: Based on the numbers, Barnett is right. In the U.S., from 1995 to 2002, 53,000 people died from fires in homes and low-rise buildings. Not counting the World Trade Center, fewer than 100 people died in high rise fires. Serious high rise blazes are rare.

In every case before 9/11, the building remained standing, and virtually everyone got out alive. So if fire is the major threat to occupants of tall buildings, do we really need expensive code changes?

JONATHAN BARNETT: I don't want to spend money if I'm not going to get value for my money. The probability of a fire that gets out of control in today's world is very small.

WILLIAM CONNOLLY: There's two sides to the traditional risk equation, the probability that something will happen and the consequences if something happens. The two of them together add up to the risk. Most of those people that talk about what has happened, statistically, in the past, are just focused on probability. It doesn't happen that often, so we don't need to worry about it.

We also need to look at the other side. It doesn't matter if it doesn't happen often, if potential consequences are huge.

NARRATOR: After Oklahoma City and two attacks on the World Trade Center, it's abundantly clear that fire is not the only threat to buildings and their occupants.

WILLIAM CONNOLLY: Terrorists, who know what they're doing, and many of them are engineers, could deliberately knock out a sprinkler riser before starting a fire, or put a chemical agent into the air handling system of the building. There are any number of possibilities related to terror.

NARRATOR: Overseas, terrorist bombings have been a serious threat for decades. To help prevent such attacks, key U.S. facilities are now protected with armed checkpoints and set farther back from roads to keep car bombers at bay.

Similar safety procedures can be found at important buildings in the U.S., as well. But as we've seen time and again, from Oklahoma City to the Middle East, if buildings are bombed, they can collapse, and deaths invariably follow.

This situation prompted N.I.S.T. to make its most expensive safety proposal.

SHYAM SUNDER: One of our most important recommendations deals with preventing progressive collapse. And essentially that means making the buildings more robust.

NARRATOR: With its sound engineering and backup support systems, the World Trade Center, unlike the Murrah Building, avoided progressive collapse, thereby saving lives. But increasing a building's structural safety, as well as its evacuation capacity, could raise construction costs five percent or more, easily many millions of dollars for a high rise building.

And since it's impossible to protect all buildings from every kind of attack, where do we draw the line?

JONATHAN BARNETT: I think that we do have to detect important and significant structures against the car bomb or whatever the terrorist attack of tomorrow is. However, it's too expensive to protect every building in this country against an unforeseen terrorist attack. Even in countries like Israel, which face terrorist attacks day in and day out, they aren't changing their building code for routine buildings.

NARRATOR: In America, the question of how, or even whether to improve big building safety is still being debated. But in other countries, significant improvements have already been made. In the burgeoning metropolis of Shanghai, the streets are jammed, day and night. And what was left of old China quickly disappears behind the spread of modernity. In a city where land is scarce and expensive, the very tall building is thriving.

WILLIAM PEDERSON (Architect, Kohn Pedersen Fox Architects): Ironically, since 9/11, there perhaps have been more tall buildings proposed around the world than at any point in history, mainly because we have greater population densities, particularly in places like Asia, we use much less material to actually house the same number of people.

NARRATOR: Shanghai will soon lay claim to the tallest building in the world, ironically called the World Financial Center, which will soar 101 stories, topped by a giant opening that frames the city. The new building will be so tall and majestic, it's destined to become the visual icon of Shanghai, as surely as the World Trade Center once symbolized New York. So perhaps it's fitting that Leslie Robertson leads an engineering team that will forge a building that defines its city.

Yet it wasn't so very long ago that Robertson wondered if he would ever work again.

LESLIE ROBERTSON: With the disaster of the World Trade Center, and the fact that the structure ultimately collapsed, I felt that it would be, kind of, the end of our company. I thought it might be the, the end of me as an engineer.

NARRATOR: But architect Bill Pedersen did not hesitate to bring Robertson on board the World Financial Center design team.

WILLIAM PEDERSON: Leslie Robertson, perhaps, had more experience in the design of the tall building than almost any other structural engineer. We had absolutely no concern about his ability to design a structure that was extraordinarily safe and efficient.

The building, in its lower levels, will be used for retail spaces, for museum spaces. We have an office space from here to here, and then above that, hotel space. The last 80 meters of the building—and it's not well represented with a wooden model—but the last 80 meters of building are all glass. So you can stand here and look all the way up to the top of the building. Here we will have observation platforms, museums, dining facilities. It is a portion of the building which, in fact, contributes to the entire city. It's like the Eiffel Tower, the place people can enjoy the city from.

NARRATOR: On any given day, more than 20,000 people will occupy the World Financial Center, making building safety paramount. And, as with all very tall structures, the threats that need to be accounted for first are the natural ones like earthquakes and, in this region, typhoon winds.

