NARRATOR: The 110-story
World Trade Center
towers were designed
to withstand the impact
of a commercial jetliner.
Yet within two hours of
the attacks, both collapse.
So why did the Twin Towers fail
so disastrously and so quickly?
Under public pressure,
the US government
commissions a $16 million
investigation from the National
Institute of Standards
and Technology
in an attempt to explain
the structural failures
of the Towers.
They examine the debris
from the disaster,
run hundreds of
computer simulations,
and conduct extensive lab tests.
The official government report
into the Twin Towers tragedy
only tackled events
on the initial impact
floors, not the physics behind
the subsequent collapse.
It left the field wide open
for conspiracy theorists, who
point to the unexpectedly
rapid collapse
amid reports of explosions.
Nuclear chemist
Dr. Frank Greening
was also surprised that the
Towers collapsed so quickly.
FRANK GREENING: I
watched this whole thing
unfold, like everybody did.
And then, of course, the
South Tower collapses.
The top section just
crushes the lower section.
A little later, the North
Tower does the same thing,
and I'm just shocked, horrified.
And I say to myself,
how could this happen?
NARRATOR: Greening felt
that until the mechanics
of the collapse were
properly understood,
the fanatical rumors
would simply not go away.
He decided to calculate
whether the Towers would
collapse under their own
weight, without the need
for explosives.
 The first thing I felt
needed to be cleared up
was could the building
collapse the way we saw it,
or did it actually need
help from explosives,
as some people were suggesting?
I really threw myself
into 9/11 research,
and I decided I wanted
to apply science, physics
to studying this problem.
I set up a computer program,
based on momentum transfer,
to look at each
impact of each floor.
NARRATOR: Greening's
momentum transfer program
calculated the weight
of the buildings
above the impact floors.
He discovered that
if one floor failed,
the weight of the higher floors
would cause each floor below
to give way exactly as seen.
As the impacts were
on different floors,
the simulation would need
to account for the differing
mass above each.
There were 15 floors
above the impact
point in the North Tower, and
22 floors above the South Tower.
Frank's calculations
would also need
to prove that the floors
below each impact point
would disintegrate
in a manner that
looks similar to a
controlled demolition.
FRANK GREENING: I
ran the program,
and it showed that the buildings
would indeed come down.
Even more interesting
was the computer program
predicted the collapse times.
NARRATOR: The North Tower
falls in 13 seconds.
But with a greater
number of floors
above the impact site
of the South Tower,
it takes only 11 seconds.
FRANK GREENING: Once the
building started to collapse,
there was no stopping it.
It was a juggernaut.
The mass of the upper
block in the North Tower
was equivalent to the
mass of the Titanic.
It's about 38,000 tons.
