9/11 demolition theory challenged
Posted: September 11, 2007
The study analysed how the twin towers
collapsed
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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 to be 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.
Once the collapse began, it was destined
to be "rapid and total"
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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.
Conspiracy theorists see evidence of
a "controlled detonation"
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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.
Article at: news.bbc.co.uk
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