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[GreenYes] Mechanics of World Trade Center collapse
Here's the answer to a query from a list member about how the World Trade 
Center buildings imploded so "neatly".  The towers were designed to 
withstand a hit from a Boeing 707 aircraft (these were 757 and 767 planes 
and the buildings clearly withstood the impacts).  Unfortunately, the same 
engineers who designed the structures to withstand impact of a jet plane 
did not also consider that jets carry jet fuel.  The jet fuel made a fire 
that was much hotter than "normal" office building fires, since the BTU 
value of carpets, paper, wood, etc. is much less than that of the thousands 
of gallons of fuel, and certainly burn faster when doused with it.  The 
steel girders were coated with a material intended to prevent the heat of 
"normal" fires from melting them.  But the jet fuel caused the fires to 
start melting the horizontal steel structures (under each floor), and many 
floors were on fire prior to each building's collapse.  All it took was for 
one floor (didn't matter which one) to give way, and it was all over.  No 
single floor was capable of carrying the weight of two floors, much less 
all the floors above the collapse.  Photos showed the top third of the 
second tower (the first one to collapse) initially leaning at an angle; 
this was because the plane went in the south end and remaining fragments 
out the east end close to the southeast corner.  That was the most stressed 
part of the building and it gave way first.  After the initial floor 
collapsed onto the one below it, each floor below collapsed one at a time, 
taking about 10 seconds to get down to the bottom.  One reason that the 
buildings seemed to disappear is that there were shopping complexes, 
subways and parking garages seven floors below grade.  There is concern 
that the "bathtub" built around this huge foundation for the complex (about 
3 blocks x 3 blocks) that holds the Hudson River out might have been 
compromised, but nobody knows yet.

BTW, I put imploded "neatly" in quotes above, because the dust and asbestos 
(yes, a local community had a playground soil tested for asbestos - since 
the City didn't do it - and they found it to exceed standards, so it was 
shut down today) exploded violently

- breaking windows in many nearby buildings, contaminating offices and 
apartments with dust

- spreading fire into all seven buildings in the World Trade Center 
complex, causing the 47-story WTC #7 building, untouched by the aircraft to 
collapse later in the day, and leaving the rest of the buildings as shells 
which are now being taken down by wrecking balls, and

-spreading thick smoke down city streets faster than people could run away 
from it.  A colleague of mine at the Department of Sanitation, down at 44 
Beaver which is probably half a mile away diagonally, said that after the 
collapse the sky was dark as they cowered in the lobby of their building.

Structural engineers interviewed on TV afterwards said it was possible to 
have covered the steel with enough of the protective coating to prevent the 
melting and collapse, but it would have cost more money.  Since the 
47-floor WTC #7 that collapsed simply because it burned all day (wasn't 
hit, and didn't have the jet fuel), it seems to me that any skyscraper that 
has a "normal" fire for several hours will also collapse unless the girders 
have this extra treatment.  This is especially true because most buildings 
do not have nearly as much steel reinforcement as the WTCs did.

I took a walk around the war zone on late Friday afternoon taking photos; 
the buildings and even their awnings more than two blocks away from the 
perimeter are still dust covered, despite several heavy rains that have 
occurred since the disaster.  Smoke is still rising from the rubble.  The 
exclusion zone is still quite large, and police are everywhere.

Maggie Clarke, Ph.D.
Environmental Scientist and Educator
New York City 
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