CBSNews.com's David Kohn reports on an intrepid Iranian engineer who's trying to reconstruct exactly how the twin towers collapsed. In Part II, Hassan Astaneh visits a New Jersey scrapyard full of WTC steel.

On a recent morning, Hassan Astaneh visited the recycling plant, a sprawling 50-acre wasteland dotted with enormous piles of scrap - everything from washing machines to suspension cables to railroad car undercarriages. The bright chilly morning reverberated with the rumble of trucks and the clank and whine of large machines. Welders trudged to their posts; truck drivers waited for loads. In the midst of this post-industrial wasteland stood Astaneh, dressed in spotless, blindingly white coveralls and a white hardhat.

A companion asked if one prominent three-story pile came from ground zero. "No, no, that is just junk," he said, as a wine connoisseur might refer to a bottle of Olde English 800. Then, mindful of his host, he smiled and amended himself. "Not junk. Steel scrap recyclables."

The plant is owned by a company called Hugo Neu Schnitzler, one of the country's largest steel recyclers. To process the World Trade Center steel, Hugo Neu hired 100 extra workers, who work on 12-hour shifts. The steel arrives from lower Manhattan by barge. Once at the plant, it is loaded onto trucks and taken to a seven-acre lot set aside for the project. Because the girders and columns range from two to ten inches thick - too wide to be cut by machine - the work is done by teams of welders, who are known as "burners."

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The burners cut the steel into two- and three-foot chunks, each weighing about six tons. Using an enormous magnet, a crane lifts these pieces onto a three-story heap. Eventually the crane loads the pieces into trucks, which carry them back to the dock, where they are put into ships bound for steel plants in Asia, Russia or Turkey.

Hugo Neu has given Astaneh a corner of the lot in which to store his pieces. To keep these from being inadvertently chopped up, he writes his name on them in fluorescent orange spray paint. To an unpracticed eye, these saved sections look no different from the run-of-the-mill mangled metal. But to Astaneh, the contrast is clear. One clue is fire damage. Only those members that were subjected to very high temperatures - hot enough to burn away fireproofing and scorch metal – could soften to the buckling point.

But the main clue, he says, is shape. "If you drop something from that 1,000-feet elevation, the bend will be random. But if a structure buckles, the buckle shape is exactly like a wave shape. That shape is a mathematical equation. It's a nice curve," he says.

"It must have happened somewhere up in the building. It can't have happened when it dropped. This must have buckled up there. When it buckles up there, it's important," he says. About half of the steel members are stamped with an identification number, so Astaneh can pinpoint exactly where in the towers they originated.

The operation - which began days after the collapse, okayed by then-New York Mayor Rudy Giuliani - goes on 24 hours a day, seven days a week. As a result, Astaneh has almost certainly missed seeing crucial pieces before they were cut up and sent overseas. The thought pains him. In his view, the building should have been reconstructed before it was recycled.

"When there is a car accident and two people are killed, you keep the car until the trial is over," he says. "If a plane crashes, not only do you keep the plane, but you assemble all the pieces, take it to a hangar, and put it together. That's only for 200, 300 people, when they die. In this case you had 3,000 people dead. You had a major machine, a major manmade structure. My wish was that we had spent whatever it takes, maybe $50 million, $100 million, and maybe two years, get all this steel, carry it to a lot. Instead of recycling it, put it horizontally, and assemble it. You have maybe 200 engineers, not just myself running around trying to figure out what's going on. After all, this is a crime scene and you have to figure out exactly what happened for this crime, and learn from it. But that was my wish. My wish is not what happens."

Sometimes he gets lucky. He came across one crucial member after it had already been cut up, just as it was to have been taken for recycling. This section has no nice curve; it is missing a semicircular chunk, about the size of a large barrel. The edges of this semicircle are serrated, indicating that the wound occurred before fire softened the metal. Astaneh is certain that a plane engine smashed into this beam during the initial collision. "If you look at this steel piece you will absolutely agree with me that it was hit by something very fast and round," he says. "When I found this, it is very clear what happened when the engine hit: It took a bite out of the column. It is like someone biting a cookie."

This jagged chunk has an aura about it. It carries the imprint of fate, of the irrevocable instant when the die was cast. But for Astaneh, the piece is important for less poetic reasons. It indicates that by itself, this collision did not cause the building to collapse; even with this grievous wound, the column held up.

In fact, says Astaneh, the towers would very likely still be standing had the impact been the only damage. But when the planes penetrated the buildings, they injected tens of thousands of gallons of extremely flammable jet fuel into each tower. "You just brought in very carefully a large amount of fuel, completely," he says. "Then let's ignite it. In this case, there was no struggle, the building sat there innocently and the plane went in. It went in like a bullet going into a flesh."

Even before Sept. 11, Astaneh was working on a solution to this - a kind of bulletproof vest for buildings. "It's not going to stop the plane like a fly bouncing off your car window," he says. "But planes are not designed to have a head-on crash. They are not like cars, they don't have bumpers. What you need is to create enough high stresses that you can fracture the body of the plane outside the building; there will be a big struggle between the building and the plane that is trying to enter. Hopefully the plane will drop the fuel outside, at least most of it."

His research has turned up other intriguing and potentially useful discoveries. He found, for example, that the collapse began when the floors gave way. In areas where the fire was hottest, the floor beams were bent downward, indicating that the floors fell before the columns. Without the floors, the columns could not support the overlying weight by themselves, and the buildings came down. Establishing this sequence is crucial, because researchers can then focus on developing techniques to shore up the weakness.

He also came across severely scorched members from 40 or so floors below the points of impact. He believes that the planes obliterated the elevator walls, allowing burning fuel to pour down into the building, igniting blazes hundreds of feet below the main fire. "When the plane hit," he says, "the walls around the elevator shaft were gone, just thrown away." These lower-floor fires may have contributed to the collapse, and certainly added to the death toll.

To turn these discoveries into practical improvements, Astaneh has been working with David McCallen, a structural engineer at the Lawrence Livermore National Laboratory in California. They are using the lab's powerful supercomputers to model how different designs would hold up to a collision with a plane. They feed the computer information about the towers, and then adjust variables to find out how the buildings might have been saved. There are an almost endless variety of possible tweaks: What would have happened if the elevator shafts had been made of concrete instead of gypsum board? What would have happened if the buildings' welds and bolts had been stronger?

Eventually, Astaneh says, this work will help limit the harm caused by terrorism. But he is less optimistic about the larger issue: terrorism itself, and the destruction and death it causes, he says, cannot be eliminated. Even the best system of security will eventually be breached, he says.

"The way the scientific community works is that as soon as you realize there is a threat, there is a germ, there is a reason that people can get killed, you look at probability. And it looks like the probability is pretty high, because this threat is not such that you can say, like malaria, that we completely eradicated it and there is no way of people getting it in this country. We haven't eradicated terrorists. We are not going to eradicate them. If terrorists attack a building and explode something you will get some damage, some casualties. We are thinking maybe society can tolerate maybe ten casualties, maybe 50 even, even maybe 100 in a major terrorist attack. But society cannot tolerate 3,000."

Written by David Kohn;