"Why the Towers Fell"

NARRATOR: By now you've seen the images and heard the stories.

BILL FORNEY (World Trade Center Survivor): ...because I remember a coworker saying, "Don't. Don't. Don't open the door. Don't go out there. It's fire out there. You're going to, you're going to burn up."

NARRATOR: But what really caused the Twin Towers to collapse? Was their failure inevitable? Or could they have stood longer, giving occupants and emergency crews a better chance for escape?

FIREFIGHTERS: When we hit the fifth floor, that's when everything happened.

It was rattling. It was rolling. It was roaring. The floor was shaking.

I remember getting knocked down the stairwell, landing like a rag doll.

That's when the building started coming down.

NARRATOR: When a blue ribbon team of forensic engineers was asked by the government to determine exactly what triggered the Towers' collapse, NOVA was there from the beginning, following their quest for answers.

W. GENE CORLEY (Structural Engineer): What we're looking for is pieces that were in the areas where fires occurred. You can get a better idea of what the strength was before the collapse occurred.

NARRATOR: From their detailed examination of the Towers' innovative design to the search for forensic evidence in the molecules of collapsed steel, the investigation team has studied every possible scenario. Could one tower have collapsed for different reasons than the other? Was there something about the Towers—built to maximize rental space—that traded safety for economy?

CHARLES THORNTON (Structural Engineer): A lot of people are saying that the structural engineering of the World Trade Center was miraculously wonderful, that the buildings stood up in the case of two 767s flying into it. I would tend to think they were not as successful as they could have been.

NARRATOR: Was the damage from the explosions and massive fires too great for any building to sustain?

MATTHYS LEVY (Author, Why Buildings Fall Down): As the steel began to soften and melt, the interior core columns began to give. Then you had this sequential failure that took place where it all pancaked—one after the other.

NARRATOR: Why could only eighteen people from the impact areas or above get out alive? Was there a problem with the emergency stairs? The escape route? And perhaps most importantly, what does this disaster tell us about the safety of all tall buildings?

JAKE PAULS (Building Safety Analyst): Public perception about evacuation of large buildings is that if they decide to evacuate that they will get out quickly. The reality is really something quite different.

NARRATOR: This unthinkable tragedy has come to define our times. The question now is, "Can we learn from it?"

LESLIE ROBERTSON (Engineer, World Trade Center): I cannot escape the people who died there. It's still, to me, up there in the air, burning. And I cannot make that go away.

Why the Towers Fell, up next on NOVA.

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NARRATOR: Even today Ground Zero has the capacity to shock, because what happened here on September 11th still seems beyond comprehension.

BRIAN CLARK (World Trade Center Survivor): He said, "You know, I think those buildings could go over." And I said, "There's no way." I said, "Those are steel structures." And I didn't finish the sentence.

NARRATOR: People come as pilgrims to the site, to honor those who were lost and to try and understand how two of the world's tallest skyscrapers could have been destroyed so quickly. A disaster on this scale raises two crucial questions: Was the collapse of the buildings inevitable? And need so many people have died?

MIKE MELDRUM (Ladder 6 Fire Crew): I still find it hard to believe that these buildings are missing. I can't explain what happened. I can't explain how we walked out of that building.

NARRATOR: One place to begin the search for answers is among piles of charred and twisted steel now lying in a scrap yard in New Jersey.

Gene Corley is leading a team from the American Society of Civil Engineers investigating the precise causes of the collapse. Corley led the investigation of the Oklahoma City bombing disaster, but the magnitude and relevance of this investigation is daunting.

GENE CORLEY: I have looked at now two major terrorist attacks, and I never want to look at another one in the future. I want the findings that we have obtained from these studies to be used to develop buildings that will provide more safety for those who are in those buildings.

NARRATOR: In the months since the collapse, the team has analyzed countless fragments of steel and pored over hundreds of hours of video tape trying to determine the timetable of the collapse and exactly which parts of the buildings failed. But to really understand how these structures performed, the team had to look back at decisions made 35 years ago, when the Twin Towers were designed and built.