WILLIAM PEDERSON: With a building of this height, one of the important aspects is to be able to relieve as much wind pressure as possible.

NARRATOR: Pedersen's solution for alleviating wind pressure is the giant aperture at the top, allowing the wind to blow through the top of the building instead of buffeting it.

But it's the elements you won't see when the building is finished that give it its great strength. Long diagonal steel members, just beneath the surface, will help stabilize the building in an earthquake. And the exposed orange trusses that belt around the building, attach to small columns on the facade, huge columns on the corners, and, through slanting girders, to columns in the core. All these elements tie together the entire structure and create an open, welcoming and protected environment.

WILLIAM PEDERSON: So, super columns, diagonals, belt trusses, outrigger trusses and the central core are all brought together in a single unit.

NARRATOR: Mammoth concrete super columns at each corner are the main anchors for the building. The concrete is embedded with giant steel girders and a thick jungle of long steel bars. These massively reinforced columns will help make this building one of the strongest on earth.

LESLIE ROBERTSON: The project was designed, not just for the typhoon winds and the great earthquake, but it was designed for a series of unforeseen events that...we really don't know what they might be. But it's extremely robust and redundant. You can take out columns, you can take out the giant diagonals and so forth, and the building's unhappy, but it stands there, even so.

NARRATOR: In China, builders of skyscrapers must also meet stringent code requirements for building safety, beginning with a concept not found in America.

LESLIE ROBERTSON: Every 12, 13, 14 floors in the building, you see a truss across the face of the building. Behind that truss is what's called a refuge floor. And the idea of a refuge floor is that the stairs, the fire stairs, come down to the refuge floor. You exit out onto the refuge floor and you're protected there. And of course, if you wish, you can go back into the stair and continue on down.

NARRATOR: On these floors, steel plates underpin the concrete flooring, ventilation shafts bring in outside air, and every structural element is given extra fire protection.

WILLIAM PEDERSON: The concept of these floors is that they will be protected with a four-hour fire rating, such that fire cannot penetrate into the floors. People can reside on those floors during the emergency.

NARRATOR: Once safely on the refuge floors, people could continue on down the emergency stairs or share special firemen's elevators located inside the thick concrete walls of the core. And in extreme emergencies, they could even use the observation elevators on the corners of the building.

WILLIAM PEDERSON: We have two elevators which run all the way up to the top observation platform, which are intended, primarily, to handle visitors that are coming to that observation platform. But after 9/11, we decided that we wanted to also allow them to stop at the individual refuge floors, such that they could assist in evacuating people from the building.

NARRATOR: Creative efforts to better protect people who might one day need to evacuate a tall building are not confined to China. Back in New York, the building known as Number 7 World Trade Center is almost complete.

Its predecessor had also collapsed on 9/11, and the new one has been constructed with an eye to lessons learned. A dense fireproofing material covers the steel, which better protects it from impact, as well as from fire. Strong steel beams help support the wide open, light filled office floors. And a thick concrete barrier protects the elevators and staircases in the core.

But it's the lowly exit stairs that are one of the building's main attractions. Carl Galioto is the building's architect.

CARL GALIOTO (Architect, Skidmore, Owings & Merrill, LLP): These stairs are much brighter than one normally thinks of fire stairs, and they have two levels of emergency power to keep the stairs well lit in the event of an emergency. But one of the features that I'm most proud of is the width of these stairs. These stairs are 20 percent wider than required by code. The walls here maintain the integrity of this enclosure. These are reinforced concrete walls.

Most buildings have one fire stamp high sprinkler riser. We have two, one in each stair. In the event that one is compromised, there is a second one delivering water to the sprinkler system. These wide stairs also have space for wheelchairs. It's called an area of rescue assistance, so that someone in a wheelchair can be brought to this space and helped down the stairs.

And we've picked all these features together. We believe this will make it one of the safest buildings in America.

NARRATOR: Although the safety improvements to Building 7 have increased its construction costs, from New York to Shanghai, many owners and designers of new buildings seem to be taking the initiative on improving safety. But how much should the specific events of 9/11 dominate our thinking?

LESLIE ROBERTSON: In my lifetime, in buildings that I've worked on, we've experienced about everything you can think of that could take place: big earthquakes, giant windstorms, the airplanes striking the World Trade Center, the bombing of the World Trade Center and so forth. But I don't think it's necessary for us to go out and start designing all of our structures for the impact of jet aircraft. It would create buildings that are not so wonderful places to live and work.

They should be robust; they should be strong...oh, yes, of course. But special circumstances should not be dictating the way we design high rise buildings.

NARRATOR: So how safe is safe enough? Certain buildings, because of their size or iconic status, will always require greater safety considerations. A case in point is the Freedom Tower, currently planned to rise from the footprint of the World Trade Center and fill the empty skyline of lower Manhattan.

