How the World Trade Center fell

 

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How the World Trade Center fell
Thu Sep 13 20:00:24 2001


How the World Trade Center fell
world_trade_structure300.gif (12009 bytes)
http://news.bbc.co.uk/hi/english/world/americas/newsid_1540000/1540044.stm


The design of the World Trade Center saved
thousands of lives by standing for well over an
hour after the planes crashed into its twin
towers, say structural engineers.

But the towers' ultimate
collapse was inevitable,
as the steel cores
inside them reached
temperatures of 800C -
raising questions as to
why hundreds of rescue
workers were sent into
the doomed buildings to
their deaths.

The steel and concrete
structure performed amazingly well, said John
Knapton, professor in structural engineering at
Newcastle University, UK.

"I believe tens of thousands of lives have been
saved by the structural integrity of the
buildings," he told BBC News Online.

"They had a lot of their structure taken out,
yet they remained intact for more than an
hour, allowing thousands to escape."

Temperatures at 800C

But as fires raged in the towers, driven by
aviation fuel, the steel core in each building
would have eventually reached 800C - hot
enough to start buckling and collapsing.

The protective concrete cladding on the cores
would have been no permanent defence in
these extraordinary circumstances - keeping
the intense heat at bay for only a limited
timespan.

"It was the fire that
killed the buildings.
There's nothing on
earth that could survive
those temperatures
with that amount of
fuel burning," said
structural engineer
Chris Wise.

"The columns would have melted, the floors
would have melted and eventually they would
have collapsed one on top of each other."

The building's construction manager, Hyman
Brown, agreed that nothing could have saved
it from the inferno.

"This building would have stood had a plane or
a force caused by a plane smashed into it," he
said.

"But steel melts, and
24,000 gallons (91,000
litres) of aviation fluid
melted the steel.
Nothing is designed or
will be designed to
withstand that fire."

Once the steel frame on
one floor had melted, it
collapsed downwards,
inflicting massive forces
on the
already-weakened floor below.

Science of collapse

From then on, the collapse became inevitable,
as each new falling floor added to the
downward forces.

Further down the building, even steel at normal
temperatures gave way under the enormous
weight - an estimated 100,000 tonnes from
the upper floors alone.

"It was as if the top of the building was acting
like a huge pile-driver, crashing down on to the
floors underneath," said Chris Wise.

Early in the unfolding horror, some office
workers were told to stay where they were -
dreadful advice, said Professor Knapton.

People's only hope was
to run and keep
running - reaching
open ground. The
building could have
fallen over sideways,
he points out,
potentially bringing
even greater
devastation.

Other buildings -
including the 47-storey
Salomon Brothers
building - caved in
later, weakened by the earlier collapses, and
more nearby buildings may still fall, say
engineers.

But the eventual collapse of the twin towers
was so predictable that the order should have
been given to withdraw emergency services
within an hour, said Professor Knapton. He
watched in horror, knowing the building would
fall within two hours.

The hundreds of dead firemen and police
officers should simply not have been there, he
said.

"I think they should not have gone in at all," he
said. "If they did decide to take the risk, they
should have been pulled out after an hour."

But in the panic and horror, the order was
never given for rescue workers to abandon the
building. "Mistakes were made," said Professor
Knapton.

"It sounds harsh - this
had never happened in
the world, so you can
hardly criticise them.

"But I would have given
the order to get out.
You would have
thought someone with
technical expertise would have been advising
them."

But he acknowledged that the sheer scale of
the tragedy probably overwhelmed the
operation commanders.

"I think everyone was not thinking. It was like
a horror film and I think people's rationale had
gone," he said.

Steel-core design
world-thecollapseap150.jpg (8579 bytes)
The building's design was standard in the
1960s, when construction began on what was
then the world's tallest building. At the heart
of the structure was a vertical steel and
concrete core, housing lift shafts and
stairwells.

Steel beams radiate outwards and connect
with steel uprights, forming the building's outer
wall.

All the steel was covered in concrete to
guarantee firefighters a minimum period of one
or two hours in which they could operate -
although aviation fuel would have driven the
fire to higher-than-normal temperatures. The
floors were also concrete.

The building had to be tough enough to
withstand not just the impact of a plane - and
the previous bomb attack in 1993 - but also of
the enormous structural pressures created by
strong winds.

Newer skyscrapers are constructed using
cheaper methods. But this building was
magnificent, say experts, in the face of utterly
unpredictable disaster.

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