It all began in 1966, with a radical dream. The World Trade Center Towers were designed to be more modern, more economical and taller than any other skyscraper in the world. The lead structural engineer on the project was Leslie Robertson, then just 34 years old.

LESLIE ROBERTSON: It was really a young person's project. It took a huge amount of energy. Did a lot of things that I don't think an older engineer would have bothered to do, because he would have had confidence in the work that he'd done in the past. And I was charging down a different highway.

NARRATOR: Earlier skyscrapers, like the Empire State Building, used a dense grid of steel girders to support the height and mass of the structure, but they all had the same drawback.

LESLIE ROBERTSON: The buildings of the past had columns spaced roughly 30 feet on center in all directions. And the issue with that is it worked well but it has columns in space that you would like to rent.

NARRATOR: To increase the rentable floor space, Robertson repositioned most of the inner columns to the exterior wall. This dense steel palisade would support half the downward weight of the building. But its main task was to resist the biggest load on any skyscraper, 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. Just the brute strength of it is the driving force behind all structures of tall buildings.

NARRATOR: The World Trade Center's exterior skeleton was exceedingly strong, capable of resisting the lateral force of the wind and the unexpected force it would receive three decades later.

BILL FORNEY: It lurched forward, back and forth. After maybe six to 10 movements back and forth of that building it was over—and it was still standing.

NARRATOR: In Robertson's design, the downward weight of the building was also supported by large steel columns around the building's inner core, which is where he placed elevator shafts, emergency stairs and other building services. But the tall vertical columns of the inner core and outer walls were like freestanding stilts until Robertson tied them together with floor trusses.

LESLIE ROBERTSON: The World Trade Center is a very large project. In essence, it still boils down to a series of small pieces, and this is an example of a top part assembly of a typical floor truss.

NARRATOR: Long and thin, these horizontal steel assemblies were connected by bolts to the columns at each end and then welded to the exterior columns for extra support. The trusses were critical for holding the buildings together, and their performance is now at the heart of the investigation into what happened.

Robertson tried to save weight and costs wherever he could. He fireproofed all steel members, including the trusses, with the latest lightweight heat-resistant foam. And he kept the core area light by walling it off with drywall or Sheetrock (TM) rather than concrete.

JONATHAN BARNETT (Professor, Fire Protection Engineering): This is very typical. We often build buildings this way, two layers of Sheetrock on either side of a steel framework. It's just like you might build a wall, except we use special Sheetrock that's particularly fire-resistant.

NARRATOR: Although drywall is indeed effective at keeping fire at bay, it has one serious drawback that would reveal itself on September 11th. It's not very strong, especially when it's been heated.

The designers of the Trade Center tried to anticipate every possible disaster. The Towers were the first skyscrapers ever explicitly built to survive the impact of a plane.

LESLIE ROBERTSON: We had designed the project for the impact of the largest airplane of its time, the Boeing 707, that is, to take this jet airplane, run it into the building, destroy a lot of structure and still have it stand up.

NARRATOR: When the World Trade Center was opened, there was little doubt these buildings were as safe as any skyscrapers in the world. Although they were initially criticized for their sterile, industrial look, over time they worked their way into the hearts of New Yorkers and became one of the busiest spots in the city.

But their very success made them a target. In 1993, Islamic extremists attacked the buildings for the first time. The terrorists exploded a huge bomb in a parking garage beneath the Trade Center complex. The blast blew a 90-foot hole through five floors of the underground structure, killing six people and sending soot and smoke racing through the building. But the Towers stood firm.

LESLIE ROBERTSON: The bombing, I think, created a lot of confidence in everyone's mind that the Trade Center was pretty sturdy.

NARRATOR: The bombing did reveal at least one serious problem.

JAKE PAULS: Remarkably, there was no study performed of the people evacuating in 1993. The evacuation time was something in the range of one to seven hours depending on how high you were in the building and what your disabilities were.

NARRATOR: After the attack, exit stairs were much improved, but no one really knew if people could evacuate the towers in less time if they had to.

