Engineers study rubble of N.Y. towers, Pentagon

Teams seek lessons for tougher buildings

Terrorism Strikes America

The Nation

September 22, 2001|By Michael Stroh | Michael Stroh,SUN STAFF

Engineer Paul Mlakar stepped through the rubble of the Pentagon this week, slipping chunks of concrete and steel into his pocket.

Mlakar, an expert in blast resistance with the U.S. Army Corps of Engineers, is one of eight forensic engineers dispatched to New York and Washington by the American Society of Civil Engineers.

Their mission: to determine whether hidden in the rubble of the World Trade Center and Pentagon are lessons on how to make buildings less vulnerable to terrorists.

In particular, engineers are keen to know: Did recent design changes at the Pentagon save lives? Did the impact of the jetliners alone cause the trade center towers to fall? Or was it the fires? If so, why did the towers collapse when other high-rises have withstood similarly intense blazes?

Answers might be 18 months away.

Each team includes experts on steel, concrete, fire behavior and explosives. Engineers plan to use tools from computer simulations of how much stress the buildings were under to microscopes to examine bits of steel and concrete.

"The scrapes and scratches each hold a clue," says W. Gene Corley, an engineer with Construction Technologies Laboratories in Skokie, Ill., and head of the New York team. One of the most difficult challenges, he said, will be to differentiate between damage caused to the steel by the crashes and damage caused by the collapses.

Another will be finding the pieces of steel that can solve the mystery. The ruins of the World Trade Center - all 1 million tons - are being hauled to Fresh Kills landfill on Staten Island, where federal investigators are raking them for bodies and evidence.

Even among engineers, there is considerable debate about what caused the buildings to come down. The south tower fell 56 minutes after impact by a Boeing 767. The north tower lasted100 minutes from impact. The working hypothesis is that the combination of fire and airplane impact weakened each building enough to cause it to collapse under its own weight.

"If there hadn't been a fire, I'm convinced the towers could have stood," says Jon Magnusson, chairman of Skilling Ward Magnusson Barkshire Inc.

Magnusson, whose Seattle firm was one of the two original structural engineering consultants for the World Trade Center, estimates that the airplane that ripped into the south tower might have sliced as many as three-quarters of the 61 steel columns supporting that face: "The fact that building was able to take the hit was amazing."

James Milke, a fire protection engineer at the University of Maryland, notes that other skyscrapers ravaged by fire have emerged standing. In 1988, the 62-story First Interstate Bank Building in downtown Los Angeles burned out of control for more than three hours. In 1991, the 38-story One Meridian Plaza in Philadelphia blazed almost 19 hours. Both skyscrapers were supported by steel frames. Neither collapsed.

"Why are the two World Trade Center towers different?" asks Milke, who is also part of the World Trade Center investigation team.

One possibility, he says, is that the thick insulation that should have protected the World Trade Center's steel support beams was knocked loose or missing in key spots, making the beams more susceptible to heat. Temperatures in a building fire, whether fueled by office paper or jet fuel, reach 2,000 degrees. Unprotected steel, says Milke, would begin to weaken at 1,200 degrees.

But even if engineers eventually pinpoint the cause of the collapse, some question whether there are lessons that can be drawn from it.

After Timothy J. McVeigh destroyed the Alfred P. Murrah Federal Building in Oklahoma City in 1995 with a truck bomb outside, forensic engineers conducted a similar post mortem. The team concluded that structural reinforcement of the building could have cut damage by as much as 85 percent. The price for such improvement: 1 percent to 2 percent of the building's original cost.

The World Trade Center might be a different story, said Magnusson: "People are getting this false sense of possibility. They don't understand that this is something that you can't design for. Even if you design for one 767, what happens if they crash two?"

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