ENGINEERS OF DREAMS: Great Bridge Builders and the Spanning of America.

By Henry Petroski. Knopf. 479 pp. $30

"Structural art" is what Petroski calls bridge design, and here he offers a spirited account of the lives and work of some of its leading practitioners. To earlier generations, the builders of great public structures were technological heroes, literally forging the unity of the nation. Petroski, a professor of engineering at Duke University, combines this half-forgotten sense of wonder with a keen analysis of the aesthetic, scientific, economic, and political choices facing his predecessors.

Focusing on five master engineers-James Eads, Theodore Cooper, Gustav Lindenthal, Othmar Ammann, and David Steinman-Petroski demonstrates that behind successful bridges lie both aesthetic vision and gritty financial and political skills. Unlike even the most ambitious buildings, bridges require agreements among municipal, state, and even national governments. For every site, there may be several plausible technologies. New designs appear, more attractive or economical, but not necessarily more durable. The imponderables include earthquake risk, future loads, and long-term maintenance. There are ugly surprises, such as the sudden collapse, in 1967, of the eye-bar suspension bridge in Point Pleasant, Ohio. And there are also unanticipated delights, including the lasting beauty, utility, and profitability of San Francisco's Golden Gate.

If graceful and economical design assured success, then bridge architecture would be a search for Platonic forms. Unfortunately, as Petroski shows, some solutions can be too elegant for their own good. Thanks to the deflec- tion theory of the Latvian-born engineer Leon Moisseiff, the builder of the George Washington Bridge (Othmar Ammann) saved millions of dollars on steel. Yet the same slender-deck design has caused bridges to sway in crosswinds. In some cases, such as the Bronx-Whitestone Bridge, the only damgae was to trusses, which ultimately had to be replaced. In others, such as the Tacoma-Narrows Bridge, the swaying caused the bridge to twist apart spectacularly.

Petroski cites research suggesting that bridge disasters occur in 30-year cycles. Each collapse promotes a new dominant design, which in turn encourages a new cadre of professionals, complete with inter- locking consultantships, to grow in confi- dence and boldness until they lose touch with their predecessors' insights. These innovators may prize the forms of older bridges, but they never read the fine print involved in their creation. Accordingly, Petroski suggests that we may be due for another debacle by the end of this century, as today's engineering elite keeps building ever-longer versions of cable-stayed bridges.

Despite its technical depth, this book is not just for admirers and protectors of our great bridges. It is also for men and women in every profession. By linking the widely publicized needs of the "physical infra-structure"--the ravages of neglect and deferred maintenance-with the more subtle but equally urgent demands of the "engineering-design infrastructure," Pe-troski shows how "neglected patterns from the past become unconscious patterns for the future." To engineers, the message is that they are "reinventing, albeit with faster and more powerful tools, the bridges of the past and of different cultures." To the general reader, it is that technological sophistication can promote a fatal illusion of discontinuity with the past. The profound contribution of Engineers of Dreams is to remind us that communication across generations may be the most important bridge of all.

-Edward Tenner