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Posted on 10/02/2001 1:44:37 PM PDT by Magician

MIT engineers say architects will have to rethink tall-building designs.

When two jetliners eviscerated the World Trade Center towers on September 11, causing them to burn and eventually buckle under their own weight, the reverberations were felt not only in New York but in the architecture and civil engineering departments of the nation’s universities, where faculty are paid to think and teach about tall buildings and the principles that keep them standing. At MIT, as elsewhere, the shock and horror shared by professors and students were quickly followed by a host of technical questions about the towers’ design and their demise. Why did the buildings collapse as quickly as they did, and in the way that they did—plunging down into their own foundations? How much structural-engineering knowledge would the terrorists have needed to predict and deliberately bring about such a catastrophic failure? Most important, what can be done in the future to protect tall buildings and their inhabitants against a brand of violence that—until recently—most people considered unimaginable?

In a number of courses offered by the School of Architecture and Planning and the Department of Civil and Environmental Engineering, scheduled lectures went by the wayside as faculty responded to students’ questions and concerns about such issues. An informal graduate seminar quickly got under way to study and simulate the disaster, and a number of research groups began preparing technical papers. “There is a lot of awareness and dedication that is going into this, in terms of thinking and talking to each other, and trying to relieve some of the pain as well,” says civil-engineering professor Oral Buyukozturk.

Tube Within a Tube

Construction of the towers was completed in the early 1970s. Institute engineers and architects describe the structures as the largest example up to that time of “tube within a tube” design, which is characterized by an inner box of steel columns supporting part of the building’s weight, with an outer tube of load-bearing steel columns joined to the floor braces in a way that stiffens the building against twisting forces produced by wind (see “Tube Design Showed Strengths and Weaknesses”). The impacts from the hijacked Boeing 767s breached the outer tubes, while temperatures approaching 1,100 °C from the blazing jet fuel—which was impervious to water from the sprinkler system—rapidly weakened the inner tubes.

“Two or three floors essentially collapsed because of the buckling of the steel,” says Buyukozturk. “The upper portion dropped onto the remaining building, and it resulted in failure because of a dynamic application of load that would have been five, six, or 10 times more than the static load. We do not consider that kind of load in building conventional structures.” Once the collapse had begun, adds civil-engineering professor Jerome J. Connor ’53, SM ’54, ScD ’59, there was no stopping it. “The pathological aspect is that the load gets higher as more stories join in the mass coming down,” says Connor. “It literally hammered itself into the ground.”

Chillingly, many MIT faculty agree that the terrorists probably knew what they were doing. “If you had a way of compromising any significant number of exterior columns and then attacking the inner tube simultaneously by essentially laying in a bomb at the middle of the building, that’s probably one of the only ways to bring those buildings down,” says John Fernandez, an assistant professor in the architecture school. Even the hijackers’ decision to aim for the area of the towers’ 90th floors may have been calculated. If the planes had hit at higher floors, “It’s conceivable that the progressive failure would not have happened,” says Buyukozturk. “Every aspect of it shows us that this was a very coordinated, knowledgeable, though terrible act. I wouldn’t be surprised if they thought about the structural implications.” In fact, the suspected mastermind behind the attacks, Osama bin Laden, earned a degree in civil engineering from King Abdul Aziz University in Jeddah, Saudi Arabia, in 1979 and inherited an estimated $300 million family fortune earned in the Saudi construction industry.

In one sense, the World Trade Center towers, designed by the late Seattle-born architect Minoru Yamasaki, performed well under the coordinated attack. Robert Fowler, SM ’65, a graduate of the civil-engineering department, went to work in 1967 at Skilling, Helle, Christiansen and Robertson, the engineering firm that oversaw the towers’ construction. “Because the size of the building was unprecedented, there was a fair amount of thought about what catastrophes could happen and how the building could respond,” he says. “And one of the things we looked at was what happens if a [Boeing] 707 hits the building. It was concluded that the buildings would survive that impact [without toppling onto other buildings]. And they met the objective in that sense.”

MIT faculty concur with Fowler’s assessment. “The structure was still good” after the planes hit, notes civil-engineering professor Franz-Josef Ulm. “Let’s be clear about it—it was fantastically designed. But then, as we all know, the stiffness and strength of materials, steel in particular, goes down with heat.”

Serious Questions

And therein, say researchers at the Institute, lie a number of serious questions about the adequacy of fire protection and evacuation planning in today’s tall buildings. Building codes in the early 1970s required that the Twin Towers’ steel cores, protected by several centimeters of cement-based insulation, last at least one hour under fire conditions before starting to deform. The south tower lasted about this long, collapsing 62 minutes after the impact, while the north tower stood for an hour and 43 minutes. But this wasn’t nearly long enough for the thousands of workers crowded into the emergency stairwells to exit the buildings. Says Connor, “Someone should have realized that one hour is pushing it when you are talking about a building with so many people.”

Buyukozturk, who also studied the 1996 fire in the English Channel Tunnel, says the Twin Towers disaster will force those designing and engineering tall buildings to consider advanced techniques for fire protection and evacuation management. More passive firebreaks—horizontal or vertical spaces containing no flammable materials—could be added to buildings to keep smoke and fire from spreading between floors, Buyukozturk and his colleagues say, and lightweight, cavity-filled materials could be used in walls or columns to absorb the impact energy from flying weapons. New cement-based or ceramic composite materials could be applied in multiple layers to structural steel, extending its longevity in a superhot fire. And independent structures containing stairwells, similar to the fireproof evacuation tunnel that runs alongside the Channel Tunnel, could be built inside or adjacent to tall buildings. (In the twin Petronas Towers in Kuala Lumpur, Malaysia—at 452 meters, the world’s tallest skyscrapers since they were completed in 1997—emergency stairwells are protected inside a pressurized, seven-square-meter concrete core that its architects call a “building within a building.”)

Buyukozturk estimates such improvements would add only 10 to 20 percent to the cost of a new building, and could extend the time available for evacuation in a calamity like the one that struck the World Trade Center to three or four hours. “It’s an expense that should not really be spared,” he says.

But protecting tall buildings and the cities around them from the acts of determined terrorists or madmen will require more than a change in design and construction methods, according to MIT researchers. Ulm and Buyukozturk, who are cowriting a paper on the World Trade Center catastrophe, say the most important task now for architects, engineers, urban planners and security experts is to build redundancy into all of the critical systems of the modern metropolis—not just structures and evacuation routes, but also transportation, communications and information networks. “This was the first attack on a mega-city in the 21st century, and as such it will go into history, like the loss of the Titanic for the 20th century,” Ulm says. “The most critical thing we have to look into now is the vulnerability of modern mega-cities.”

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