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Graphic look { October 29 2002 }

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October 29, 2002
In Data Trove, a Graphic Look at Towers' Fall

There is a computer image that captures the jetliner's nose as it splays open like a log being split, its wings shearing apart, the entire aircraft dissolving into a hail of steel and aluminum buckshot during its deadly plunge through one of the twin towers.

There is, too, a meticulously annotated, color-coded map that tracks how the fires in the towers moved on the morning of Sept. 11 window by window, floor by floor, timed to the minute as they chased trapped office workers.

And there is the alien landscape of ground zero the cliffs of debris, the weird meadows of tumbled columns, and the somehow uncrushed subterranean passageways that is captured in thousands of never-before-seen photographs.

This trove of material, including videotape, compact discs, witness accounts, mathematical analyses and high-tech imagery, was kept confidential until last week, when a federal judge in the World Trade Center insurance case made it clear that he had no objection to its public release. Taken together, the dozens of boxes represent the largest single repository of raw data and expert analysis on the Sept. 11 attack in Lower Manhattan, dwarfing the analysis compiled by the government in its first examination of how and why the towers collapsed.

The expert consultants, who have marshaled this material as part of a $3.5 billion lawsuit, consistently reach conclusions that support the side that hired them: either a consortium of insurance companies or Larry A. Silverstein, the trade center leaseholder. Regardless of their legal allegiances in that case, the forensic scientists and engineers are undisputed authorities, and the value of their labor will undoubtedly transcend the legal battle of the moment and perhaps reshape perceptions of the tragedy. The material contains, for example, a stunning new analysis asserting that except for a quirk in the path of the second jetliner, the south tower could have caved in and fallen the instant it was struck, taking many more lives.

Simulations of the planes' passage through the buildings may also explain the mystery of why some people on the very floors the planes struck could survive the impact: instead of exploding horizontally, the shattered planes were actually somewhat compacted by the dynamics of the crashes.

But the experts found that a much wider and more rapid dispersal of burning jet fuel in the north tower may explain why dozens of people on floors below the plane impact died in that building.

The opposing teams of experts also produced impressively comprehensive, but ultimately completely different answers to the rarely asked question of what might have been the implications for the Trade Center if only one tower had been struck and destroyed.

One set of experts determined that the damage caused by the one tower's collapse would have rendered the entire complex useless. The other experts, using the same raw data, dismiss suggestions that a single collapse would have caused serious structural damage, permanent environmental contamination or ignited widespread fires in the other tower.

These are just a few of the revelations and disagreements that emerge from the thousands of pages of reports by the experts.

Feat of Forensic Engineering

"Taken in the aggregate, it represents a milestone in the forensic engineering of a disaster," said Jeremy Isenberg, a member of the National Academy of Engineering and president of Weidlinger Associates, where some of the work was done, who believes the information can be used to build safer skyscrapers and to better understand the risks posed by existing ones. "I have never seen this level of technical knowledge and experience brought to bear on a single problem."

The mass of documents and analysis was compiled over the last year by a kind of dream team of engineering experts as the two litigants weighed in on the question of how much Mr. Silverstein should be compensated for the loss of the towers. Mr. Silverstein says that he is owed about $7 billion, the insurance companies half that.

Both sides, recognizing the extraordinary public interest in what would normally be an esoteric insurance debate, say they always intended to make the work public, and agreed to discuss their findings.

The Sept. 11 disaster began as two jetliners, each weighing more than 200,000 pounds with their fuel, cargo and doomed passengers, hurtled into the towers and disappeared forever from the view of the outside world.

But a powerful computer simulation led by Matthys Levy, an engineer and founding partner at Weidlinger Associates, has created a three-dimensional rendition of the mayhem that probably took place in less than a second before most of the plane fragments came to rest inside the towers. The simulation created ultra-slow-motion movies, each frame separated from the next by less than a thousandth of a second, as the plane and the structure of the towers broke up.

Although the simulation does not include the people who, tragically, were on the floors that were struck, the movies do hold new revelations about their immediate fate.

The planes were moving at such great speeds up to 586 miles an hour in the south tower impact and almost 500 miles an hour in the north that the aluminum of their wings and fuselage and the steel of their engines passed through the perimeter steel columns of the towers almost without slowing down, the simulation shows.

"It was able to go through the outer wall quite easily," Mr. Levy said.

Once inside, the aluminum of the planes was hacked to pieces by the concrete slabs of the floors, which acted like great axes when struck from the side. The heavier steel of the engines punched ahead until striking sturdy structural elements or plunging all the way through the building and soaring out the other side. As the plane slowed, the concrete floors themselves were pulverized to dust. Whole sections of the light steel support trusses that held up the floors a web of thin bars and steel strips were annihilated.

Shrapnel Compressed

Surprisingly, though, most of the shrapnel created from the planes stayed in a relatively confined path and was even compressed slightly. Seen from the side, the hail of debris formed a tapering cavity, like a worm burrowing into an apple, rather than exploding in all directions. This compression may explain why relatively few people were immediately hurt outside the floors of impact and why a handful of people on those very floors survived and escaped from the south tower.

The mangled planes finally barreled into a forest of crucial structural columns in the cores of the buildings, the simulations show. In both towers, the damage to those columns was severe so severe, in fact, that the simulations predict that the south tower should have, by this calculation, collapsed immediately.