The test would come on September 11, 2001. On that day British transport consultant Paul Neal was sitting at his desk on the 63rd floor of the North Tower.

PAUL NEAL (North Tower Survivor): The day was a beautiful, clear day which I'm quite sure was significant because it meant that the hijackers of the aircraft would have perfect visual conditions, so they'd've been able to see those twin towers probably 60, 70 miles away.

NARRATOR: Down below, in the World Trade Center's underground station, the morning rush hour was underway. It's now estimated that there were just 14,000 people in the two towers at that time in the morning, far fewer than the 40,000 who would normally fill them later in the day.

DISPATCHER: Ladder 6, Ladder 6 only, Box 215, 120 Mulberry Street.

NARRATOR: A few blocks away, the Ladder 6 fire crew was going about its normal duties when one of them heard the roar of aircraft engines.

MATT KOMOROWSKI (Ladder 6 Fire Crew): We started pulling out of quarters and I distinctly remember hearing from the dispatcher, "All Lower Manhattan Units respond to the World Trade Center."

SYNC: Go. Go to the Trade Center.

NARRATOR: Inside the stricken North Tower, just ten floors beneath where the plane had hit, was commodities trader Bill Forney.

BILL FORNEY: There was a high-pitched scream. There was a tremendous change in the air pressure. The building lurched forward, back and forth. It was a scary situation. It was actually the first time that I had truly ever thought that I might die.

NARRATOR: The 767 that flew into the North Tower was larger than a 707 and moving fast. It struck the building between the 93rd and 98th floors, instantly killing scores of people in the plane and tower.

It also created a huge void across six floors on the impact wall. You can see the outline of the wing tip on the upper right. Two-thirds of the supporting columns were completely severed, but the building stood firm.

GENE 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: Leslie Robertson's radical design seemed to have worked, but there was more devastating damage hidden inside. Although the aluminum aircraft shattered on contact with the exterior wall, the speed and force of the fragments and the intact steel engines severely damaged the columns and stairwells in the core, and jet fuel began saturating the building.

PAUL NEAL: Almost immediately after the impact, somewhat bizarrely, I smelled an overwhelming stench of aviation fuel, Jet A1 gas, which I recognized because I'm a private pilot and I'm used to airfield environments. I recall smelling it and almost instantly dismissed it as being illogical and didn't have any place in the World Trade Center.

NARRATOR: In an instant, the fuel ignited a massive fire that quickly engulfed the damaged area, and this was something even Robertson had not considered.

LESLIE ROBERTSON: With the 707, to the best of my knowledge, the fuel load was not considered in the design. Indeed, I don't know how it could have been considered.

CHARLES THORNTON: They didn't have the mathematical models in the computers to model a fire as a result of the fuel in a 707. I was asked in 1986 what would happen if a plane flew into the Trade Center. And I said it would not knock the building down from the pure physics of the mass hitting the building. But we...none of us really focused on that kind of a fuel fire.

NARRATOR: Initial reports described the fire as "super hot" due to the thousands of gallons of jet fuel carried by the plane. But the fire experts on the study team found those reports to be wrong.

JONATHAN BARNETT: The role of the jet fuel...although it was hot, it only lasted a short period of time. It's very similar to using lighter fluid on a charcoal fire. It ignites the charcoal and then burns out. Its main role was to ignite other combustibles and really start the whole space burning at once.

NARRATOR: The fuel served to flash start the fire on several floors instantaneously. And since sprinkler piping in the core was completely destroyed, there was no water to slow down the blaze. Even worse, when the core was struck, the building's three emergency exits were also destroyed. So 950 people above the impact became trapped with no way out of the growing inferno.

LESLIE ROBERTSON: The people above...obviously they were 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: It now appears that people could not get past the crash area because the drywall used to protect the stairs had been blown off leaving the staircases destroyed or in flames.

One remarkable story illustrates just how weak the drywall was. The crash caused an elevator to jam between floors trapping six people in the middle of the building.

