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To Forgive Design: Understanding Failure by Henry Petroski

2012, Belknap Press of Harvard U. Press, Cambridge, MA, and London, 410p. (last 50 p. are notes and indexes) At this writing, available for US$16.83 on Amazon.com.

Petroski is Professor of Civil Engineering and Professor of History at Duke University.

He is perhaps best known for his earlier book, To Engineer is Human: The role of Failure in Successful Design (1992 paperback version; at this writing, available for US$10.74 on Amazon.com).

Prof. Petrosky has credentials. I’ve seen his name and earlier book mentioned frequently across the years. 

Theme: Engineers should study and frequently consider failure. This is not only about engineers. Designers of all types can benefit from failures, including but not limited to software, business modeling, organizations, instruction, and women’s apparel (wardrobe malfunction J).
Petrosky’s background and special interest is in bridge design, so there’s much discussion about bridges. The Takoma Narrows bridge collapse (1940, Washington State) is a standout example. A videos of the bridge’s gyrations before collapse is easily found on the Internet (e.g. YouTube) and well-worth watching.

While bridges are interesting enough, Petrosky thankfully mentions other notable disasters such as:

Sometimes failure—controlled failure—is intended. Several example designs for failure are:

Why engineering failures? Following are three major causes from the book.

Design Changes

Making any change to a system that has worked should be a red flag. Beware unintended consequences. For example, software updates—in part to fix bugs—often introduce new bugs.

Major bridge disasters seem to occur every 30 years or so. This is about once every professional generation. Because some space projects require decades, NASA makes an effort to write very detailed reports so that future engineers do not have to “figure out what steps we took and why we took them (p. 341).”
(NASA is the U.S. National Aeronautics and Space Administration.)

Insufficient Attention to Odd Behaviors and Near-Misses

The book has many instances of early warnings not being investigated, including:

Situations Not Considered in the Design

Sometimes, a failure mode involves science principles unknown at the time. Metal fatigue was unknown until the latter 1800s. It took perhaps 70 years more to understand fatigue reasonably-well and to routinely consider fatigue in design. Meanwhile, railway axels kept breaking, and bridges fell down (p. 110-121). And this continues. A German high-speed train was destroyed in 1998 due to a fatigue failure of a wheel. In 2000 a train in Great Britain was derailed because fatigue destroyed the track.
Trade-Offs in Decision Making

Petrosky’s first design textbook in college (Shigley's Mechanical Engineering Design, now in its 9th edition) began “To design is to make decisions.” Later it added “Decisions are always compromises.” (p. 245)

He acknowledges multi-criteria decision making. He recounts decisions about design that were driven by cost, schedule, performance (from a purely functionality perspective), aesthetics, area economics, and politics.

“While structures can be designed to resist every possible and imaginable onslaught, judgments have to be made about whether that is practical and affordable (p. 75).”

“And the design “invariably (needs) to take into account and weigh, implicitly or explicitly, the cost and affordability of fail-safe improvements against the value of human life (p. 76).”

Petrosky’s Recommendations

“Because we are human and by nature fallible we can count on bad things continuing to occur, often when we least expect them (p. 4).”

He offers several recommendations:


To Forgive Design is an interesting book. Think all persons doing "design" work should read about failures, and this book is a good place to start.

—John Schuyler, May 2013

Copyright \A9 2013 by John R. Schuyler. All rights reserved. Permission to copy with reproduction of this notice.