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Schrock: Why some bridges collapse


Only four months after completion in 1940, the Tacoma Narrows Bridge in Washington State collapsed.

This new suspension bridge was based on what were presumed to be the full set of engineering requirements of the time, and was considered a state-of-the-art structure that had eliminated all possible risks. But unlike other bridges that served as models, this bridge was subjected to regular high winds.

Suspended bridges experience “resonance,” a physics principle most of us encountered when we used a swing on a playground and additional pushes at the right time cause us to swing higher and higher.

In the case of the Tacoma Narrows Bridge, when the sideways winds reached 42 miles per hour, it would push again and again accelerating the swing of the bridge until it collapsed. In my library of books on biology, education and China, I have a few books on engineering, all by Henry Petroski.

This explanation of the Tacoma bridge collapse is highlighted in his first book, To Engineer Is Human: The Role of Failure in Successful Design in 1982. He wrote 17 more books on engineering. And while the Francis Scott Key Bridge collapse was not caused by wind resonance, many of the concepts he explained can help us understand current dilemmas in engineering without having to go back to graduate school.

Henry Petroski (1942–2023) was an American engineer and popular author in a field that rarely produced literature understandable to the untrained public. As a professor of civil engineering and history at Duke University, he specialized in failure analysis. Had he not died of cancer last June, he would have been central to today’s analysis of the Baltimore bridge collapse.

While the impact of a large cargo ship on the slender concrete piers, not wind, was the cause of this bridge collapse, he would have noticed how the entire truss collapsed rapidly. He would be interested in whether the design was minimal to known safety levels, or whether additional strength had been built in to anticipate unknown forces.

One of his books provides an example of how new computer capabilities allowed the design of new roof supports that were just sufficient to hold up the roof, allowing cost savings by using less metal. But an unexpected event takes out one of the many supports, and the whole roof then caves in. Petroski explains: “There is clearly no guarantee of success in designing new things on the basis of past successes alone, and this is why artificial intelligence, expert systems, and other computer-based design aids whose logic follows examples of success can only have limited application.”

Petroski continues: “One of the paradoxes of engineering is that successes don’t teach you very much. A successful bridge teaches you that that bridge works.”

Petroski’s insight into the value of analyzing failure made him very valuable in our new age of highly computerized design, our blind acceptance of digital answers, and our current obsession with AI.

He first came to my attention when I read his article in American Scientist on the building of the gigantic Three Gorges Dam in China. While it was only partially completed, China invited him to come inspect their mammoth project that was underway. To his surprise, he found the huge dam to be well-constructed with abundant extra structuring to maintain its safety.

Since then, China has continued on a massive building of bridges, now over 45% of the world’s total bridges, and they span rivers and bays far longer than any American bridges. In viewing those bridges, their concrete piers appear far more robust than the slender piers of the bridge that collapsed in Baltimore. Because of their much larger population of newer bridges, Chinese engineers will be watching our response to this disaster and re-examining their own blueprints.

Was the “continuous truss” design and slender concrete piers of the Baltimore bridge a factor in the bridge collapse? Today’s engineers will eventually know, and they will use the wisdom described by Petroski. The real science of engineering relies on understanding failure.

John Schrock is retired from training biology teachers at Emporia State University and in China. He is editor emeritus of Kansas Naturalist magazine.