Its design has been a matter of great controversy, but from its strong exterior walls to its hardened interior staircases, this building promises to be one of the safest in America.

DAVID CHILDS (Architect, Skidmore, Owings & Merrill, LLP): We've done all sorts of extremes to make this a building that is responsible in every possible way, that has all of the safety redundancy features that one can imagine, that is safe long enough for people to get out if there is a fire, or a hurricane that breaks glass, or whatever disasters could happen. So I think we've made tremendous leap forward for how we build skyscrapers in the United States.

NARRATOR: Clearly, the Freedom Tower has a special reason to strive for a higher safety standard. But the real question is whether all the safety issues raised by 9/11 will finally be addressed with new national building codes.

WILLIAM CONNOLLY: The thing that frustrates me the most about all of our reaction to the World Trade Center disaster is how long it has taken to take action. I think we need more staircases. We need more-fire-resistant buildings. We need structures that are more resistant to progressive collapse. We need better command and control systems built into the building to support fire department operations. Those things were obvious within the days after the event, yet we have taken a very long time to get to the point of even talking about getting them into our codes.

NARRATOR: There is still much debate on whether to accept the N.I.S.T. recommendations for changing our safety codes. A final decision will have to be made by state and local building officials.

But, to the survivors of 9/11, the decision should already be clear.

JOHN D'ANTONIO: I think it's important to improve the way new buildings are built, so that as buildings kind of change over, and the out-with-the-old-and-in-with-the-newer ones, the newer ones are better. They're better for my kids. They're better for everybody's children.

NARRATOR: Will 9/11 leave a legacy of safer buildings for America? Five years after the tragedy, the answer is still unknown.

On NOVA's Building on Ground Zero Web site, follow a detailed analysis of the towers' collapse, and hear recommendations for safety improvements on future skyscrapers. Find it on

Educators and other educational institutions can order this or other NOVA programs, for $19.95 plus shipping and handling. Call WGBH Boston Video at 1-800-255-9424.

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Building on Ground Zero

Written and Produced by
Larry Klein

Narrated by
Liev Schreiber

Edited by
David Espar

Narrated by
Liev Schreiber

Associate Producer
Jonathan Loewald

Gordy Waterman
Tom Kaufman
Edward Marritz
Brian Dowley

Sound Recordists
Paul Rusnak
Paul Flinton
Mark Roy
Sean O'Neal
John Cameron
Jeff Hayash


Assistant Editor
Ben Howard

Post Production
Henninger Media Services

Online Editor
Jef Huey

Dave Markun

Audio Mix
David Hurley

Archival Material
Associated Press Television News
Federal Emergency Management Agency
ITN Archive
New York City Police Department
The Port Authority of New York and New Jersey
James A Bonner Productions - Philadelphia Fire Films
KWTV News 9
Oklahoma City National Memorial

Special Thanks
American Society of Civil Engineers
Construction Technologies Laboratories, Inc.
Institution of Civil Engineers
Isolatek International
Kohn Pedersen Fox Architects
Leslie E. Robertson Associates
The Millenium Hilton Hotel
Mori Building Co., LTD
National Institute of Standards and Technology
Natasha Sealy-Fraser
Silverstein Properties, Inc.
Skidmore, Owings & Merrill, LLP
University of Maryland,
James Clark School of Engineering
Worcester Polytechnic Institute,
University of Science and Technology

Producer and Director for "Why the Towers Fell"
Garfield Kennedy

Production Manager for the BBC
Anna Mishcon

Series Producer for the BBC
Matthew Barrett

Executive Editor for the BBC
John Lynch

NOVA Series Graphics
yU + co.

NOVA Theme Music
Walter Werzowa
John Luker
Musikvergnuegen, Inc.

Additional NOVA Theme Music
Ray Loring

Post Production Online Editor
Mark Steele

Closed Captioning
The Caption Center

NOVA Administrator
Dara Bourne

Eileen Campion
Olivia Wong

Gaia Remerowski

Production Coordinator
Linda Callahan

Unit Managers
Carla Raimer
Karen Lally

Raphael Nemes

Legal Counsel
Susan Rosen Shishko

Jessica Ghofrani
Gregory Scott
Sara Slavin

Post Production Assistant
Joshua Thurston

Assistant Editor
Alex Kreuter

Associate Producer, Post Production
Patrick Carey

Post Production Supervisor
Regina O'Toole

Post Production Editor
Rebecca Nieto

Post Production Manager
Nathan Gunner

Supervising Producer
Stephen Sweigart

Producer, Special Projects
Susanne Simpson

Coordinating Producer
Laurie Cahalane

Senior Science Editor
Evan Hadingham

Senior Series Producer
Melanie Wallace

Managing Director
Alan Ritsko

Senior Executive Producer
Paula S. Apsell

A BBC/WGBH Boston Co-Production


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