Had you crossed the Francis Scott Key Bridge just a few weeks ago, as millions of motorists did before a wayward container ship struck it in the early hours of March 26, you’d have passed below a familiar steel superstructure of triangulated sections about half a mile long, a sight that was part of Baltimore’s skyline for decades.
A similar drive sometime in the future, engineers say, will offer a different view. A replacement for the fallen Baltimore landmark could be anchored by a pair of massive concrete towers, cables descending from their upper reaches in fan-like configurations.
“Cable-stayed” bridges, as they’re known, are the kind most commonly used today in places where car and truck traffic must pass across wide, heavily navigated waterways — like the 1.6-mile gap between Dundalk and Hawkins Point where the Key Bridge once stood.
There are 36 bridges of this design in the United States and hundreds more across the globe. The nearest to Baltimore are the Senator William V. Roth Jr. Bridge that carries Delaware Route 1 over the Chesapeake & Delaware Canal near St. Georges (which cost $57.9 million to build and opened in 1995) and the Indian River Inlet Bridge between Dewey Beach and Bethany Beach in the same state ($150 million, 2012). The world’s longest, at 5.1 miles, the Sutong Yangtze River Bridge, opened 50 miles north of Shanghai in 2003 with a price tag of $1.7 billion.
Among its other advantages, this comparatively new bridge style makes a longer central span possible. That’s the main portion of a bridge most maritime traffic passes below. A longer span creates more generous horizontal clearance for vessels at a time when ships like the Dali, the one that destroyed the Key Bridge, are longer, heavier and harder to maneuver than ever.
“I’m sure designers will submit many proposals for replacing the Baltimore bridge, and they will want to make it something very special, as well as very safe,” said Andrzej S. Nowak, a professor of structural engineering at Auburn University in Alabama. “I will be very surprised if a cable-stayed design is not chosen.”
For those who have not followed the conversation among engineers since last month’s disaster, the Key Bridge, built in 1977, was of the “continuous truss” variety. That type was widely used and considered effective in many settings between the 1870s and the mid-20th century.
Characterized by load-bearing superstructures made of trusses — structural units consisting of beams typically connected to each other in triangular form — they allowed for spans, or clearance through their main portals, long enough to suit the needs of their time. But the longest spans topped out at a little more than 1,000 feet.
In the case of the Key Bridge, designers also took the cost-effective option of making it continuous, meaning the trusses were so rigidly attached to each other that if one collapsed, the others followed.
Such a design, engineers say, was a safe, sturdy and economical choice for the time. But the challenges faced by bridges across busy waterways as recently as 50 years ago were minuscule compared to what they encounter today.
When Key Bridge was built, the biggest container ship in the world in storage terms, the German-made Hamburg Express, could carry 2,950 20-foot units of containers. Such TEUs are standard measurement for the industry, though most containers are 40 feet. The Dali, built in 2015, could hold nearly 10,000 TEUs. The biggest in the world today, the Chinese-built MSC Marina, carries 24,346.
“At the time the Key Bridge was designed, such vessels didn’t exist,” said Ian Firth, a British structural engineer and bridge designer, in a telephone interview with The Baltimore Sun. “Nobody had the foggiest idea that such a vessel could possibly ever exist. No way the designer could have imagined this, and indeed, not just the designer, but those who commissioned the bridge and the project and decided on the size of the span.”
In horizontal clearance terms, the Baltimore span did not meet a standard set in 2000, when American structural engineers Mike Knott and Zoltan Prucz wrote in their reference work, the Bridge Engineering Handbook, that “analysis of past collision accidents has shown that bridges with a main span less than two to three times the design vessel length … are particularly vulnerable to vessel collision.”
By that measure, the Dali, which is 984 feet long, would have needed a span of between 1,968 and 2,952 feet to maneuver safely.
Firth agreed that the authors’ standards were reasonable.
“These huge ships are notoriously difficult to maneuver,” he said. “The fundamental principle should be to give them as much room to maneuver as they can have.”
