Poor design, ageing, increasing load are the easons for bridges collapsing. But with high performance steel now being used and special attention on design, giant spans of steel and concrete may one day soar across the Strait of Gibraltar and the Bering Sea, uniting Africa and Europe, Asia and the Americas !
The collapse of the eight-lane steel-deck truss bridge recently in Minneapolis focused attention on deficiencies in many older spans around the nation. Despite these troubles, engineers say, new ideas and technologies are making bridges safer -- not to mention longer and more beautiful -- than they have ever been.
The advances are founded on better steel and concrete, smarter rules and designs and bursts of creativity that turn raw materials into works of art.
"It's usually a case of slow and steady progress," said Henry Petroski, a professor of history and civil engineering at Duke University. But past innovations, he said, suggest that the field has gone through periods of rapid progress and may do so again. Giant spans of steel and concrete may one day soar across the Strait of Gibraltar and the Bering Sea, uniting Africa and Europe, Asia and the Americas.
Today's advances develop against a backdrop of decay as the nation's aging bridges buckle under the strain of poor upkeep and steadily increasing traffic. In this decade, at least six have collapsed, often after being hit by trucks or boats. The failures have taken dozens of lives.
Experts say part of the problem is poor design, apparently a contributing factor in the Minneapolis collapse.
Another difficulty, Petroski said, is that America is in some ways a victim of its own early innovation. Early designers created the world's most comprehensive system of bridges. Many spans set records, but they are now antiques.
Petroski said much of the progress in bridge design and construction is occurring in China and other countries undergoing rapid growth, not in developed nations with fewer opportunities for new construction. He said he recently travelled to China and saw many "world class bridges" going up on the Yangtze River, with some quite striking in design and "approaching record spans."
"If there's good news," Petroski said, "it's in the Far East."
China is building thousands of bridges every year.
In the United States, many triumphs of bridge building occurred in the late 1800s and early 1900s. In 1874, the Eads Bridge opened over the Mississippi at St. Louis to become the world's longest arch bridge. It was the first use of steel as a main structural element, making possible its extremely long arches.
The pace of innovation accelerated in 1883 with the opening of the Brooklyn Bridge over the East River. It was the world's longest suspension bridge and the first hung from steel wires.
In 1931, records shattered again with the opening over the Hudson River of what became known as the George Washington Bridge. The elegant main span of the suspension bridge was the world's longest, nearly twice the nearest rival.
The secret of its success lay in a novel idea, making the roadway deck much thinner and lighter, allowing the central span to be unusually long and graceful.
The theory worked over the Hudson, but it was a disaster elsewhere. In 1940, a bridge over the Tacoma Narrows in Washington, of the same design but with an even lighter deck, collapsed in high winds. No one died because worried officials had closed it earlier that day.
Innovation
The biggest risk for bridges turned out to be not innovation but aging and increasing loads. In 1967, the Silver Bridge over the Ohio River, nearly 40 years old, collapsed, killing 46 people. It was one of the deadliest failures. The span had been designed for Model T Fords, but the weight of automobiles had more than tripled in the ensuing decades. The disaster prompted a national program of regular inspections, especially of the 1,100 highway bridges designed for Model Ts.
Today, innovation is again accelerating. Increasingly, designers use high-performance steels that are unusually strong and tough. Among other factors, their strength allows for the creation of spans of great length and beauty that are also more durable.
"They give you lots of building and design options that we didn't have 20 years ago," said Conn Abnee, executive director of the National Steel Bridge Alliance, an industry group based in Chicago.
The nation, he added, has some 400 bridges that use high-performance steel.
In 2000, a graceful bridge made from the superstrong material opened in Pennsylvania over the Allegheny River, its main span more than a football field long.
Designers and engineers are also starting to make use of a new generation of highly fluid concretes that flow better into constricted spaces. M. Myint Lwin, director of bridge technology at the Federal Highway Administration, recently said that the materials had "extraordinary potential for improving the quality of the finished product."
Colin MacDougall, a professor of civil engineering at Queen's University in Kingston, Ontario, said many advances in bridge design centered on subtle changes in how the elements are assembled. For instance, researchers are developing new recommendations for where to weld, lessening the risks of fatigue and cracks.
Putting together
"A lot of the problems for old bridges are not in the strength of the steel, but how things are put together," he said. "It comes down to design."
A final improvement is forcing improvement whether designers like it or not. In October, the Federal Highway Administration will begin enforcing new rules for bridge design meant to make new structures more efficient, more reliable, safer and longer lasting.
Among other improvements, the rules should produce bridges better able to withstand peak traffic loads and brutal weather, as well as extreme events like ship collisions and earthquakes.
