Engineer discusses structural failure in Katrina, other mishaps

Learning from past failures is an important part of the design process for future structures, engineering professor Henry Petroski said during a Voices of Discovery lecture Nov. 29. He also discussed engineering aspects of the New Orleans levee failures during Hurricane Katrina. Details...

Petroski, a professional engineer who serves as the Aleksandar S. Vesic Professor in the department of civil and environmental engineering at Duke University, studies the role of failure in bridge and building design. Petroski also talked about the challenge of rebuilding New Orleans’ levee system in a way that would protect the city and save its unique character. Petroski said engineers could probably design a system of levees and small hills that would offer protection from rising waters for people unable to evacuate the city.

“There are a lot of people who believe the character of the city should be preserved and raising the city, building small hills, would fundamentally change that,” Petroski said. Unlike buildings and bridges, whose flaws and failures have been studied and avoided in later construction, Petroski said engineers have “no real successful models to look at for levees. One problem with levees is that the foundation is not something which can be changed. You pretty much have to take what you get.”

Petroski believes considerations other than engineering factors will probably determine how the levees are rebuilt.

“It’s a classic engineering problem. Engineering questions don’t just revolve around technical issues,” Petroski said. “Ultimately, somebody is going to have to make some tough decisions, and that’s probably going to be made by somebody in Washington. How the levees are built will ultimately not be a question of engineering, but ultimately of politics and emotion.”

The author of “To Engineer is Human: The Role of Failure in Successful Design,” Petroski has a particular interest in the evolution of suspension bridges, which fell out of favor in Europe in the 19th century because they were vulnerable to wind. American bridge builder John Roebling studied the failure of these bridges and determined that adding more weight, guides, trusses and stays to suspension bridges would make it harder for the wind to move them. Roebling went on to build the Niagara Gorge suspension bridge in 1854 and the famous Brooklyn Bridge in New York.

As engineers became more comfortable with suspension bridges, however, they pushed the limits with little regard to potential failure, Petroski said. Suspension bridges of the early 20th century became lighter and narrower, inviting disaster. Engineers were forced to take a hard look at suspension bridge designs after the Tacoma Narrows bridge in Washington collapsed in 1940 in winds of only about 40 mph.

“What tends to happen is that as these structures become more common, they become less and less challenging,” Petroski said. “The idea of designing on failure is based on thinking about all the ways a bridge could fail.”

Petroski’s presentation was part of the Voices of Discovery science speaker series, sponsored by Elon College, the College of Arts and Sciences. The series invites noted scholars in science and mathematics to Elon to share their knowledge and experience with students.