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Comparing twin towers { October 23 2002 }

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October 23, 2002
Comparing 2 Sets of Twin Towers

Over many months, reams of analysis have piled up a sometimes competing mix of calculation and deduction that suggests an array of answers to the questions of why and how the steel skeletons of the twin towers suddenly came apart in raging fires on Sept. 11. And with that increased understanding, the storms of emotion surrounding the grim, disturbing questions have quieted some.

But as scientists and engineers have gained these hard-won glimpses into the mechanics of a tragedy, there is one other question that almost all of them have carefully avoided asking: could another building, indeed any building, no matter how stoutly or cleverly built, have stood longer than the twin towers did, let more people escape or perhaps never collapsed?

At a time when public officials across the country are considering sweeping revisions to building codes, and some tenants are struggling to regain confidence in the safety of skyscrapers, many technical experts say there are good reasons they have dealt so gingerly with such a volatile, if perhaps valuable, question. Those experts worry about putting victim families through wrenching what-if thoughts, they want to avoid highlighting new potential targets for terrorists and they are awed by the sheer complexity of actually producing an answer.

It turns out, though, that in little-noticed lectures at academic and professional institutions in the last year, one man, Charles H. Thornton, who is among the world's most renowned engineers, has been both asking and answering that lingering question.

In statements that some experts see as bold and others as self-serving, Dr. Thornton has argued that at least two enormous buildings would have withstood the Sept. 11 attacks far better than the World Trade Center towers did: the Petronas Towers in Kuala Lumpur, the world's tallest buildings and, in a structural sense, alter egos of the trade center.

Dr. Thornton, who collaborated on the design of the Petronas Towers, says this has nothing to do with an exercise in ego. He says his two buildings in the capital of Malaysia are an extraordinary guide for engineers and architects who are now rethinking tall buildings from top to bottom. Dr. Thornton's claims have produced both anger and admiration. Some have accused him of grandstanding. Others say he has, with considerable courage, helped force a continuing examination that is sorely needed.

"In a situation like this, some people are hesitant to raise any issue," said Michael J. Chajes, a professor and structural engineer who is chairman of the department of civil and environmental engineering at the University of Delaware, where Dr. Thornton spoke last spring.

"Having someone at his level saying these things and bringing up the discussion is certainly a good thing for the engineering community."

For example, in the Delaware talk, Dr. Thornton contrasted the lightly protected, wallboard-encased stairwells of the World Trade Center which were severed by flying debris, trapping hundreds of people above with the thick concrete walls enclosing the Petronas stairwells. In the talk, Dr. Thornton said "concrete-encased stairwells probably would have survived that and allowed people from above to get down," Dr. Chajes recalled.

Dr. Thornton, who is co-chairman of the Thornton-Tomasetti Group, an engineering company, is one of the few living builders who can match the prestige of Leslie E. Robertson, the engineer who designed the twin towers. In 1999, they were both named among the top handful of engineers of the last 125 years by the venerable Engineering News-Record, along with people like Emily and Washington Roebling, who built the Brooklyn Bridge, and Gustave Eiffel.

Guy Nordenson, who leads his own engineering company in Manhattan and is an associate professor of architecture and civil engineering at Princeton University, where Dr. Thornton gave a talk shortly after 9/11, has an information-age comparison of the two titans. "Les is a Steven Jobs to Charlie Thornton's Bill Gates," Dr. Nordenson said, calling Mr. Robertson "a brilliant thinker" and Dr. Thornton pragmatic.

Leveling a criticism at the assertion that the Petronas Towers would have stood longer than the World Trade Center towers did, Dr. Nordenson said, "If you're claiming that such and such a building would have performed better than the World Trade Center, then the answer is, `Well, you're an engineer; show me the calculations.' "

In an interview, Dr. Thornton said, "common sense and intuition can replace a lot of calculations." For instance, he said, there is little doubt in his mind that the concrete-enclosed spine of the Petronas Towers with walls 30 inches thick at the bottom of the building would have performed better against blasts and fires than the lightweight gypsum wallboard of the trade center.

