THE RIGHT STUFF
Though the World Trade Center towers ultimately fell, they each withstood a massive impact and remained standing for an hour or more -- time that enabled thousands of people to flee to safety.
Part of the reason the towers held up was their immense width, 208 feet. That girth meant that some of the outer steel columns remained in place even after the jets lodged inside the buildings. The severely damaged tops of the towers leaned on those remaining columns for support. If the ensuing fire hadn't weakened those columns, the buildings would most likely be standing today.
"It's time for people to stop asking what went wrong at the World Trade Center, and to start asking about what went right," says Ron Klemencic, president of Skilling Ward Magnusson Barkshire, a Seattle engineering firm, and chairman of the Council on Tall Buildings and Urban Habitat.
But despite the World Trade Center's solidity under attack, many engineers believe there's room for improvement. In the early 1970s, when the Twin Towers rose to the top of the New York City skyline, any building more than 20 stories tall was made primarily of structural steel. Back then, even the strongest concrete would never have been able to support the weight of anything higher, let alone skyscrapers that stood 104 stories above the ground. Today, however, builders have at their disposal concrete that's 10 times sturdier -- its every square inch can withstand 10,000 pounds of pressure before crumbling.
Innovative kinds of concrete lend further structural benefits. For example, one new concrete contains recycled stainless steel fibers. The fibers increase the concrete's strength and its ability to absorb energy. In case of a building-shattering catastrophe, this type of concrete would cling together in larger chunks. The better the concrete stays together, the less likely it is to break from the structure and plunge down onto pedestrians below. Steel-mesh concrete was pioneered as an earthquake-proofing technique, but it could also protect against the lateral force of an airplane crash.
Concrete is far more heat-resistant than steel; it's also less flexible, which makes it relatively blast-proof. As a result, engineers say, encasing a skyscraper's steel beams in concrete will better protect the building's frame from intense heat and explosions.
In addition to fire protection, concrete provides stability. Whereas the World Trade Center towers derived their structural support from the steel columns around their perimeters, many engineers now endorse a skyscraper blueprint that features a concrete core that runs down the center of the building. Such a core serves as a building's spine, supporting its weight. It can also serve as a safe haven within which designers can place emergency escape routes: fireproof elevators and stairwells.
A case in point is the Petronas Towers in Malaysia, twin buildings that opened in 1998 and that at 1,242 feet are the tallest in the world. Each tower possesses a vertical inner core, 41 feet in diameter, which houses the building's exit routes, wrapped around a smaller concrete cylinder.
Anyone who remembers reading survivors' accounts of escaping down the World Trade Center stairwells, only to encounter exhausted firefighters making their way up, will understand the utility of a fireproof emergency elevator, accessible to rescue workers. "The idea of building a blast-proof elevator shaft so that emergency personnel can get to a fire quickly isn't new," says Arthur E. Cote, senior vice president and chief engineer of the National Fire Protection Association, which establishes fire codes for skyscrapers. In light of recent events, though, he says, "I'm sure it will receive a lot more attention."
Meanwhile, at the University of California at Berkeley, structural engineer Abolhassan Astaneh-Asl is experimenting with bolting half-inch-thick steel plates to the outer walls of concrete buildings. Though Astaneh-Asl's work aims to lessen earthquake damage, many engineers believe such a facade could have absorbed the enormous energy of the recent airplane crashes, preventing a jet fuel fire from igniting inside.
The balance between building a secure structure and one that's inviting is always tenuous at best. Creating a skyscraper that's a beacon of safety is expensive, for one thing, and it also tends to destroy the allure of prime real estate. Redundant columns block luxurious views. "We could build fortresses," says Klemencic, "but would people want to work in them and would builders want to pay for them? I'm not so sure."
One need only to look at a structure designed to withstand a 767 crash -- the nuclear power plant -- to understand the aesthetic argument. Nevertheless, in today's climate, tenants will be unlikely to sign leases for high floors without some assurance that the building is as secure as possible. "The commercial world will look into this more seriously," Klemencic says, "and they'll begin to use safety to enhance their marketing position, quite frankly."
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.