When the 1,200-foot-long Francis Scott Key Bridge opened in 1977, its enormous steel-truss design was the state of the art for river crossings. But in the decades since, a more modern design that has gained favor could replace the span that was destroyed in March when a cargo ship struck it.
The National Transportation Safety Board is expected to release itspreliminary investigation this month into the circumstances of the Key Bridge's collapse into the Patapsco River, which killed six construction workers. Meanwhile, authorities must decide how to replace the structure, which used to carry 40,000 vehicles each day.
What type of bridge, the construction timeline, how much it will cost and who will pay all remain open questions. But projects elsewhere in the U.S. offer clues to what might lie ahead, including what's known as a cable-stayed bridge.
Habib Tabatabai, director of the Structural Engineering Laboratory at the University of Wisconsin-Milwaukee, says that in the past three or four decades, this newer bridge design has all but eclipsed steel-truss designs. "Since about 1990, as far as I know, all the new crossings of the Mississippi have been cable-stayed bridges," he says.
Cable-stay bridges offer ways to save time and money
Tabatabai describes cable-stayed bridges as "a modern but mature technology" that "optimizes material usage while minimizing fabrication and labor costs." As a result, he says, "cable-stayed bridges [have] been predominant in low-bid contracts of major new river crossings."
A good analog to Baltimore's situation is the Arthur Ravenel Jr. Bridge, a cable-stayed structure over the Cooper River in Charleston, S.C., that was completed 18 years ago. It replaced an older steel-truss design and has roughly the same dimensions as a replacement for the Key Bridge would require, Tabatabai says.
Flatiron, a Broomfield, Colo.-based heavy civil infrastructure construction firm, worked on the Ravenel Bridge and the replacement for the I-35W bridge in Minneapolis that suddenly collapsed in 2007. Ural Yal, Flatiron's senior vice president for preconstruction, envisions a cable-stayed bridge as the most likely option in Baltimore — a sentiment echoed by other experts, including Tabatabai.
The extensive use of easily formed concrete, which is less expensive than steel, is key to the cost equation for cable-stayed bridges, Yal says. With concrete, he says, "there are opportunities to cast a lot of the pieces off-site while you're building the foundations" of the bridge, thus speeding up construction. Flatiron, which says it likely will submit a proposal for Baltimore's project, thinks it would need about 80 acres along the Patapsco River for precasting, and Yal says the firm has already begun scouting locations in case it wins the contract.
Other examples of cable-stayed bridges include the John James Audubon Bridge in New Roads, La., and the Harbor Bridgein Corpus Christi, Texas, which is under construction. The same type is being built across the Detroit River, linking the U.S. and Canada. Flatiron has been involved in all three projects, Yal says. Another, which didn't involve the company, is New York City's Governor Mario M. Cuomo Bridge. It opened in 2018 as a replacement for the older Tappan Zee Bridge over the Hudson River.
Another cable-stayed bridge, the Sunshine Skyway Bridge in Tampa, Fla., completed in 1987, has eerily similar parallels to Baltimore's Key Bridge. In 1980, a cargo ship hit a support pylon of the original steel-truss bridge in Tampa, causing one of the spans to fall into the water below, killing 35 people. As NPR member station WUSF recently reported, the replacement Sunshine Skyway Bridge set the standard for how to protect such structures from similar accidents.
Safety features offer some protection from future collisions
Sameh Badie, a professor at George Washington University's School of Engineering & Applied Science, says major bridges are designed to last a minimum of 75 years, assuming they are properly maintained, though most are expected to last much longer. Anticipating future capacity over many decades necessitates a certain amount of crystal-ball gazing, he says.
For Baltimore's Key Bridge, doubling the number of automobile lanes from four to eight would be a good place to start, he says. A new bridge will also need to factor in the future size of commercial ships. "Every day we are getting bigger and bigger cargo ships like the one that hit the [Francis Scott Key] Bridge. And by the way, that was a middle-sized cargo ship," Badie says.
Protecting a new bridge from another calamitous ship strike could be accomplished in a few ways, he says. One method is to create artificial islands that surround bridge pylons. Any hit would be absorbed by the island, not the bridge. Another option: Install a robust system of defensive structures known as dolphins — circular concrete pillars located near a bridge's central supports that are designed to absorb the impact of a careening ship. Yet another system, known as fender rings, acts like a car's bumper, shielding bridge pylons from most of the impact in the case of a collision. The structures are "anchored at the bedrock" and stand off about 40 feet from the pylons, Badie says.
Some bridges use multiple systems, such as the Sunshine Skyway Bridge, which has both artificial islands and dolphins protecting its pylons, he says.
How much will it cost to replace Baltimore's bridge?
Any of these systems, properly designed and installed, would likely have prevented the catastrophic Key Bridge collapse, Badie says. But the measures don't come cheap. They could add $100 million to $200 million to the cost of a new bridge, he says.
Corpus Christi's new 1,660-foot Harbor Bridge, set to open next year, has a projected cost of $1.2 billion, according to Lynn Allison, the project's public information manager. But the bridge itself accounts for roughly half the total price tag, she says. The other half includes rebuilding 6.5 miles of interstate highway and an interchange approach. The project, begun in 2016, has had its share of problems and setbacks, even resulting in a lawsuit over alleged design flaws, which was settled last year for $400 million.
There's also the question of aesthetics, says Martin Lessard, vice president of construction, West Coast Division, at Dragados USA, which is in a joint venture with Flatiron on the Harbor Bridge project. The balance is between producing "a signature bridge" or building something rapidly, he says.
"If the client wants a signature bridge with a diamond-shaped pylon," for example, "that takes time." In Baltimore's case, where restoring a critical city artery is paramount, designers and the builder may be asked to "keep it simple. Keep it straightforward," he says.
Normally, an impact study under the National Environmental Policy Act (NEPA) can take three to four years before a new bridge project can move forward, Yal says. "That is the most critical piece of it," he says. That typically results in a hefty and time-consuming report in which "you refine, you go back out to the public and you come back."
It is possible to streamline the process, though. "There's a lot of ways that they can get variances and exceptions [to] push the project forward, which is what happened in I-35 in Minnesota, where you can actually clear a project relatively quickly and waive all those NEPA requirements," according to Yal.
Timothy Witman, NEPA program manager for the Environmental Protection Agency's mid-Atlantic region, which includes Baltimore, says, "We will be working together to try to streamline things as much as we can."
Yal says with an "aggressive schedule," a bridge with protections from vessel strikes and with roadway connections could be built by 2027 at an estimated cost of $2 billion to $2.5 billion, depending on the final design and other factors.
But first, there's the business of clearing the channel of debris from the collapsed span and the other unknowns that will inevitably creep into the project. How much all that adds to the final cost is difficult to anticipate, Badie says.
"I don't think that anybody right now can put a figure on the indirect costs," he says.
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