Despite such criticisms, Dr. Thornton's words have had such resonance partly because he is generally careful to say he believes Manhattan's twin towers were built as well as they could have been in their day.

He describes Mr. Robertson's design as visionary and points out that the advanced, high-strength concrete holding up all of the Petronas Towers not just the core had not been developed when the trade center went up in the 1960's and 1970's.

In that respect, the Petronas Towers, built in 1998, were made possible by rapid improvements in the most mundane of materials, said Franz Ulm, a professor of civil and environmental engineering at the Massachusetts Institute of Technology. "In the '80's and '90's, there occurred a real revolution in concrete materials that is almost not known to the public," he said.

He said that high-strength concrete generally resisted both blasts and fires better than steel like that in the trade center, where the plane impacts probably knocked loose a lightweight form of fireproofing that had been sprayed onto columns and beams, which then buckled in the heat.

Advocates for steel construction, which has long battled concrete in the marketplace, dispute the claim that there is any real difference in properly constructed buildings that use either of the materials especially since even the high-strength concrete is reinforced with embedded steel bars. "There's this perception that just because we wrap something in concrete, that it's protected from the fire," said Charlie Carter, chief structural engineer for the American Institute of Steel Construction in Chicago. "That's not true."

For a combination of historical, cultural and economic reasons, tall, concrete-core buildings dedicated to office use are unusual in New York, where builders prefer the wallboard-enclosed cores with steel frames that Mr. Robertson pioneered in the trade center.

But Patricia J. Lancaster, an architect who is the city's building commissioner, said that despite the possibility of higher costs, the city should look at revising its building codes. "Certainly, the hardening of core areas including elevator shafts and stairway enclosures is something that needs to be looked at," she said.

The escape system of the Petronas Towers has other advantages, Dr. Thornton said. The bridge running between the towers, he said, offers a backup plan that perhaps only a twinned skyscraper or, at least, one very near another tall building can take advantage of: if a catastrophe in one tower somehow blocks escape, the bridge opens an alternate route. He recommends finding new ways to put this kind of redundancy into high-rise escape plans.

Another idiosyncrasy of the Petronas Towers, one that may make them more stable against catastrophic collapse, could be hard to emulate in other structures.

The World Trade Center towers were box-shaped, and tightly clustered rows of steel columns around the facade not only held up nearly half the towers' weight but also provided all of the stiffness needed to resist the force of high winds. Each of the floors that were bolted and welded to those columns both held up the weight of people and office equipment inside and provided lateral support for the columns, preventing them from buckling.

But 16 massive concrete columns are arranged in a circle around the outside of each Petronas Tower. And the concrete core not only shares the task of holding up the building's weight with those columns but also provides about half the stiffness against the wind. Floors, of course, still run from core to exterior, but both the strong core and the circular exterior are stable and can stand largely on their own, Dr. Thornton said.

When floors began falling in the heat of the fires at the trade center, federal engineers investigating the attacks determined earlier this year, the columns were prone to buckle, setting the conditions for a total collapse. In Dr. Thornton's view, the Petronas Towers could lose several floors and remain standing. Some experts, working for Larry A. Silverstein, the leaseholder at the trade center, have recently offered a rival version of why the towers collapsed, one that says the floors did not contribute to the collapse.

"A circle is a stable shape much more stable than a square shape," said Dr. Thornton, adding that the choice of that motif originally had nothing to do with stability. Instead, the builders were looking for shapes consistent with Islamic imagery.

Which only goes to show, said Richard Tomasetti, co-chairman of the Thornton-Tomasetti Group, that there is likely to be more than one path to safer skyscrapers of the future. "I do think that there are features in the Petronas Towers that give you an arrow to the future," Mr. Tomasetti said. But he added: "Every building is an individual. I don't think there's a Utopian solution out there."

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