-Caveat Lector-
Nighttime
September 25, 2001
Years to Build and Moments to Destroy: How the Twin Towers Fell
By WARREN E. LEERY
Little is left but rubble and dust. The degree of destruction at the World
Trade Center after the Sept. 11 terrorist attacks seems almost beyond
comprehension.
But understanding the natural forces that contributed to the disaster can
help explain the destruction of the twin towers, why they came crashing
down the way they did and why enormous clouds of dust erupted from the
site, experts say.
The buildings contained more than 200,000 tons of steel, 425,000 cubic
yards of concrete and 600,000 square feet of glass in 43,000 windows. Each
floor, a reinforced concrete pad on a metal deck supported by steel cross
beams, was about one acre and weighed about 4.8 million pounds.
The general outline of the towers' collapse was well known soon after the
terrorist attack: engineers believe that the crash of the 767's and the
intense heat generated from thousands of gallons of burning jet fuel
weakened and buckled the buildings' steel framework, causing the upper
floors to collapse and beginning an unstoppable chain reaction borne along
by the force of gravity.
Since the crash, engineers have continued to analyze the catastrophe. The
buildings came down, engineers said, much as they would have in a
controlled demolition - although a controlled collapse would have started
with the strategic placement of small explosives.
The size and weight of the concrete floor pads, and the way they collapsed
onto one another, helps explain why so much of the mass of each building
was smashed into small pieces, demolition engineers said.
As each concrete floor panicked into the one below, their combined weight
grew so large that they and everything in their path broke into smaller and
smaller pieces, until mostly dust and small pieces of rubble remained.
Dr. Frank A. Muscatel, a professor of physics at Swarthier College in
Pennsylvania, calculated that the total energy released in the disaster was
equivalent to the explosion of 600 tons of TNT. This energy discharge,
which takes into account the weight of the airplanes that struck the
buildings, their speed at impact, the jet fuel and the mass of the
buildings, is about one-twentieth of that released by the 10-kiloton atom
bomb dropped on Hiroshima, he said.
"And most of this energy was supplied by gravitational collapse," Dr.
Muscatel said. "People don't think of an item sitting still as having
energy, but it does. The energy from gravitational collapse is what the
controlled demolition people use to take down structures, and it is very
powerful."
Once the mass of the upper part of a building begins to fall, Dr. Muscatel
said, it gains momentum that adds to the force the structure
below has to resist. "If the rest of the building can't do it, everything
begins to fall straight down," he said.
Jon D. Magnusson, chairman and chief executive of Skirling Ward Magnusson
Berkshire in Seattle, a structural engineering company that worked out the
original World Trade Center design, said the enormous energy released in
the collapse was enough to pulverize the 110-story towers and their
contents.
As the buildings fell, large plumes of dust billowed from the structures
in what appeared to be high-speed jets. Engineers said compression of the
buildings' air propelled the dust.
"People don't think about it, but there is a large amount of air in a
structure," said James Redeye, president of Dykon Blasting, a demolition
company in Tulsa, Okla. "A building is like an accordion, and when it
collapses the air has to exit someplace. It exits at a pretty good volume,
carrying all the dust from concrete, sheetrock and other things inside."
An initial analysis of the World Trade Center disaster released last week
by Risk Management Solutions of Newark, Calif., a company that analyzes
catastrophes for the insurance industry and others, estimated that the
pancake collapse of the tower floors produced a major airborne "debris
surge." This cloud, which it said initially spread at more than 50 miles
per hour, dumped significant deposits of dust more than a half mile from
the site and small amounts up to two miles away.
Larry Glimmer, president of Controlled Blasting, a demolition company in
Norcross, Ga., said the large cloud of dust coming from the fall of a tall
structure was from more than the displacement of air inside.
Watching tapes of the New York disaster, he said he noticed another
phenomenon seen during controlled demolition's.
"As a building falls, it pulls the atmosphere down with it, creating a
vacuum behind it," Mr. Glimmer said. "Air rushes into the space that has
been evacuated above and creates a downdraft that hits the debris below,
pushing even more dust out with great force."
Impact of the collapse coupled with this powerful downdraft provided the
force to propel the giant dust cloud down nearby streets and around
buildings, he said.
Mr. Magnusson and others said they were initially surprised that the
buildings went straight down without sliding to the side or toppling.
Pictures of the collapse indicate that the buildings' outer support
structures may have helped guide the falling floors, Mr. Magnusson said.
The buildings had a so-called metal tube structure design, which consisted
of hundreds of steel columns spaced around the outer face of each tower to
give the structures stiffness and to support much of their weight,
including the floor pads. This outer ring was connected by steel trusses
to a cluster of metal columns in the center that supported core components
such as elevators and stairs, as well as part of each concrete floor.
Mr. Magnusson said the outer tubes of the buildings, consisting of 61 steel
columns on each of a tower's four sides, seemed to keep the concrete floors
aligned as they fell.
"As you look at the videotape, it seems that as the building frame peels
back, it stays long enough for the floors to pass," Mr. Magnusson said.
"The reason the buildings stood and the reason they went straight down was
that strong exterior tube."
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Kadosh,,, YHVH, TZEVAOT
FROM THE DESK OF:
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