The answer lies in a partially classified government study of the bombings that have come before. Scott Pelley reports.
"Five minutes before the plane hit, somebody said, 'Come over, you gotta see what's going on,' so I walked over and I was looking at the television and I walked back to my desk and I called my wife," recalled John Yates, a civilian security manager in the Pentagon. That day, he was in his office watching the World Trade Center crashes on TV.
"She asked me to do the rest of my work for the remainder of the day from underneath my desk. She was just joking, so I said 'Yeah, OK, honey, I will' and I walked back over to watch some more of what was going on in New York."
Then a plane hit the Pentagon. "The sound, it was deafening, absolutely deafening and I just remember his bright ball of fire that just went right over my head."
The fire burned 35 percent of his body. The plane hit directly below his office. But the floor held, giving some, but not all, a chance to live. There were people standing next to Yates who died.
He's here, in part, because his floor was reinforced and held up for half an hour. In an astonishing stroke of luck, the terrorists had hit the only section of the Pentagon designed to resist a terrorist attack.
"We made several modifications to the building as part of that renovation that we think helped save people's lives," says Lee Evey, who runs a billion-dollar project to renovate the Pentagon. Theyve been working on it since 1993. The first section was five days from being finished when the terrorists hit it with the plane.
The renovation project built strength into the 60-year-old limestone exterior with a web of steel beams and columns.
"You have these steel tubes and, again, they go from the first floor and go all the way to the fifth floor," says Evey. "We have everything bolted together in a strong steel matrix. It supports and encases the windows and provides tremendous additional strength to the wall."
When the plane hit at 350 miles an hour, the limestone layer shattered. But inside, those shards of stone were caught by a shield of cloth that lines the entire section of the building.
It is a special cloth that helps prevent masonry from fragmenting and turning into shrapnel. The cloth is also used to make bullet-resistant vests.
All of this, especially the steel, held up the third, fourth and fifth floors. They stayed up for 35 minutes. You can see them through the smoke, suspended over the hole gouged by the jet. Only after the evacuation did the heat melt the new steel away. Evey says that without the reconstruction, the floors might have collapseimmediately.
Lt. Gen. Bob Flowers commands the Army Corps of Engineers. The Corps helped design the Pentagon's new protection. The engineers studied past attacks, including the 1983 Marine barracks bombing in Lebanon, Oklahoma City in 1995, the Khobar Towers barracks in Saudi Arabia and the U.S. embassies in east Africa.
"At Khobar Towers, for example, most of the damage and casualties were caused by flying debris from the structure and the glass, et cetera," says Flowers. "And so based on that, we worked, designed, things to prevent flying debris and flying glass. At Oklahoma City, the bulk of the casualties were caused by the collapsing structure. So one of the things we studied was how to put redundant capability in a structure to prevent it from collapsing if it was attacked. So by applying the lessons that you learn from doing those studies, you can better protect structures in the future."
It was a tough way to learn a lesson. But there is an easier way. The Corps is making a study of safer buildings by setting off its own bombs at a research center in Mississippi.
Reed Mosher is the technical director for survivability. They have developed a team of specialists that goes to these terrorist strikes as soon as they happen.
The buildings tell the team a great deal. "We want to find what performed well, what didn't perform well, try to characterize the size of the bomb, the blast," says Mosher.
Mosher also designs his own terrorist bombings in miniature with exacting scale models of reinforced concrete buildings.
Recently, Mosher's team tested a common interior wall, particle board, steel wall studs and sheetrock. The wall is set in a steel frame with instruments inside.
Then they set off a bomb. Mosher has done hundreds of these, in an effort to create new building materials. The Corps of Engineers runs these experiments through its super computer center, which is one of the most powerful in the nation. The computer can test various kinds of bombs against different buildings without breaking any glass.
In a special 3-D imaging room Mosher showed how the super computers recreates the blast wave that hit Khobar Towers. It predicts the path of every shard of glass from a single breaking window.
These studies have helped create a new generation of window that can stand up to tremendous force. The window is strapped in, almost as if it had a seat belt. The Corps has tested more than 100 window designs.
The renovated section of the Pentagon had just been fitted with blast resistant windows
U.S. Army lieutenant Colonel Vince Kam was sitting next to one of them. "I was sitting basically about three feet away from the window. I can see with that loud bang that accompanied the fireball that came up toward the window," he recalls.
But neither the heat nor the force broke through the windows right next to the impact site. The Pentagons new windows cost $10,000 apiece. No one has counted how many lives hey saved. "If I was sitting in the same window on the unrenovated part of the Pentagon, I probably would not have survived it," Kam says.
Evey's team finished tearing down the section that they had just put up 400,000 square feet. It is less than 10 percent of the building.
Already the new Pentagon is rising. These steel rods that will support the new columns are being sunk into the old pilings. Sept. 11will always be an essential date for the Pentagon. It was Sept. 11, 1941, that ground was broken for the building and on Sept.11, 2002, they intend to open the new section. Evey says that when its finished, it will look just as it did before.
They're already pulling the limestone out of the same Indiana quarry of 60 years ago.
In Mississippi, the super computers are analyzing the Sept. 11 attack to improve the defenses. Yates has already been told that he will be moving back into the very same office.
"It's a testament to the work that the people in the renovation did and the engineers. If it hadn't been done, if there'd been no structural hardening, I can't imagine what the death and destruction would be. It would have been more catastrophic that what it is. Ten times, 100 times worse."
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