ENGINEERS

Observers say fire may have felled towers

By Gareth Cook and Thomas C. Palmer Jr., Globe Staff, 9/12/2001

As the World Trade Center building filled with smoke some people that were trapped decided to jump to their deaths as opposed to dying from the damage caused by the impact of the plane. (AP PHOTO)

hen terrorists reduced the twin towers of the World Trade Center to a plume of collapsing steel and glass, they brought down two of the world's strongest buildings.

Even the impact of a passenger jet, a force one specialist estimated to be the equivalent of a half-ton of TNT, was not enough to demolish either tower.

But less than two hours after yesterday's initial collision, both of the buildings had almost totally collapsed, with terrible loss of life. And engineers were searching for an answer to the question everyone was asking: How could it have happened?

Although nobody knows exactly what took place on the planes or in the towers, specialists suggested the buildings may have fallen victim to the fire that followed the initial impact.

Once the planes hit the buildings, fires fed by jet fuel could have begun to melt the massive steel columns that held the building up and girders that support each floor. And then, engineers said yesterday, one floor probably gave way, slamming into the one below and setting off a chain reaction - a sickening process, driven by gravity, that engineers call ''progressive collapse.''

''You have a domino effect,'' said Mysore Ravindra, president of LeMessurier Consultants, a structural engineering firm in Cambridge that designs skyscrapers. ''I can't even watch it on television.''

For the engineers and architects charged with designing skyscrapers that hold thousands of people, yesterday's tragedy was especially potent.

The Trade Center towers were a stunning 110 stories tall: One was 1,362 feet high, the other 1,368. The highest structures in New York, together they housed more than a thousand different businesses.

When the towers were finished in 1973, they were the world's tallest, a testament to the advancing technology of vertical construction. Only a year later, the Sears Tower took the height record away. The Petronas Towers in Kuala Lumpur, Malaysia, are now the tallest buildings in the world, at nearly 1,500 feet.

The Trade Center buildings, though, remained the highest point in New York City, and a piece of the skyline every bit as memorable as the Statue of Liberty.

''Iconic buildings like the World Trade Center towers are more than places to work,'' said Jerold Kayden, a professor at Harvard's Graduate School of Design and author of a recent book on Manhattan's skyscrapers and public spaces. ''They represent much of what New York stands for - its vitality, its optimism, its brashness. ... It's as if a piece of us has been torn away.''

They were constructed like a metal tube, with the perimeter of the building constructed of support columns, joined by reinforcing crossbeams. In the center of each building, inside this metal tube, was another rectangular arrangement of columns called the ''core.''

The exterior support columns, punched by the jets, were set so close that virtually the only space left was for windows. Even several of these columns could be removed and the building would still stand, engineers said.

In the 1993 bombing of the Trade Center, the bomb - detonated in a parking lot at the base of one of the buildings - was not set close enough to a support column to threaten the structural integrity of the building, said Mihanj Kirmani, the principal in charge of the Boston office of Weidlinger Associates, an engineering firm that has helped design US embassies abroad to withstand terrorist attacks.

In 1945, a B-25 bomber, which is a smaller plane, crashed into the Empire State Building, but the building was not permanently damaged.

But in this case, the crucial difference seems to have been the fire. The amount of fuel carried by the planes when they hit is not known, but a Boeing 767-200 can carry 16,700 gallons of fuel. When the planes hit, their fuel would have been spread by the impact. Fed by oxygen from the outside and fuel and other material inside, the building became a chimney.

At 1,200 degrees Fahrenheit, steel loses about half of its strength. The steel could buckle, bend, and eventually break, setting off a progressive collapse that would bring the building pancaking down in a column.

Engineers cautioned, though, that it will take time to determine how the buildings fell. It was impossible to say with certainty whether the planes - even loaded with fuel - would have been enough to set off the collapse.

Pedro Sifre, a consulting engineer with Simpson Gumpertz & Heger of Arlington, Mass., formerly worked in New York and is familiar with the World Trade Center towers. He said that he was having trouble imagining that the buildings came down without an assault with additional explosives.

''I see the scenario of a progressive collapse being a lot worse than the immediate damage of the crash,'' said Sifre, ''but I still don't see it bringing down the entire building.''

The vulnerability that the nation now feels is in part justified, engineers said, because it would be almost impossible to design tall buildings to withstand the kind of massive, coordinated attack that was unleashed yesterday.

The country, they said, does not have an unlimited amount of money to spend on its buildings.

''Are we only going to build bunkers?'' asked Kirmani.

If terrorists bring enough force to bear, they can bring down whatever engineers can put up.

''They tried to blow up this building before, at the columns in the basement, but the building survived,'' said William J. LeMessurier, who founded LeMessurier Consultants and teaches structural engineering at Harvard. ''This time, evil won.''

Anthony Flint of the Globe Staff contributed to this report.

This story ran on page A8 of the Boston Globe on 9/12/2001.
© Copyright 2001 Globe Newspaper Company.