Mr. Levy conceded that the simulations do have some significant limitations. They take into account only the tower's structural steel and not the partitions and other contents of the offices inside, which must have absorbed some of the plane's impact. So the estimated damage to the structure itself is an upper limit, "the worst thing that could happen in terms of the results," Mr. Levy said.

John Osteraas, director of civil engineering practice at Exponent Failure Analysis, who has been retained by the insurance companies, said that the incorrect result cast doubt on some of its predictions.

But Mr. Levy, who is working for Mr. Silverstein's side of the suit, said he did not believe that the erroneous prediction of the south tower's collapse revealed any shortcoming in the computer work. Rather, he said, it showed how close the tower came to falling even before the fires broke out. Subtleties in the path of the plane, which the simulation may not have captured, could have been the difference, he said.

"A slight change in the direction of the plane could have caused more damage, could have caused immediate collapse," he said.

Next, of course, came the fire. By assembling thousands of photographs, videos and witness accounts, Richard L. P. Custer, the national technical director of ArupFire, a Massachusetts fire science company, prepared a color-coded map of each face of the two towers that shows the spread of fire and smoke from the moment the fireballs erupted until each of the towers collapsed.

What emerges from this analysis and a separate fire survey by Exponent Failure Analysis may help explain why everyone in the two floors just below the plane impact in the north tower ultimately died, even if they survived the initial impacts. In the south tower, most people below impact survived and were able to flee.

As the American Airlines Flight 11 rammed into the north tower, the jet fuel was sprayed into a much larger area within the tower, the analysis shows. It documents office workers who reported burning ceilings, floors and elevators at locations throughout the lower reaches of the north tower. Flames even reached the north tower lobby, where several people were severely burned as they stood near the elevators.

The rapid and wide dispersion of the fuel apparently ignited fires on the 92nd and 93rd floors of the north tower, just below the impact zone, where Carr Futures and Marsh & McLennan had their offices. The fires also engulfed another series of floors just above impact and they somehow spread to the offices of Cantor Fitzgerald in the tower's upper reaches, possibly through a mechanical shaft, the analysis finds.

Huge Fireball, Less Damage

The experience in the south tower, at least with regard to the fire, was quite different. First, a much larger fireball in the south probably consumed more of the fuel, and, spectacular as it was, did little damage itself. Second, the path of the plane was angled away from elevator shafts and stairwells, probably leading to a more confined area of spillage, said Craig L. Beyler, a fire expert who is technical director at Hughes Associates.

"The north tower was a very central hit," Dr. Beyler said. "The south tower was more asymmetrical."

The fires in the south tower were largely confined to the tight area around the plane impact, Mr. Custer's report finds. And no fire at all is seen from the western face of the tower, even in the impact zone, which was the one area where a stairwell survived, allowing 18 people to get out of the building before it fell the only people from either building at or above impact who survived.

A statistical accounting by Mr. Custer bears out those conclusions. At one time or another, fire appeared in approximately 390 windows in the north tower, compared with 151 in the south. The reports do not directly address what these differences in the fire patterns meant for the trapped worker. Still, the findings may explain why so many more people jumped or fell from the windows of the north tower than from the south.

Steadily, the fires weakened the structure of the towers. The Weidlinger analysis created a series of diagrams for the towers, showing how stresses were distributed before they were struck, then after. Immediately after impact, the stress on remaining columns shot up, over a butterfly-shaped pattern around the impact zone on the facade and throughout the core. But none of the columns were stressed to the breaking point.

As the fires burned and the columns heated and weakened, the bland matrix of numbers measuring stresses shifted to critical levels, indicating the inevitable approach of the catastrophe the world soon witnessed. Finally, according to the Weidlinger analysis, the columns heated to the point at which the laws of physics dictated the next act: they lost their strength and failed, leading to collapse.

Not everyone agrees with those conclusions. Other analysts believe that the trade center's floors, supported by the lightweight trusses, sagged and snapped in the heat, removing critical supports for the columns, which then buckled and led to collapse. The issue remains unresolved, Dr. Osteraas said.

A Catalog of Disaster

Either way, said Daniel A. Cuoco, an engineer who is president of the Thornton-Tomasetti Group, "the central portion collapsed on itself and the facade just peeled off," a conclusion he reached after his company, which worked for the city at ground zero beginning on Sept. 11, examined hundreds of photographs of the ghastly patterns of destruction and debris that remained where the giant towers had stood.

Those photographs, each annotated to specify where and when it was taken, form perhaps the largest repository of ground zero images ever assembled. "They present a catalog, so to speak, to anyone who has an interest in understanding the disaster," said Richard Tomasetti, co-chairman of the Thornton-Tomasetti Group.

A darkened, subterranean train station where tumbling debris has ripped open the ceiling and fouled the tracks with twisted bars and pulverized concrete. An abandoned, dust-choked, underground newsstand, gutted ductwork and burned-out wiring dangling over shelves still neatly stocked with candy and magazines. A steel canyon carved into what had been the trade center's plaza, the charred and ruddy steel columns that had held up the towers strewn about like tree branches after a hurricane.

It is a world that has vanished. But through this strange, adversarial court proceeding, its images remain.

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