Medill News Service article - Published: Oct. 10, 2001

NU engineers draw lessons from WTC fall

By Patrick Cooper
Medill News Service

A member of a panel of civil engineers Wednesday termed the .collapse of the World Trade Center towers in New York "the most significant event in civil engineering" in years. And members of the panel agreed that little could have been done to prevent the fall.

But they said the catastrophe shows that engineers now have an even more significant role to play in future planning.

The panel from Northwestern University's Technological Institute examined the towers’ fall, the perils of the ongoing clean-up efforts and what lies ahead for engineers.

"Despite the atrocity of the mass murder in New York, we have to learn from the disaster," professor Zdenek Bazant said in his opening remarks. He called the collapses "the most significant event in civil engineering" in years.

Bazant examined the physics of the terrorist-flown jets hitting the tower and the subsequent collapses, and concluded that only fire could have caused such a disaster. The fuel on board the jets, which were traveling across the country, would have pushed the fires between 1400 and 1800 degrees Fahrenheit.

While the impact of the planes would have done a large amount of damage, the force of the jets would not have been nearly enough to trigger a collapse. Instead, agreeing with other experts on the collapse, Bazant said the multi-story fires inside the towers weakened their structures to the point where the vertical steel columns buckled and broke.

Fires of such size and heat were unexpected, he said. "In the 1960s when the building was designed, it could not have been predicted." Once the steel columns began to collapse, the top of the building fell on the bottom part, a weight that was "totally unsustainable."

But the experts said building a type of skyscraper that could withstand attacks such as these would be "astronomically expensive," Bazant said.

Some measures could have helped: a hardened outer structure of the building and better insulation of the columns and inner stairwell system. But he said even these steps would not have ultimately prevented the collapses.

With the enormous weight of the towers hitting the ground, the infrastructure below the ground still remains a danger. Professor Raymond Krizek warned that excavators at the site would have to be extremely careful in their work, especially around one massive underground wall.

"That wall is holding back the ground water, particularly the Hudson River," Krizek said.

The wall was held in place by supports when it was first built, but the restraints were cut after the floors underneath the complex were completed, with the floors then taking over all of the wall’s support, he said. Citing estimates, he said only 40 percent of these supporting floors remain after the attacks.

"What is holding the wall up now is all the debris inside the hole," he said. "If that wall moves enough to crack ... the Hudson River is only a couple blocks away."

Krizek said engineers have already plugged the subway tunnels on the site, a move that would prevent the train lines from flooding if the wall collapsed.

While the workers at the Trade Center do recovery planning, professor Dave Schulz said civil engineers and planners across the country have to do planning of their own.

"There’s clearly a major role for civil engineers to secure the country and to strengthen and rebuild the infrastructure," said Schulz, director of Northwestern’s Infrastructure Technology Institute and former Chicago budget director.

He said all aspects of building design and procedure need to be examined, especially in Chicago -- a major city and transportation center that has many tall and symbolic buildings.

Paying attention to the placement and layouts of buildings is more important now, he said, and emergency and ventilation systems deserve greater thought.

Despite the massive scale of the disaster, he urged engineers not to abandon their long-range custom of tackling projects.

"These are very early days," Schulz said. In the field of civil building, "we ought not to be making any long-term decisions based on 28 days of experience."