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Continued From Above
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 date, 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
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.
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 news-stand, 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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