AL SMITH (Mail Services): "If we don't get out of here, are we going to suffocate in here? Will the elevator move again?" I think a lot of thoughts just raced through our mind at that particular moment.

NARRATOR: They pried the doors open and found themselves facing drywall which Jan Demczur attacked with nothing more than his window cleaning squeegee.

JAN DEMCZUR (Window Crew): Whatever you have, you have to try. And this particular time there was not brick or concrete or something, there was drywall.

AL SMITH: He takes the handle of the squeegee, takes the rubber out to make a device to work with. I focused on this guy digging into the wall like there was no tomorrow.

JAN DEMCZUR: I was chopping...I don't know...my hand was tight or something and I was...and the squeegee went straight through the hole. And I lost my squeegee.

NARRATOR: Others in the elevator took over and by kicking the drywall enlarged the hole. Al Smith, the slimmest, went through first.

AL SMITH: I went head first, then my shoulders, which was a tight squeeze. Then I hollered back into the elevator for them to push my feet.

NARRATOR: All six people got out alive by breaking through the weak drywall. For those higher up that weakness proved fatal. Some safety experts believe that stronger walls might have allowed many more people to get down the stairways.

JAKE PAULS: If the stairs had been more hardened the walls would have been less able to be breached by the collision of the aircraft. Perhaps one or two of the stairs would have survived the impact. And that would have meant people from above maybe could have passed through the impact area.

NARRATOR: For 6000 people below the impact the stairways were clear and they quickly began leaving the building. In the early stages all eyes were on the fire and the victims stuck high in the Tower.

CROWD SYNC: I saw it. I saw it.

NARRATOR: No one was thinking that there might be even worse to come, but the seeds of destruction that would eventually bring the tower down had already been sown.

These images reveal that spray-on fireproofing was completely blown off critical load-bearing steel, and several of the floor trusses were destroyed. Inside, additional trusses would have been weakened or dislodged, and fireproofing everywhere would have been obliterated.

CHARLES THORNTON: Once the plane hit and the fragments of the plane came through the building, we know it knocked out floors. We also know that it knocked spray-on fireproofing off a lot of the components. Once you lose the spray-on fireproofing you have bare steel. Once you have bare steel you don't have a fire rating anymore.

NARRATOR: Without fireproofing the steel in the core was now exposed to intense heat.

MATTHYS LEVY: So that fire caused the steel to soften up. The columns in the interior of the core began to soften, buckle, fail. And I saw that the building had really a good chance of collapsing at that point.

NARRATOR: Unaware of the danger that lay ahead, firemen began assembling to enter the crippled North Tower. Among them was the crew from Ladder 6 who faced a brutal climb of 93 floors.

MATT KOMOROWSKI: It was very slow. We took our time going up because we have heavy gear...very crowded. The civilians were coming down on our left in a single file.

JAKE PAULS: The stairs were narrow. They were crowded and they have firemen coming up. Now, every time a firefighter comes up heavily loaded with gear, the people coming down the stairs have to stop or twist to the side, so the firefighters, if anything, tended to slow down the evacuation. In perfect hindsight, firefighters should have focused on facilitating people getting out of the building at the bottom as opposed to trying to help them at the top of the building. Those people were already lost.

NARRATOR: Controlling the fire was also hopeless. It was too high up and spread over too big an area. Had firemen even reached the blaze, the equipment they carried would have made very little difference.

By now, more than 2000 people had managed to escape the burning North Tower. And many occupants had also decided to leave the undamaged South Tower, a decision that probably saved countless lives. But not everyone left the South Tower. Brian Clark, a volunteer fire marshal, stayed on the 84th floor.

BRIAN CLARK: I am strictly guessing but I would think we were perhaps down to about 25 people left on our floor. There was an announcement came over the system and said, "If you are in the midst of evacuation, you may return to your office by using the re-entry doors on the re-entry floors and the elevators to return to your office."

NARRATOR: It is not known how many, but some people turned around and went back up. Just five minutes later the terrorists struck again. A second hijacked 767 crashed into the South Tower, hitting it between floors 78 and 84. Brian Clark's office was on the 84th floor where the upper wing of the aircraft struck.

