Tacoma Narrows Bridge

Cited as exemplar within Petroski's *To Engineer is Human* thesis that engineering progresses through failure. "Galloping Gertie"; "only the dog died."

[Tom holds up Petroski's To Engineer Is Human.] I wanted to talk about how failures lead to code changes — this is basically Henry Petroski. In the early-to-mid-1980s, he's a civil engineer at Duke University, and he wrote this book To Engineer is Human. He has something in here about the Hyatt Regency, and mostly civil engineering things — bridges and buildings, famous failures. To Engineer is Human comes from to err is human. His thesis is that the only way we really progress in engineering is by failure. We build a bridge, we build a new way, it collapses, and we decide that's not a good way to build it. There's the Tacoma Narrows Bridge, Galloping Gertie — you've all seen the vibration study where only the dog died. The casualty was the dog.

Cited as Petroski's central example of progress-through-failure. Not developed here.

As Petroski goes through and talks about the factor of safety in chapter nine — "Safety in numbers. While engineers can learn from structural mistakes what not to do, they do not necessarily learn from successes how to do anything but repeat what they already did." This is his thesis, that we learn from failures. He uses the Tacoma Narrows Bridge and other things. We have something called a factor of safety: if we think the required load is going to be a hundred pounds, we make it so it will support 150 pounds. That gives us a 1.5 factor of safety, fifty percent extra, because there are uncertainties in what we design.

"Galloping Gertie" — referenced as the freshman-physics canonical case for resonant failure. One casualty (a dog). Used as an example of Petroski's thesis that codes are rewritten in response to mistakes.

We build something, it blows up, it breaks, and we go in and we figure out why. There's a famous picture of Galloping Gertie, that bridge they teach in freshman physics when they talk about waves. They made that too thin, it was too slender, and so when the winds came, it wasn't stiff enough, and it would start to buckle and the waves got into resonance. How many casualties in Galloping Gertie, anybody remember?

Cited via Petroski's *To Engineer Is Human* as the canonical example of engineering progressing through failure.

There are still plenty of other industries where, until they have a big failure, they don't start requiring things. A guy who was elected to the National Academy of Engineering the same year I was, from Duke University, wrote a book called To Engineer Is Human. That's what made him famous — he didn't do anything famous in engineering, he basically wrote a book about engineering. He said that engineering progresses historically, over thousands of years, due to failures. We have a failure like the Tacoma Narrows Bridge — if you've seen Galloping Gertie — people keep pushing the limit and pushing the limit until they push it too far, it causes a major failure, and then everyone says, well, why did that occur. They start developing codes and standards to keep it from happening again.

Petroski's canonical failure example. Long slender suspension bridge lacking torsional stiffness; resonant or wind-coupled failure (cause still contested). Second iteration of the design failed. Solved by stiffening webs under the deck.

Petroski wrote this around 1990 or early '90s, saying we progress because we make something bigger, or faster, or lighter, or cheaper, and we all of a sudden have a major failure. Anybody know the story of the Tacoma Narrows Bridge? Most people have seen Galloping Gertie, the bridge that kind of twisted. Not everybody had a camera there back in the 1940s to take a picture of this thing. Galloping Gertie was a bridge, and they basically didn't have enough stiffness. They made a long suspension bridge that was very slender. The roadway, with the cables up here holding this thing — if you go look on Google for the Tacoma Narrows Bridge, you can see that 1940s movie, and this was very thin. Compare it to any other bridge like the Golden Gate Bridge, or the Verrazano-Narrows Bridge. One of the things they did after this — it was actually the second failure — they basically put some stiffening webs underneath to increase the bending strength, so they wouldn't get into the resonance.

Cited as example of design improvement (slenderness for lightweight) exceeding code limits — wind resonance failure leading to code revision.

Codes were developed historically, and they tell us the design rules to keep prior failures from happening again. That's a very valuable function. However, as we try to improve designs, we often go beyond the limits of that design — just like the galloping Gertie, that bridge they made slender, and all of a sudden they got it so lightweight and so slender it didn't have enough stiffness for the wind resonance. So they go back and rewrite the code.