Another take on why the towers collapsed

Louis Proyect lnp3 at
Tue Sep 11 16:57:17 MDT 2001

Why the towers collapsed
The jetliners hit the World Trade Center buildings at a vulnerable

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By Bill Wyman,

Sept. 11, 2001 | The World Trade Center's twin towers were the
tallest buildings in the world at the time of their opening in 1970.
They each stood 110 stories and more than 1,300 feet tall. They are
the dominant features in an enormous office complex totaling more
than 9 million square feet of office space and together make up one
of the most recognizable architectural landmarks in the world.

Today they were reduced to heaps of rubble after one of the worst
catastrophes in U.S. history. A pair of jetliners crashed into them
Tuesday morning -- at precisely the points at which they would do the
most damage, according to architectural experts. The impacts created
fires and, ultimately, brought about the collapse of both buildings.

Why did the buildings collapse?

According to Gregory Fenves, a professor of Civil Engineering at the
University of California at Berkeley, the planes weakened the
buildings' structures at key points. Fenves, working on information
gleaned from preliminary TV reports, stressed that he was
speculating. He said that if the planes had hit the structures
higher, they could have merely damaged their tops; if they had hit
lower, they would have been up against the enormous weight and
resistance of the base of the buildings.

The buildings were architecturally interesting in many ways. Each
structure is based on a central steel core, which is surrounded by
the outside wall, a 209-foot by 209-foot cube of 18-inch tubular
steel columns, set 22 inches apart. The cores and "tube walls" share
the enormous physical weight of the structures and protect them
against the extraordinary wind forces of buildings that tall. There
are trusses that support each floor, but no other columns between the
cores and outside walls. Some floors contain nearly 40,000 square
feet of open office space.

News reports said the planes were jetliners, a 757 and a 767. The 757
has a 124-foot wingspan, is 155 feet long and can weigh 100 tons. A
767 is bigger, with a 156-foot wingspan and 159-foot length and can
weigh a maximum of 200 tons. (A 747 is more than 200 feet long and
can weigh 400 tons.)

The planes hit the buildings near the 70th or 80th floors. Their
impact severely damaged the tube walls, which carried a large
proportion of the buildings' weight. CNN footage of the second plane
hitting a tower appeared to show that a large part of the jetliner
went all the way through the building, suggesting that the interior
core was also damaged.

Once a building like a World Trade Center tower loses some of its
support, the building in effect goes to work, Fenves said. "The loads
are trying to redistribute," he said. "The loads are figuring out how
to get back down to the ground." At the same time, he noted, the
fires are deforming the physical properties of the support steel.

"It's a very rugged system," he said. "It takes a long time for the
collapse mechanism to develop. It's not like kicking the leg out from
underneath a chair. The building is 200-foot square and there's a lot
of structural system there."

But once the upper floors began to give way, terrible force was set
in motion. Each floor of a building that big might weigh 6 million
pounds, he said. Once impact is factored in as well, he said, the
force becomes irresistible.

The disaster is a terrible echo of another disaster involving a New
York landmark.

On July 25, 1945, a B-25 bomber slammed into the north side of the
Empire State Building, then the tallest building in the world. A
reckless pilot was flying over Manhattan in poor visibility; it was
apparently an accident. Thirteen people died, mostly in fires started
by burning gasoline.

The Empire State Building, Fenves noted, was built during the
Depression, and made with a much heavier structural system. The
bomber in that accident was also a much smaller plane, said Fenves.

The WTC buildings' official names are One and Two World Trade Center;
their respective heights are 1,368 and 1,362 feet tall. They are part
of a massive seven-building complex near the southeastern end of
Manhattan. The center's architect was Minoru Yamasaki. The engineers
were John Skilling and Leslie Robertson of Worthington, Skilling,
Helle and Jackson.

The complex cost $350 million in 1966, or nearly $2 billion in
today's dollars. Ground was broken in 1966, and the buildings opened
in 1970, but the complete center was not finished until 1974; there
are now seven total buildings, a large shopping mall, and an enormous
garage. An observation deck is a popular tourist destination. Beneath
the center two New York subway lines converge; there is also the
Manhattan terminus of PATH commuter trains from New Jersey.

The center has been the target of an attack before. On Feb. 26, 1993,
terrorists linked to Osama bin Laden planned and carried out a truck
bombing in the parking garage. Prosecutors said the weapon was a
1,200-pound truck bomb. Six people died and more than 1,000 were
injured in the attack. The explosion created a five-story crater
beneath the building, but its structure held.

After the center opened in 1970, for several years it was feared the
complex would become a real-estate white elephant. But for decades it
then reigned as one of New York City's premier office buildings. A
recent press release from the New York and New Jersey Port
Authorities, which own the building, says that more than 430
companies from 28 countries are tenants. The authorities said that
40,000 employees work in the buildings daily, besides 140,000 daily

The World Trade Center lost its position as the world's tallest
building in 1974, when the Sears Tower in Chicago opened. In 1998 the
two Petronas Towers in Kuala Lumpur, Malaysia, opened; they are each
more than 100 feet taller than the World Trade Center structures.

Louis Proyect, lnp3 at on 09/11/2001

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