BRIAN CLARK: Our room fell apart at that moment—complete destruction. For seven to 10 seconds, there was this enormous sway in the building, and it was all one way, and I just felt in my heart that, "Oh my gosh, we're going over."

NARRATOR: The plane sliced into the South Tower at an angle to the right. Unlike in the North Tower, the core was not hit dead on. But in one crucial respect the South Tower was hit in a far more damaging way than the North. It had been struck far lower down which meant the wounded section was having to bear a far heavier load.

LESLIE ROBERTSON: And I kept saying to myself, "What's going on inside? How bad is it inside?" And there's no way to measure it.

NARRATOR: Although the path of the impact did not compromise the core as severely as in the North Tower, here the plane acted like a snowplow, pushing office contents and debris into the northeast corner and starting a raging fire at that spot.

When the plane struck, there were about 2000 people left in the South Tower, 500 above the impact line and some 1500 below.

For those above the crash site, two of the three staircases were completely destroyed. But a lucky few somehow found the one that was passable.

BRIAN CLARK: So we started down that stairway and we only went three floors, and...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, by comparison, a rather frail male. She said, "Stop, stop. You've got to go up." And she labored up to join us, moving very slowly. She was such a big woman. 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: Clark then heard cries for help coming from a damaged office nearby. It was banker Stanley Praimnath. Clarke pulled him free, and together they decided to chance the smoky stairs. But their progress was hampered by one of the things that was meant to protect them, the fire-resistant dry wall.

BRIAN CLARK: Drywall had been blown off the wall and was lying propped up against the railings here, and we had to move it, shovel it aside. You could see through the wall and the cracks and see flames just licking up, not a roaring inferno, just quiet flames licking up and smoke sort of eking through the wall.

NARRATOR: Clark and Praimnath were two of only eighteen people to escape the towers from the impact zones or above.

Less than a quarter of an hour after it had been hit, all the conditions for the collapse of the South Tower were in place. The huge weight of the top third of the building was bearing down on the weakening structure. Analysis of the steel from this part of the building reveals that the fire here reached 2000 degrees Fahrenheit, a temperature that would definitely have caused the steel to buckle.

Inside, the fire was weakening the floor trusses. Some were starting to soften and sag, pulling on their bolted connections to the columns.

CHARLES THORNTON: They had two 5/8-inch bolts at one end of the truss and two 3/4-inch bolts at the other end, which is perfectly fine to take vertical load and perfectly fine to take shear loads, but once the floor elements start to sag during a fire...okay...they start exerting tension forces because it becomes a catenary, like a clothesline, and those two little bolts just couldn't handle it.

NARRATOR: The trusses were essential for holding the building together. If too many failed the building would collapse.

LESLIE ROBERTSON: I think the structures were stalwart, but they were not that stalwart. There was no fire suppression system that could even begin to deal with that event. Nothing. Nothing. So I didn't know whether they would fall or not fall.

NARRATOR: The South Tower had now been burning for 50 minutes. The North Tower for over an hour. Office workers from both buildings were reaching ground level in a steady stream.

PAUL NEAL: There was burning debris. Over this whole plaza level were, well, bodies and body parts, and I'm assuming these were the people who had been jumping.

NARRATOR: Paul Neal made it out to the street. Others were led below ground.

BILL FORNEY: They were ushering us forward, "Let's go. Let's keep moving." We walked down these escalators down to the tunnel system, the concourse underneath the World Trade Centers.

NARRATOR: Seven minutes later, Brian Clark and Stanley Praimnath made it out of the South Tower and were four blocks away when they both looked back.

BRIAN CLARK: And Stanley said to me, "You know, I think those buildings could go over." And I said, "There is no way." I said, "Those are steel structures. That's furniture and paper and carpeting and draperies and things like that that are burning."

NEWSCASTER: We're standing next to the World Trade Towers. The police and firemen...

