Engineers of Dreams: Great Bridge Builders and the Spanning of America
by Henry Petroski
Knopf, 479 pp., $30.00
Big bridges are not as big as tunnels, dams, and canals, but they hold a special place in the human imagination. It is not just that the experience of crossing a bridge is memorable (in a different way, so is driving through a tunnel), or that a bridge, like a dam, performs an important economic function. The Suez Canal, over one hundred miles long, is an extraordinary engineering achievement, but it is, after all, a long and deep ditch. Great bridges like the Brooklyn or the Golden Gate, on the other hand, are not merely larger-scale versions of interstate overpasses, they are in a class by themselves.
Whether or not you know anything about bridge design, you can’t fail to be touched by their form. It is certainly possible for a large bridge to be ugly. The Quebec Bridge holds the record as the longest steel cantilever bridge in the world, but, to my eye, it has an awkward shape. It lacks the brooding presence of the second-place runner-up, the Firth of Forth Bridge in Scotland, which Alfred Hitchcock used to such good effect in The 39 Steps. The best bridges have an evocative beauty that is unlike anything else. They can be stolidly robust or soaringly graceful. They are utilitarian, but unlike transmission towers or fiber-optic cables, they have character. They also manage to give a sense of place to their surroundings—rural or urban. Transportation devices, civic symbols, landmarks, and sculptures—bridges are all of these.
Bridge building has a long history. Crude cantilever bridges of overlapping logs were known in ancient China, and the Mesopotamians and Egyptians built bridges of corbeled stone. It was the Romans, however, who perfected the technique of building stone arches, which they used to great effect in bridges, as well as in aqueducts and amphitheaters. Surviving Roman bridges in Italy, Spain, and France attest to the skill of these engineers. Then bridge building stagnated for several hundred years. Medieval bridges such as the Pont d’Avignon or the Ponte Vecchio in Florence are attractive but do not fundamentally improve on Roman techniques. Nor do Renaissance bridges such as the Pont Neuf in Paris or the Rialto Bridge over the Grand Canal in Venice. Like their Roman antecedents, these stone-arch bridges rarely exceed one hundred feet in span.
Bridges are sometimes ranked by their overall length, by the height of their towers, or by their height above water level, but the measure that really counts is the span—the clear distance between supports. Achieving as big a span as possible is often a major goal of bridge designers. This is partly a practical question. There are advantages to eliminating intermediate piers that are expensive and complicated to build (especially with underwater foundations) and that interfere with river traffic. But it is also the challenge of defying gravity. Spanning greater distances is a distinct measure of engineering prowess. Just as the early airplane builders were challenged to create machines that could fly farther …