American Scientist

Many people have asked me how long a bridge can last. The answer to that question can range from days to months to decades on the one extreme and from centuries to millennia—and possibly even longer—on the other, depending on such diverse and interrelated factors as design, construction and maintenance, all of which are affected by the vagaries of economics, politics, weather and luck. Examples are legion. London's Millennium Bridge stayed open only three days before it had to be closed for a major reconsideration of its design. The Tacoma Narrows Bridge lasted only four months before it fell to the wind. Just about everywhere we drive, we see interstate highway bridges built barely 20 years ago being replaced by wider and stronger spans. Moving toward the opposite extreme, the world-famous Brooklyn Bridge is well over a century old. In England, the first iron bridge, completed in 1779, still carries pedestrians over the Severn River. In southern France, the Pont du Gard stands as a two-millennium-old monument to Roman engineering.