BILL FORNEY: A tidal wave of destruction just flowed. I remember tightening my eyes as tight as can be and grimacing and hoping that I wasn't going to die.

GENE CORLEY: The South Tower over here and the damage to the side...

NARRATOR: As they searched the visual record for the precise moment and trigger for the South Tower collapse, engineers Gene Corley and Bill Baker found crucial evidence in video shot by a nearby firm of architects. It reveals that much of the central core remained momentarily intact when the outer walls fell.

If the core remained standing, something else must have triggered the collapse.

CORLEY AND BAKER:...starts to collapse. It spreads over to here and the top of the building now is on its way down.

NARRATOR: These pictures show that the South Tower fell away from the impact wall and toward the side where the fire had concentrated. To the team, this suggested a particular mechanism for the collapse, which the video helped confirm.

The plane slammed along the eastern wall, starting a fierce fire in the northeast corner and severely damaging many of the steel columns in this area. The heat of the fire would have softened both the floor trusses and the outer columns they were attached to. When the steel became weak, the trusses would have collapsed. And without the trusses to keep them rigidly in place, the columns would have bent outward and then failed.

CHARLES THORNTON: As you start to lose the lateral support due to the floors, the exterior just crumples like a piece of paper. Or if you took a sheet of cardboard and you put some weight on it and you take out the lateral supports it will just bow right out.

NARRATOR: This footage shows the process in action. A line of columns in the outer skeleton snaps. The top of the building then lurches outwards and falls. As it does so, it dislodges many more floor trusses. Once the trusses fail, the floors they were holding cascade down with a force too great to be withstood. The result is what's called a "progressive collapse," as each floor pancakes down onto the one below.

In all, 600 people died in the South Tower, and those numbers could have been much worse.

JAKE PAULS: Had the evacuation occurred an hour or so later when the buildings were more occupied, then the story would have been very, very different. Because then the evacuation time for those extra people would have exceeded the time during which the towers were actually standing after the impact, and so there would have been many people who would be trapped in the stairways or even on the floors at the time of the collapse.

NARRATOR: It's almost impossible to overstate the shock of that first collapse.

PAUL NEAL: So I came back out onto the surface and came out into what would be my idea of what a nuclear winter would have been like.

BRIAN CLARK: The building I had worked in for 27 years was gone. And it was just a staggering thought. I mean there was then silence. People just couldn't believe it.

NARRATOR: There was now one terrible implication. If the South Tower had fallen, the North was likely to follow. An urgent message was radioed to all firemen in the building. The Ladder 6 team had reached the 27th floor when they got the word to evacuate immediately.

MIKE MELDRUM: We heard someone yelling on the radio, "It's time to start back down now."

MATT KOMOROWSKI: We are trained to go and save people and go into dangerous situations. And then when we're told to abandon our assignment it's a very odd thing

NARRATOR: By now, most of the people who could have gotten out of the burning tower had gotten out. The men of Ladder 6 had only made it down to the fourth floor when, at 10:28, the tower came down.

MATT KOMOROWSKI: I felt an incredible rush of air at my back.

SAL D'AGOSTINO (Ladder 6 Fire Crew): I remember hearing the boom, the boom. As the floors are pancaking, I'm hearing that.

MIKE MELDRUM: It was like standing in between two heavy freight trains in a tunnel going by you.

NARRATOR: Over 1400 people died in the North Tower but somehow the Ladder 6 team survived.

MIKE MELDRUM: I said, "Captain," I said, "there is a light above us." I thought it was somebody with a flash light. And I said, "What is it?" And he said, "Mick, there is a beautiful blue sky above us." And I said, "Captain, there is a 105-story building above us." He says, "No." He says, "I think we are the top of the World Trade Center right now."

SAL D'AGOSTINO: Yeah.

NARRATOR: Nearly 3000 people perished in the attack. Four hundred seventy nine were from the emergency services. One hundred fifty seven were on board the two jets. The majority of the casualties were office workers who had been trapped in the crumbling towers.

BILL BAKER: The fire may be related to the initial impact.

