New York State's longest bridge is in dire straits. “At times, you can see the river through the cracks in the pavement,” President Barack Obama said at a press conference in front of the Tappan Zee Bridge in May 2014. “Now, I'm not an engineer, but I figure that's not good.”
It's not. The three-mile-long Tappan Zee carries 138,000 vehicles a day over the Hudson River. It is also “functionally obsolete” and as such exemplifies America's crumbling infrastructure: about one in 10 bridges in the country merits the disturbing designation of “structurally deficient,” according to a 2013 report by the U.S. Department of Transportation. Built in 1955, the Tappan Zee has aged beyond its 50-year design life, like many of the steel crossways constructed during the nation's most fervent bridge-building days in the 1950s and 1960s. And now the cantilever bridge—a structure that distributes weight over beams anchored to the shore—costs $50 million a year to maintain. It is in such bad shape that Obama fast-tracked a replacement: a cable-stayed bridge, which distributes weight with cables and towers.
The cable-stayed bridge debuted in its full form in the U.S. in the 1970s, decades after engineers in Europe honed the design. Today, because of improvements in structural modeling, this method is often a civil engineer's first choice for bridges up to 3,000 feet long. They go up faster than alternative approaches and cost less because they use less material. The Tappan Zee's replacement, currently called the New NY Bridge, will take shape this spring as crews begin to work above the water on the steel underpinnings of the road. Cable-stayed bridges are also under construction in Portland, Ore., Louisville, Ky., and Los Angeles. “They're becoming a go-to type in the U.S.,” says Andrew Herrmann, a former president of the American Society of Civil Engineers.
The design's closest relative is the suspension bridge—the difference between the two lying largely in how the cables are strung. In a suspension structure, such as the Golden Gate Bridge, there are two sets of cables: primary cables that connect the towers to one another and secondary cables that hang from the first set and hold the roadbed in place. A cable-stayed bridge, in contrast, has only cables that run directly from the towers to the road. Suspension bridges also require large anchors—typically huge blocks of concrete—at either end to hold them in place, whereas the weight of the road deck of a cable-stayed bridge is balanced evenly on each side of its towers and so does not need anchors.
When completed, the New NY Bridge will have eight traffic lanes and be the widest cable-stay in the world at a cost of approximately $3.9 billion. Its erection is long overdue. Bridges throughout the Northeast, which have tolerated 50 or more years of harsh winters, are in worse shape than most. “Even though this bridge will be the first cable-stay in New York State, it won't be the last,” says David Capobianco, a project manager for the new bridge. “Cable-stays are definitely here to stay.” The construction company expects that the structure will last for 100 years and that the first vehicles will cross it in late 2016.
FURTHER READINGS AND CITATIONS ScientificAmerican.com/feb2015/advances