When a bridge collapses, our reaction is one of incredulity.

After all, humans have been putting up bridges for thousands of years with increasingly strong and resilient materials, guided by ever-more sophisticated design, engineering and construction tools. Shouldn't catastrophe have been eliminated by now, the way medicine has eradicated smallpox?

In "To Forgive Design: Understanding Failure," Henry Petroski, a professor of history and civil engineering at Duke University, explains why bridges and other structures will continue to fail: They're commissioned, funded, designed, built and maintained by humans.

The book inspects a cross section of disasters: the destruction of the space shuttles Challenger and Columbia, the 2010 explosion of the Deepwater Horizon oil rig in the Gulf of Mexico, Boston's Big Dig and the collapse of the Kansas City Hyatt Regency walkway in 1981. The bulk of these pages, however, address bridge failures.

Because bridges are "products of pure engineering," Petroski believes disasters such as the rush-hour collapse of the I-35 bridge in Minneapolis in 2007 and the failure of the Point Pleasant Bridge over the Ohio River in 1967 offer the keenest lessons in human arrogance, neglect, complacency and failure to learn from the past. These, he writes, can be as dangerous as inferior steel and neglected maintenance.

It is human nature to overestimate the reliability of what we build. Managers at NASA predicted that the failure rate for the space shuttle would be one in 100,000 -- absurdly low for such a complicated machine. Tragically, the real failure rate proved to be one in 100. The estimates were off by a factor of a thousand.

In his 17th book, Petroski argues that failure is the sine qua non of improvement. This notion animated his original 1992 work, "To Engineer Is Human."



"Successful change comes not from emulating success and trying to better it, but from learning from and anticipating failure, whether actually experienced or hypothetically imagined," he writes in the current volume.

Imagine the Titanic had steamed back and forth across the Atlantic for 30 years without ever encountering an iceberg. Its supposedly unsinkable design -- which featured a thin steel hull for greater speed, insufficiently high bulkheads and a paucity of lifeboats -- would have been deemed a success. Other shipbuilders would have copied the design and might have made hulls even thinner and carried even fewer lifeboats.

But because the Titanic sank, its design flaws were exposed and ship design improved as a result. In engineering, technology and design, failure defines the scope.

Nonengineers needn't worry that the book will be too dense with details; Petroski makes the science easily understandable. (In 2009, Petroski wrote a piece for the Washington Post taking a shot at the customary privileging of science over engineering.)



In "To Forgive Design," it's a slog through the author's recollections of his student days at the University of Illinois, and he takes an unnecessary detour into a Canadian engineering society.

However, those flaws are minor in a book that satisfactorily explains why our determination to push the boundaries guarantees both failure and triumph.

James F. Sweeney is a critic in Fairview Park, Ohio.