NARRATOR: The team now believes the North Tower collapsed in a different manner than the South. The main clue lies in what happened to the TV antenna, which rested directly on top of the core.

GENE CORLEY: Looking at the films of the North Tower, it appears that the antenna starts down just a little bit before the exterior of the building. That suggests the core went first.

MATTHYS LEVY: It was very much like a controlled demolition when you look at it, because the building essentially fell almost vertically down, as if someone had deliberately set a blast to take place to cause the building to fall vertically downward.

NARRATOR: The reason the core failed first in the North Tower can be explained by the way it was hit. The 767 had smashed through the outer wall and impacted the inner core directly, damaging or destroying essential load-bearing columns and their fire protection.

In this scenario, the fire would have softened the already weakened core columns to the point where they could no longer carry weight from above. When these columns finally failed they immediately precipitated another progressive collapse.

Knowing how the towers collapsed does not fully explain what specific components failed. For that, the investigators needed to examine the remains of the buildings.

ENGINEER: The first numbers identify which building it's in.

NARRATOR: Most steel components had locator numbers stamped on the surface.

GENE CORLEY: This part of the column extended from Floor 39 to 41.

NARRATOR: If those numbers survive, investigators can actually pinpoint where each piece of steel came from, even in this surreal and mangled pile.

Central to the investigation is finding the floor trusses and their connections.

ENGINEER: This is the bottom piece of the truss connection.

NARRATOR: These lightweight but critical supports have been prime suspects in the collapse, and many observers have been outspoken on this issue from the beginning.

CHARLES THORNTON: Had the floor system been a more robust floor system 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: From the evidence found at the steel yards, and from computer modeling of applied forces, the team now believes the truss connections probably did fail from the force of the impact, the heat of the fire, or both. But the study concludes that there was a more fundamental reason for the overall collapse.

GENE CORLEY: We found that the types of fireproofing that were used were damaged by the aircraft hitting them. If the fire resistance of the building was increased so that the material in there could burn out before a collapse occurred, then you could come back in quickly afterwards, stabilize the building and save it from collapse.

NARRATOR: The team concluded that the fire resistant foam was blown off with ease. If it had remained intact the steel would have kept its resistance to the damaging effects of the heat. And since the contents would have eventually burned out on their own, had the steel been better protected, the twin towers might not have fallen.

JONATHAN BARNETT: We have a long history of successful steel construction in this country and, in fact, the world. And one of the great successes is that under normal fire conditions we don't have building collapse. In fact, until 9/11, I was unaware of any protected steel structure that had collapsed anywhere in the world from just a fire.

GENE CORLEY: It was the combination of the impact load doing great damage to the building, followed by the fire, that caused collapse. We need to look for types of fireproofing that can take the impact and can stand up to the impact and stick to the steel after the impact.

NARRATOR: The report also points to failure of the drywall construction to protect the emergency exits in the core.

MATTHYS LEVY: The core in concrete might have actually stood for a much longer period of time, allowing many, many more occupants to leave the building. It would certainly have allowed the occupants on the upper floors to have a safe passage through at least one of the vertical stairwells. The core in concrete might have actually stood through the fire and survived.

NARRATOR: The official report suggests that, in the future, architects and engineers should consider hardening stairwells; toughening fire protection on all steel members, especially their points of connection; and creating back up supports in case key load-bearing systems fail.

Given the recommendations of the study team, it is hard to imagine that these are the ruins of buildings so stalwart and strong that they actually saved people's lives. Yet this is the central conclusion of the report and its most controversial finding.

CHARLES THORNTON: A lot of people are saying that the structural engineering in the World Trade Center was miraculously wonderful, that the building stood up that long in the case of two 767s flying into it. I would tend to think that they were not as successful as they could have been. I think the buildings, had they been a different floor design, probably would have lasted longer.

NARRATOR: Although the World Trade Center collapse will be studied for years to come, Gene Corley stands by his team's assessment.

GENE CORLEY: The buildings, we found, performed well. They demonstrated that they could take the hit of a large aircraft and not immediately collapse, and there was no trade off of safety for economy in construction.

