On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
If you catch a train heading north from New York City, you’re bound to enjoy a view of the Tappan Zee Bridge spanning the mighty Hudson River’s second-widest point. The cantilever bridge exemplifies one of the most popular structural styles from the 1950s and 60s, America’s most fervent bridge-building days. Seeing the span from a riverside viewing platform, the Tappan Zee’s aged, slanting beams bolster one another in a thick steel grid. The bridge outlasted its original 50-year service life but now costs $50 million a year to maintain. So, in 2012 the New York State Thruway Authority asked teams of engineers to submit plans for a replacement. Three teams drafted competing proposals but they all embraced the same bridge design—a cable-stay. The winning proposal by Tappan Zee Constructors for what currently is called the New NY Bridge will stretch five kilometers across the Hudson at a cost of $3.9 billion. The first steel to break the waves was installed just off the Westchester County shore in September—two vertical columns of thick rebar that will hold up a section of the new bridge's approach.
Elsewhere—in Greenville, Miss., Louisville, Ky., and Tacoma, Wash.—civil engineers are also choosing cable-stay designs as they update the nation’s aging infrastructure. "Twenty years ago, each state wanted to get its first [cable-stayed bridge] but now you're seeing a lot more of them,” says Andrew Herrmann, a former president of the American Society of Civil Engineers. “They're becoming a go-to type in the U.S."
Cable-stay’s closet relative in bridge design is the suspension bridge, and the difference between the two is mostly in the cables. In a suspension structure such as the Golden Gate Bridge primary cables are strung from tower to tower and secondary cables drop down from those to hold the roadbed in place. Cable-stayed bridges, by contrast, have cables that run directly from the tower to the road. They essentially eliminate the cables between towers.
Today the cable-stay method is often an engineer’s first choice when designing a bridge to carry traffic, provided that the longest section of road deck to be installed ranges between 150 and 915 meters. At these lengths the thinner deck and minimalist pattern of cable-stays almost always beats out the alternatives in cost, time and material savings. Cable-stay is inadvisable for bridges with a main span longer than 915 meters because the towers would have to soar twice as high as the towers of a suspension bridge of the same length to string enough cables to hold the road deck in place. For very long bridges, traditional suspension wins out.
The longest strip of road to be lifted into place on the New NY Bridge will be 365 meters. “It falls really within the sweet spot of what a cable-stay design enables,” says David Capobianco, an NY Bridge project manager. Two wide strips of road, hung side by side for east and westbound traffic will more than double the capacity of the Tappan Zee and render the New NY Bridge to be the widest cable-stayed bridge in the world. Construction on the new bridge began last summer and the first cars are expected to cross west on it by the end of 2016. Eastbound lanes are set to open a year later.
The cable-stay method was the favorite for the Tappan Zee replacement for additional reasons. Suspension bridges like the George Washington Bridge farther down the Hudson require large anchors—typically huge blocks of concrete—on either side to hold the bridge in place. “A cable-stay bridge is self-equilibrating,” says David Goodyear, chief bridge engineer at T.Y. Lin International. “You don't have the large horizontal forces like you do with a suspension bridge.” The weight of the road deck in a cable-stayed bridge is balanced evenly on either side of a tower, rather than hanging straight down from taut lines like in a suspension.
The Hudson site for the New NY Bridge poses another problem—doughy layers of clay and silt are stacked atop the bedrock. To build the bridge, construction crews have already started to install pilings—long steel rods sunk up to 380 feet deep to reach rock below. The balanced weight of a cable-stayed bridge eliminates much of the downward pressure on pilings and piers—which is especially helpful in those soft soils. "You don’t need as many piers,” says Jerome O’Connor, director of the Institute of Bridge Engineering at the University at Buffalo, S.U.N.Y. By building fewer piers, engineers save time and money during construction. “It’s a rule of thumb—a third of the cost of the project is underground or under the water,” he adds. “You can spend a lot on the piles and piers."
Cable-stayed bridges debuted relatively recently in the U.S., even though they’ve been in vogue elsewhere for hundreds of years. An engineer named Arvid Grant built one of the first cable-stayed bridges in the U.S. across the Columbia River in Washington State between Pasco and Kennewick in 1978. Grant was trained in Europe, where bridge-builders had been honing cable-stayed methods for decades and where the one of the earliest modern bridges using this design was erected in Sweden in 1955. Here in the U.S. a French engineer named Jean Muller completed another major cable-stay project, the Sunshine Skyway Bridge in Saint Petersburg, Fl., in 1987. Improvements in stress tolerance, corrosion resistance and computer modeling around the same time helped their popularity grow as engineers built them cheaper and more efficiently.
Cable corrosion and vibrations remain two of the primary concerns for cable-stayed bridges. This is particularly true for the New NY Bridge, which will endure high vibration-causing winds along the Hudson and generous amounts of salt and snow that accumulate on the road deck in winter. To combat this, the bridge will be covered in sensors that can detect corrosion of joints or cables and track cable vibration. These sensors will wirelessly transmit data to software that analyzes it in real-time.
The New NY Bridge is expected to last for 100 years—twice as long as the original Tappan Zee was slated to be operational. The average age of U.S. bridges is 42 years old, Herrmann says, and many modern steel bridges like the Tappan Zee have aged beyond their 50-year design life. Those bridges in New York State—which were among the first to go up in the country and have tolerated 50 or more years of harsh northeastern winters—are in worse shape than most around the country. “Cable-stays are definitely here to stay,” Capobianco says about the New NY Bridge. “Even though this bridge will be the first cable-stayed in New York State, it definitely won’t be the last."