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 complied 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 owned 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 two 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 that 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 relative 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 that 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. Lev 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.

    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 onto 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 finding 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.