NARRATOR: In the meantime, what is left is a fierce human tragedy and thousands of people trying to come to terms with it.

BRIAN CLARK: We lost 61 dear friends that we worked with and laughed with for years. I'm deeply saddened that they aren't here.

BILL FORNEY: You know, it scares me to think about going into a tall building. One of the big visions that I have is that the building is going to fall. And in the past I probably would have written that vision off, thinking that could never happen. But now I know that it can happen.

MIKE MELDRUM: I ride the ferry home at night and I still find it hard to believe that these buildings are missing. I can't explain what happened. I can't explain why anybody would go to that extent. I can't explain how we walked out of that building.

NARRATOR: But for Leslie Robertson, the man who built the World Trade Center, there is a special kind of torture: his office overlooks what was once his greatest achievement.

LESLIE ROBERTSON: Ground Zero is a very disturbing place for me. I mean I probably have more emotional attachment to it than maybe any other person now alive. And I cannot escape the people who died there. 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.

In this program, you met Brian Clark, one of only a few survivors who got out from above the impact zones. On NOVA's Website, hear the complete story of his escape, at PBS.org or America Online, Keyword PBS.

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

Next time on NOVA:

"It was unbelievable how catastrophic it was."

"A deadly inferno."

"The whole country was on fire."

"A war with nature."

"The fire across the ridge there."

"This summer, I think, was a warning."

Fire Wars.

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PRODUCTION CREDITS

Why the Towers Fell

Produced and Directed by

Garfield Kennedy and Larry Klein

Edited by

Bob Ede and Sam Green

Narrated by

Will Lyman

Associate Producer

Matt Barrett

Production Managers

Eileen Maguire

Marie Wiljanen

Research

Colin Davies

Alison Richards

Susie Salmond

Animation

Pictures on the Wall

Camera

Mark Molesworth

Tom Kaufman

Greg Andrake

Sound Recordists

David Roche

Michael Boyle

Mark Roy

Additional Editing

Jim Allison

Additional Music

Ray Loring

Post Production Services

Henninger Video

Archive Material

ABC News

Associated Press Television News

BNN TV

CNN Image Source

Discovery Communications, Inc.

Federal Emergency Management Agency

Gamma Press USA

ITN Archive

Molesworth Enterprises, Inc.

NBC News Archives

New York City Police Department

The Port Authority of New York and New Jersey

WABC-TV Channel 7

Special Thanks

American Society of Civil Engineers

ARUP

Fire Department of New York

Institute of Civil Engineers

Leslie E. Robertson Associates

Metal Management Northeast, Inc.

Pell Frischmann Group

Skidmore, Owings & Merrill

Worcester Polytechnic Institute, University of Science and Technology

For the BBC

Production Manager

Anna Mishcon

Series Producer

Matthew Barrett

Executive Editor

John Lynch

For NOVA

NOVA Series Graphics

National Ministry of Design

NOVA Theme

Mason Daring

Martin Brody

Michael Whalen

Post Production Online Editor

Mark Steele

Closed Captioning

The Caption Center

Production Secretaries

Queene Coyne

Linda Callahan

Publicity

Jonathan Renes

Diane Buxton

Katie Kemple

Senior Researcher

Ethan Herberman

Unit Managers

Holly Archibald

Sarah Goldman

Sharon Winsett

Paralegal

Nancy Marshall

Legal Counsel

Susan Rosen Shishko

Business Manager

Laurie Cahalane

Post Production Assistant

Patrick Carey

Associate Producer, Post Production

Nathan Gunner

Post Production Supervisor

Regina O'Toole

Post Production Editor

Rebecca Nieto

Supervising Producer

Lisa D'Angelo

Senior Science Editor

Evan Hadingham

Senior Series Producer

Melanie Wallace

Managing Director

Alan Ritsko

Executive Producer

Paula S. Apsell

A BBC/WGBH Boston Co-Production

© BBC MII

Additional Program Material © 2002 WGBH Educational Foundation

All rights reserved