Tuesday, April 05, 2005

Petroski: To Engineer Is Human—Failure As a Design Consideration

Engineers today... are not superhuman. They make mistakes in their assumptions, in their calculations, in their conclusions. That they make mistakes is forgiveable, that they catch them is imperative.
Henry Petroski has written a number of very enjoyable essays on the art and practice of engineering design. In To Engineer Is Human, he turns his gaze on a different and more serious aspect of engineering. In building structures and mechanisms, it is not enough to ensure success—the engineer must also "forsee failure" and design to avoid it.

Petroski looks at structures and designs that have failed, and also those that avoided failure. Along the way, he examines what we mean by terms like "strong enough," and why it doesn't make sense to overdesign in the attempt to eliminate the possibility of failure. He tells us how the collapse of the Hyatt Regency walkways in Kansas City was related to the failure of "Galloping Gertie" (the Tacoma Narrows bridge) in Washington state four decades earlier.

In much the same way as Petroski celebrated the design of everyday things like The Pencil, he discusses the way new materials changed the way engineers could deal with the problems of design, and how these changes gave rise to new methods for avoiding catastrophic failure. The chapter on the design of London's Crystal Palace describes the methodical way in which the new structural design was tested, and how even individual members were tested before being included in the building.

A chapter on brittle fracture looks at buses, nuclear power plants, stainless-steel utensils and the Liberty Bell to explore how crystalline materials may be stressed beyond their capacity to bear. And in a chapter titled "Forensic Engineering and Engineering Fiction," Petroski refers to Nevil Shute, whose novel No Highway tells the story of a forensic engineer who sabotages an airplane he fears may be subject to fatigue failure, to prevent its flying before he can uncover evidence to substantiate his fears. Petroski feels this story reflects the real brittle-failure issues faced by the DeHavilland Comet, although he quotes Shute's own disclaimer.
This book is a work of fiction. None of the characters are drawn from real persons. The Reindeer aircraft in my story is not based on any particular commercial aircraft...
—Nevil Shute, author's note to No Highway
My favorite chapter, "From Slide Rule to Computer," looks at the reliability of numbers and our willingness to extend more credibility to them when they are computed by machine rather than "the human brain." (Although my enjoyment is perhaps as much for my fellow-feeling with Petroski that learning how to assign "sig figs" in order to calculate with a slide rule ought to be still a required part of engineering education.)

We all rely on the conscientious nature and diligent design of engineers as we go about our daily business. We literally put our lives in the hands of these men and women dozens of times a day. Petroski's work goes a long way to reassure us that, despite the notorious failures, the vast majority of structures and designs function safely, as they should. As they are designed to do.

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