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Why the Towers Fell

Elevators diagram The towers' pioneering "skylobby" system, which separated express and local elevators, maximized efficiency of transport and economy of space.
Towers of Innovation
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Elevators like subways
The second problem that Tozzoli's team addressed concerned elevators. Ironically, while the invention of elevators had made skyscrapers possible, elevators were thought to limit how high skyscrapers could go. The more floors you have, the more people you have; the more people you have, the more elevators you need; the more elevators you need, the less space you have to rent to pay for all those floors. This conundrum was one of the reasons, if not the chief one, why skyscrapers rarely reached beyond 80 floors.

Undaunted, Tozzoli's group devised a solution. They would design the elevator system to mimic a subway system, with express and local elevators.

In the World Trade Center, giant express elevators, each capable of carrying 55 passengers and rising at 1,600 feet per minute, zipped up to "skylobbies" on the 44th and 78th floors. Here passengers exited on the side opposite from where they had entered and crossed the lobby to pick up local lifts. Each tower also had a single express elevator that went all the way to the top. The one in the South Tower went to the observation deck, that in the North Tower to the Windows on the World restaurant.

The beauty of this system lay in its economy of space. Local elevators for the lower, middle, and upper zones of the building sat one atop the other in the same shafts. And since the express elevators to the skylobbies traveled no farther than the 44th and 78th floors, respectively, the higher one ascended in the building, the less space had to be given over to elevator shafts.

It was, as Angus Kress Gillespie, author of the book Twin Towers, put it, "a pioneering translation into the vertical of horizontal mass transportation." The result: 75 percent of the floor space in each tower was rentable, a significant improvement over 62 percent, the highest yield achieved in earlier skyscrapers.

A tube of a tower
That 75 percent was also made possible by another innovation. Previous high-rises had relied for their structural integrity on a forest of supporting columns on each floor. Typically, architects spaced these 30 feet apart throughout the interior. The exterior walls of such buildings were merely curtain walls, which let light in and kept weather out but provided little support.

Wall and core diagram The World Trade Center's tube-style construction, with steel columns found only along the exterior wall and within a central core, freed up nearly an acre of space on each floor for offices.
Such was not the case in the World Trade Center. Consulting engineers Leslie Robertson and John Skilling invented an entirely new method of construction. The forest of interior columns vanished; such columns only appeared in and around the central core of elevator shafts, stairwells, and bathrooms. Then it was nothing but open space -- 60 feet of it on two sides, 35 on the other two sides -- before one reached the outside walls. These were not curtain walls but cages of steel columns spaced just over a yard apart, with 22 inches of glass in between. (Minoru Yamasaki, the building's architect, designed it this way in part because he was insecure with heights and felt more comfortable with such narrow windows.)

The shafts of steel in the exterior walls shouldered not only gravity loads pressing down from above but also lateral loads caused by gusty winds nudging the building from the side. Such tube-style architecture relied on high-strength steel, which was only then becoming available. It resulted in up to an acre of rentable space on each floor, and it became the pioneering style of frame for a whole new generation of buildings.

Damping the sway
As sturdy as these towers would be, Robertson and Skilling knew they would still be flexible in high winds. Indeed, they designed them to be so. But they realized the swaying effect, especially in strong gusts, might bother tenants high in the building. So they fashioned yet another innovation, a state-of-the-art damping system. Like door closers or car shocks, the dampers absorbed the wind's punch, easing the towers one way or the other so smoothly that office workers hardly noticed the movement.

Towers Each of the Twin Towers had 11,000 built-in shock absorbers to lessen the buildings' sway in strong wind.
The dampers were made of visco-elastic material. "These are materials that are partially viscous, that is, partially flowable like oil, and also elastic, which means they act somewhat like steel, in that if you strain them they return to their original shape," Robertson says. "So the material is in between those two materials -- it's not like oil, it's not like steel, it's visco-elastic."

Robertson's crew placed the dampers, 11,000 of them in each building, between the bottom of the floor trusses and the columns -- two parts of the building that tended to move with respect to each other when the edifice swayed. When it did so, those two parts would shear the visco-elastic dampers. This shearing caused the material to heat up, and that heat was transferred to the building. "So we take the energy of the wind, and we heat the building with it," Robertson says with a note of pride in his voice.

Such innovations meant nothing to the tower's critics, however. Both before and after the World Trade Center's official dedication in April 1973, certain vocal members of the American intelligentsia went after it as assiduously as those who let their feelings about the Eiffel Tower be known by signing a petition against its construction. (These included the writers Guy de Maupassant and Emile Zola.)

The philosopher Lewis Mumford, a noted architectural critic who died in 1990, railed against the building's "purposeless gigantism and technological exhibitionism." The architect Charles Jencks went so far as to liken the use of redundancy in the towers' design to fascist methods. "Repetitive architecture can put you to sleep," he wrote. "Both Mussolini and Hitler used it as a form of thought control knowing that before people can be coerced they first have to be hypnotized and then bored."

The jabs came not just from architects. New York Times columnist Russell Baker noted that the towers "seem to go on and on and on endlessly in the upward dimension, as though being constructed by battalions of exuberantly unstoppable madmen determined to keep building until the architect decides what kind of top he wants."

Tribute in Light Yamasaki saw the World Trade Center, here memorialized in light, as a monument to peace.
Yamasaki, the architect, must have been stung by such comments. He saw his creation in a completely different light. In his book Architects on Architecture, the author Paul Heyer quotes Yamasaki as saying, "World trade means world peace, and consequently the World Trade Center buildings in New York ... had a bigger purpose than just to provide room for tenants. The World Trade Center is a living symbol of man's dedication to world peace."

Tragically, since the heinous attacks of September 11th, 2001, the towers have become instead a symbol of international terrorism. Apart from the loss of life, Yamasaki would surely have been appalled and horrified if he had had any idea that such a fate awaited his "monument to peace," as he once called it. Had he lived to witness that awful day, he might have gone on to design differently in the future, for such Eiffelesque grandeur was not his natural inclination. As he once wrote, "As an architect, if I had no economic or social limitations, I'd solve all my problems with one-story buildings. Imagine how pleasant it would be to always work and plan spaces overlooking lovely gardens filled with flowers."

Fortunately, Yamasaki did not have to watch his beloved towers fall. He died in 1986 at the age of 73, with his best-known work still standing tall above Manhattan, "grand in its own right."

Peter Tyson is editor in chief of NOVA Online.

Photo credits

Further reading
Twin Towers: The Life of New York City's World Trade Center, by Angus Kress Gillespie. New Brunswick, N.J.: Rutgers University Press, 1999.

Divided We Stand: A Biography of New York's World Trade Center, by Eric Darton. New York: Basic Books, 1999.

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Surpassing Facts

Number of stories in each tower: 110

Height of Tower One: 1,368 feet

Height of Tower Two: 1,362 feet

Height of TV mast on Tower One: 330 feet

Amount of fill excavated beneath the WTC site: >1 million cubic yards

Weight of contract drawings for the steel used in WTC: 650 pounds

Amount of structural steel used in WTC: 200,000 tons

Weight of each tower: 500,000 tons

Amount of rentable space: about 10 million square feet

Amount of masonry walls: 6 million square feet

Amount of painted surfaces: 5 million square feet

Number of lighting fixtures: 200,000

Number of windows: 43,600

Number of elevators in both towers: 198

Number of "passenger movements" per day in WTC elevators: 450,000

Distance seen in every direction from observation deck: 45 miles

Amount each tower swayed from true center in strong winds: 3 feet

Cost to build the WTC: about $1 billion

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