9/11 demolition theory challenged
Date: Wednesday, September 12th, 2007 (CST )
Topic: Conspiracies
 An analysis of the World Trade Center collapse has challenged a conspiracy theory surrounding the 9/11 attacks. The study by a Cambridge University engineer demonstrates that once the collapse of the twin towers began, it was destined to be rapid and total.
One of many conspiracy theories proposes that the buildings came down in a manner consistent with a "controlled demolition". The new data shows this is not needed to explain the way the towers fell.
Over 2,800 people were killed in the devastating attacks on New York. After reviewing television footage of the Trade Center's destruction, engineers had proposed the idea of "progressive collapse" to explain the way the twin towers disintegrated on 11 September 2001.
This mode of
structural failure describes the way the building fell straight down
rather than toppling, with each successive floor crushing the one
beneath (an effect called "pancaking").
Resistance to collapse
Dr Keith Seffen set out to
test mathematically whether this chain reaction really could explain
what happened in Lower Manhattan six years ago. The findings are
published in the Journal of Engineering Mechanics.
Previous studies have
tended to focus on the initial stages of collapse, showing that there
was an initial, localised failure around the aircraft impact zones, and
that this probably led to the progressive collapse of both structures.
In other words, the damaged
parts of the tower were bound to fall down, but it was not clear why
the undamaged building should have offered little resistance to these
falling parts.
"The initiation part has
been quantified by many people; but no one had put numbers on the
progressive collapse," Dr Seffen told the BBC News website.
Dr Seffen was able to
calculate the "residual capacity" of the undamaged building: that is,
simply speaking, the ability of the undamaged structure to resist or
comply with collapse.
His calculations suggest
the residual capacity of the north and south towers was limited, and
that once the collapse was set in motion, it would take only nine
seconds for the building to go down.
This is just a little longer than a free-falling coin, dropped from the top of either tower, would take to reach the ground.
'Fair assumption'
The University of Cambridge
engineer said his results therefore suggested progressive collapse was
"a fair assumption in terms of how the building fell".
"One thing that confounded
engineers was how falling parts of the structure ploughed through
undamaged building beneath and brought the towers down so quickly,"
said Dr Seffen.
He added that his
calculations showed this was a "very ordinary thing to happen" and that
no other intervention, such as explosive charges laid inside the
building, was needed to explain the behaviour of the buildings.
The controlled detonation
idea, espoused on several internet websites, asserts that the manner of
collapse is consistent with synchronised rows of explosives going off
inside the World Trade Center.
This would have generated a
demolition wave that explained the speed, uniformity and similarity
between the collapses of both towers.
Conspiracy theorists assert
that these explosive "squibs" can actually be seen going off in photos
and video footage of the collapse. These appear as ejections of gas and
debris from the sides of the building, well below the descending rubble.
Other observers say this
could be explained by debris falling down lift shafts and impacting on
lower floors during the collapse.
Dr Seffen's research could help inform future building design.
Copyright: BBC News
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