Prism Magazine - Novmber 2001
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- By Henry Petroski

Onward but perhaps not Upward

The era of the signature building may very well have ended on Sept. 11, 2001.

Two different attack sites in two different cities—one with its buildings arranged vertically, in celebration of America's towering supremacy, the other horizontally, crouched symbolically in defense—have posed two very different challenges for the rescue teams at work there. And after the rubble is cleared, they will provoke two very different responses from architects and engineers.

The twin World Trade Center towers provided a large amount of office space on a relatively small plot of land. The risks inherent in such a design became dramatically clear on September 11, not so much during but after the airplane attacks, when the towers succumbed to the heat and collapsed upon themselves.

The Pentagon, by contrast, is an example of how millions of square feet of office space can be constructed close to the ground. The building continued to burn two days later, but the weight of the rest of the structure did not bear down on the affected area. Firefighters were able to tackle the blaze right away—even if with the wrong equipment—rather than remaining helplessly on the street, 80-odd stories below the point of impact and the resulting conflagration.

The Pentagon will very likely be restored to its full geometry long before any decision is made about what to do with the land on which the World Trade Center once stood. Indeed, the terrorist attack on the towers may well mark the end of new skyscraper construction for the foreseeable future.

Computer and other models can test a design's strengths and weaknesses—thereby obviating many potential disasters—but sometimes it takes a life-changing event to demonstrate a structure's true vulnerabilities. Just as the 1986 explosion of the space shuttle Challenger—the haunting images of which came to mind again last week—led to missions being postponed until the spacecraft was redesigned with improved safety features, so, too, new skyscrapers will probably remain on the drawing board until their designs have been reconsidered.

It is not that structural engineers cannot build tall buildings to withstand the impact of an airplane. That they could was proved as long ago as 1945, when the Empire State Building survived being struck by a bomber flying off-course in fog. The World Trade Center attacks demonstrated that even newer, taller buildings can take the impact of large aircraft. What the twin towers could not survive was the ensuing inferno fed by enormous amounts of jet fuel.

Can any skyscraper be designed to stand up to what the World Trade Center towers could not? Perhaps. Most super-tall buildings are made of steel, but other materials are more fire-tolerant. The concrete-lined Channel Tunnel connecting England and France was able to withstand a horrendous fire in 1996. Nontraditional structural materials, such as ceramics, might someday provide the framework for new fire-resistant skyscrapers. And current research into nanotechnology—the manipulation of structures on the atomic scale—might in the distant future yield new materials suitable for building toward the sky. But ceramics are much more brittle than steel, more susceptible to snapping under impact; and nanotechnology is still in its infancy.

Furthermore, the engineering of tall buildings involves much more than just structural materials. Tall buildings present enormous problems in vertical transportation when fire breaks out, for example, and elevators stop working. In the wake of the 1993 World Trade Center bombing, people had to find their way down dark and smoke-filled stairwells, which called attention to the importance of evacuation plans. Kuala Lumpur's twin Petronas towers, the tallest in the world, are concrete structures, completed in 1996 with a sky bridge between them—an alternative exit from the highest floors.

The World Trade Center tragedy is likely to affect future building plans in even more fundamental ways. What business will want to rent space in a new skyscraper if the very word evokes images of airplanes flying into the upper stories and deaths numbering in the thousands? Will employees be constantly on the lookout for a surprise attack? Will clients want to visit an office in which they fear they may be trapped?

Then there is the question of why anyone would want to build in the heavily populated city at all. Now that the Internet and World Wide Web have provided the means for efficient and instantaneous communications throughout the world, there is a diminished need for compact contiguous space. There may no longer be any reason to build super-tall structures, raising above surrounding rooftops the most easily recognizable and tempting targets for terrorists.

The era of the signature building may very well have ended on Sept. 11, 2001, and America's skylines—as well as many others around the world—may remain for the next several decades as they are today. It is not likely that the competition to build the world's tallest building will draw many participants while terrorists are the architects of disaster.

Henry Petroski, A.S. Vesic Professor of Civil Engineering and a professor of history at Duke University, is the author of "Remaking the World: Adventures in Engineering."

Reprinted from the Washington Post