As builders in Turkey construct the world’s longest single-span suspension bridge across the Canakkale Strait and U.K. Prime Minister Boris Johnson hopes to build a 22-mile bridge and tunnel combination across tumultuous waters to connect Scotland with Northern Ireland, these kinds of ambitious projects beg a crucial question: Just how long can a bridge actually be?
The answer isn’t so straightforward; it’s dependent on the topography of the environment, the materials used, and the available technology. But the basic length can be … well, never-ending. Here’s why.
Take It to the Limit
With ridiculously long bridges, you’re either talking about the longest total bridge length or the longest single span. The only limitations for bridge length come from topography; the Eyre Highway in Australia, for example, which contains a 90-mile section deemed the straightest, flattest roadway in the world, could have been built as a 90-mile bridge with no great feats of engineering. It would have been a wild waste of money to elevate a bridge over flatlands, of course, but it is possible from an engineering point of view.
For single spans, however, we have several limits, often defined by dead load weights, steel wire strengths, suspension bridge tower heights, and gobs of money.
This is where engineers show their expertise. In engineering terminology, the span is the unsupported length between the towers and piers, which could be hundreds or even thousands of feet, says Marwan Nader, T.Y. Lin International Group’s senior vice president and an engineer on some of the world’s trickiest bridges, such as the new East Span of the Bay Bridge.
The world’s longest single span is the 1.2-mile main suspension section that’s part of a three-span bridge crossing the Akashi Strait in Japan. The Akashi-Kaikyo Bridge, which opened in 1998, requires 190,000 miles of wire cabling—enough to circle the globe seven times. The 6,532 feet of the main span of the Japanese bridge won’t hold the record once Turkey opens its Canakkale 1915 Bridge in 2022, with a span of 6,637 feet. But what allows these spans to go longer?
“Can we engineer a bridge,” asks Nader, “to resist the loads? There are many governing loads we deal with.” Bridges must be able to handle seismic loads, wind loads (we see you, Tacoma Narrows), and impact loads from the potential of a vessel hitting a tower in the water. But pure gravity becomes the rub—the dead load of the actual bridge.
The Longer the Span, the Bigger the Load
Vessel impact is about tower design, not span length, says Nader. And the odd thing about seismic loads is that suspension bridges tend to be less impacted seismically the longer they are, as they naturally become isolated from the seismic forces with care concentrated at higher frequencies.
Sure, wind is tricky—wind loads have a lot to do with bridge stiffness—and offer challenges, but these are all solvable issues. The tough one, then, is the dead loads.
“The longer the span, the bigger the loads we have to span, and thereby comes the direct challenge,” Nader says. “For shorter bridges, maybe the dead load is 50 percent of the actual load and the live load (traffic on the bridge) is another 30 percent of that. As the bridge gets longer, the dead load becomes 90 percent or higher.”
Suspension bridges have the best ability to hold long spans, using super-tall towers—as tall as 1,000 feet—to hold bridge decks with steel wire. And it’s that cable, composed of thousands of wires, that carries the bridge. As technology has improved in steel cabling, so have bridge spans.
The early days of bridge design gave us cabling with ksi (kilopounds per square inch) strengths in the 30s. Then, carbon steel and structural steel bumped it to 50 ksi. Technology started extruding steel from rods and creating extremely small wire (5 millimeters in diameter) with strength that has now risen to 285 ksi, allowing tens of thousands of these super-small, super-strong wires to hold massively heavy—and long—bridge spans. Still, tensile strength and cable diameter remain the limiting factors.
“We can recognize the longer the span, the bigger the stress on the cable,” Nader says. “The limitations lie within the actual available material that a suspension bridge is made of.”
Burly Materials and Bold Ideas
If technology can change the rod strength from 36 ksi all the way to 285 ksi, what if that strength gets us another 10 percent—or more?
The biggest jumps in history in terms of bridge spans, whether the Brooklyn Bridge or Golden Gate Bridge, Nader says, comes with increases in wire strength.
“The self-weight of the bridge starts driving what we are doing,” says Thomas Cooper, a complex bridge expert with WSP. “At some point, we’re getting materials not strong enough to support their own self-weight.”
Of course, these super-long spans require bold owners and engineers willing to take a risk, and funding that increases exponentially the longer they go.
“In Japan, they went for the world record and they did it,” Nader says of the Akashi-Kaikyo. “They wanted to span the bay there and there was a need for it, but the challenge seemed to just fit within the available technology we had. You could potentially challenge yourself a bit further. Is it going to be a 5K [3 miles] suspension bridge? Is it going to be more than that? Ultimately, you are limited by the strength of the wires.”
Suspension bridges use anchorages to bear the load of the cables, which are supported by towers, whereas cable-stay bridges use the towers themselves to bear the loads. So while a cable-stay bridge may be the more economical choice for smaller spans because they require less infrastructure and steel, they often can’t bear the load of the super-long spans alone, making a suspension bridge the span of choice because of the strength of anchorage points on land. A hybrid design can marry up both styles into one bridge, bringing in multiple points of benefit.
The Super Bridges of the Future
To go the longest, Cooper says the hybrid idea allows the best solution, especially as technology advances materials and steels. And more technology may be on the way as terrain plays a factor. Cooper cites bridges in Nordic canyons that have water flowing through areas where tower foundations may need to drop 3 miles into the canyon floor, requiring the use of technology from the offshore oil industry to make it possible. “Those are really long spans in rather extreme conditions that are trying to bring in technology developed in other industries,” Cooper says.
“I don’t know that we would say that something isn’t possible, only because certainly something like 100 miles would seem pretty ridiculous,” he continues, “but we’ve gone in the last couple hundred years from several hundred feet to thousands of feet of span. The idea that you can have a 3-mile-long span seems ridiculous, but it is out there. It takes this odd blend of someone who has vision and someone who has technical capability.”
Along with the 3-mile Strait of Gibraltar crossing that T.Y. Lin hypothesized in the 1980s, Nader believes a hybrid suspension bridge with new materials in the lengths of up to 6 miles “will be achievable in the not-too-distant future.”
The longest bridge in the world covers 34 miles in Thailand. The Bang Na Expressway, which opened in 2000, hovers over a highway and is meant to reduce traffic. Meanwhile, the Jiaozhou Bay Bridge opened in China in 2011 as the longest oversea bridge, spanning 26 miles. These super-long bridges are possible, even via basic engineering, when environmental conditions allow.
But it becomes much harder to build bridges like the one Boris Johnson envisions across the Sea of Moyle when the soil depth dips so deep that the bridge now requires multiple styles of construction—no longer just the ease of a box girder style, but mixed with suspension bridge spans requiring unprecedented tower heights because of the depth of the ocean floor.
And then there’s the psychological concern of going long. While it might just be one span after another from an engineering perspective, Cooper wonders if people would actually even use a bridge of extreme length.
“If you are going to be on a bridge for 50 miles,” he says, “are you going to be comfortable with that? Is it economically feasible? It has a completely different aspect.”
Tim Newcomb is a journalist based in the Pacific Northwest. He covers stadiums, sneakers, gear, infrastructure, and more for a variety of publications, including Popular Mechanics. His favorite interviews have included sit-downs with Roger Federer in Switzerland, Kobe Bryant in Los Angeles, and Tinker Hatfield in Portland.