As the U.S. grew in the 19th century, so did the need to build bridges across wider spaces. Structural engineers invented the suspension bridge to answer the challenge. At spans like the Brooklyn Bridge (which opened in 1883) and the Golden Gate Bridge (1937), heavy cables were run across the tops of a row of massive support towers. Lesser cables were draped over those and ran down to the bridge deck to hold it up.
Before long, European designers were working on a variety that was lighter, cheaper to build, and supportive of even longer spans. Those are cable-stayed bridges. They might seem similar in appearance to their suspension predecessors, but they feature a basic difference: their cables are attached directly to the tops of their support towers and descend directly to the bridge decks they hold up. The towers themselves bear the load of the decks.
According to Nowak, because steel is so expensive and they use less of it, cable-stayed bridges are less costly than their counterparts. Also, given the sheer number of cables, if one or two fail, others accept their load. They’re also lighter and use less of their power to support their own weight, more to support vehicle traffic.
Such cable-stayed creations as the Sunshine Skyway, which spans Lower Tampa Bay in Florida, the bridges in Delaware, and the Arthur Ravenel Jr. Bridge in South Carolina, showcase the design’s most striking visual feature: cables that fan downward in a striking geometric pattern.
“One of the more famous bridge engineers said that however economical and good it is, an ugly bridge is a bad bridge,” Nowak said. “These just look more beautiful. You see the cables descending from above, like the strings of a harp.”
Invented in the 1950s in Sweden and developed in Europe, cable-stayed bridges were still in their early stages — and largely unknown in the United States — until 1980, when a freighter called the MV Summit Venture slammed into the original Sunshine Skyway bridge in a heavy fog. The impact sent a 1,200-foot section crashing into the bay below, killing 35 people.
Florida’s governor at the time, Democrat Bob Graham, eschewed U.S. tradition by proposing a cable-stayed version as a replacement. The idea was approved, the new bridge opened in 1988, and the new Sunshine Skyway has served as a model in the industry ever since.
Cable-stayed bridges now also span the Ohio River northeast of downtown Louisville, Kentucky (the Lewis and Clark Bridge), the Hudson River between Tarrytown and Nyack, New York (the new Tappan Zee Bridge, which opened in 2017), the Charles River in Boston (the Leonard P. Zakim Bunker Hill Memorial Bridge), and the Sacramento River in Redding, California (the Sundial Bridge), among other sites.
Twenty-eight states and one U.S. territory are home to cable-stayed bridges today and more than 1,000 exist around the world. Maryland does not have any.
It’s too early to predict the cost of replacing the Key Bridge, but experts forecast a price tag in the hundreds of millions of dollars.
Linda Romero, a spokesperson for the U.S. Coast Guard at the Key Bridge Response Joint Information Center, said Friday it’s too soon to comment on what kind of bridge will be erected or who will decide.
The Maryland Transportation Authority did not respond Friday afternoon to a call seeking comment, but Maryland Transportation Secretary Paul Wiedefeld, who oversees the MDTA and other agencies, told its board at a meeting Thursday that the goal is to rebuild with a bridge that “meets current standards.”
“That work is moving just as aggressively as other things that you’re seeing, just behind the scenes,” Wiedefeld said.
MDTA officials told the board that the authority is working with the U.S. Department of Transportation and Federal Highway Administration on bridge planning.
Even if the new bridge is cable-stayed, it won’t come risk-free. Though its support towers would be far stronger than the original’s, Firth says a vessel like the Dali would collapse any bridge in the world if it struck an anchor pier directly, as the 112,000-metric ton ship did to the Key last week. So, strong barriers around any main supports will be essential.
Still, he estimated that a new bridge will need a span of 1,500 feet to bring an acceptable level of safety to the Port of Baltimore, and that points to the design he said best fits the bill.
“If one were to do a study, to determine the optimum span of structure — and that would mean the optimal type of structure — it would almost certainly be a cable-stay bridge,” he said.
Baltimore Sun reporter Darcy Costello contributed to this article.
