Ship strikes on U.S. bridges like the one that brought down the Key Bridge are more likely to happen than most of us realize—and dozens of bridges across the U.S. have just been identified as needing safety evaluations. The Chesapeake Bay Bridge is among them.
That’s according to two reports that have been released in recent days. First, the National Transportation Safety Board (NTSB) recommends that 68 bridges in 19 states should undergo vulnerability assessments to determine the risk of a bridge collapse from a vessel collision.
And Monday morning, a group of structural engineers and civil engineering risk experts from Johns Hopkins University released preliminary findings indicating that the chances of a commercial ship hitting a major U.S. bridge is higher than previously thought.
Both reports come just in time for the somber anniversary of the deadly Key Bridge collapse. On March 26, 2024, the container ship M/V Dali lost power and struck the bridge, killing six construction workers.
As part of its ongoing investigation into the bridge disaster, the NTSB found that the Key Bridge was almost 30 times above the “acceptable risk threshold for critical or essential bridges,” based on calculations used by the American Association of State Highway and Transportation Officials (AASHTO). The NTSB gathered the names of 68 bridges (with 30 owners responsible for them) that don’t have a current vulnerability assessment. The agency says the 30 bridge owners should evaluate whether their bridges are above the AASHTO’s acceptable level of risk. If a bridge is above the acceptable level of risk, the NTSB says bridge owners should make and implement a plan to reduce risk.
The bridge vulnerability calculation was developed in 1991, in response to the Sunshine Skyway Bridge collapse in St. Petersburg, Florida, in 1980. A wind-blown freighter crashed into the bridge, causing it to collapse and kill 35 people. The risk assessment calculation was intended for newly constructed bridges, but recommended to be used on existing bridges, too.
The Key Bridge was built before AASHTO’s calculation was established, and before the Federal Highway Administration began requiring new bridges to be designed with vessel collision protections that take into account size, speed and other characteristics of vessels navigating the channel under the bridge.
In its new report, the NTSB says that if the Key Bridge’s owner, the Maryland Transportation Authority (MDTA), had conducted a vulnerability assessment based on recent vessel traffic, MDTA would have been aware the Key Bridge was above the acceptable risk and could have potentially prevented a catastrophic collapse when the M/V Dali struck the bridge.
With an eye towards avoiding collapses at risky bridges that are going unstudied, the NTSB recommends that the Federal Highway Administration, the Coast Guard, and the U.S. Army Corps of Engineers establish a team to guide bridge owners on evaluating and reducing the risk of catastrophic ship strikes.
In the Johns Hopkins University study, researchers looked not at the risk of collapse of major U.S. bridges from an impact, but simply at how likely the bridges are to be struck by a large ship.
The assessment was inspired by a simple email exchange shortly after the Key Bridge disaster, between Johns Hopkins Associate Professor of Civil and Systems Engineering Michael Shields and Structural Engineering Professor Ben Schafer. It asked, “What do you think the actual probability of this happening was?”
They were quickly able to get funding from National Science Foundation’s rapid response program, intended for disaster response. In what is still a preliminary assessment, Shields, Schafer, and their team zeroed in on about 50 of the largest U.S. ports and the waterways leading up to them, cross-referencing that list with the National Bridge Inventory of major bridges where large ships traffic. The team considered “large” ships, anything over 150 meters, and “mega ships”, anything larger than 300 meters.
Shields’ first startling finding was that the Dali’s allision with the Key Bridge “was not an aberration,” he tells Chesapeake Bay Magazine. “There have been a dozen collisions that induced a collapse in the last half-century in the U.S.,” he says. That’s on the scale of every 3-5 years.”
In assessing collision risk for other major bridges, the researchers asked, “If a ship goes off course, what are the chances that it hits one of the piers?” They considered how wide the span is relative to the shipping channel and size of vessels that pass under it. They also looked at what protections, like dolphins to protect the piers, are in place.
The Hopkins assessment found that at 14 major U.S. bridges, a collision could be expected once every 100 years or less. The highest risks are at the Huey P. Long Bridge in Louisiana, where a collision is expected every 17 years; the San Francisco-Oakland Bay Bridge, where a collision is expected one every 22 years; and the Crescent City Connection in New Orleans, where a collision is expected one every 34 years.
Not far down the list is the Chesapeake Bay Bridge, where a collision is expected once every 86 years. (On the Delaware Memorial Bridge, a collision is expected once every 129 years). At the Bay Bridge, the assessment finds a lack of pier protection. “There are no dolphins. That would be one of the primary defenses,” Shields says. “Another way is to build up around the piers.” While the Bay Bridge does have fenders, Shields explains, fenders are a last defense, and aren’t especially good protections against huge ships.
When we asked Shields what his team hopes comes of their assessment, he said he hopes it starts a conversation about how to protect the bridges, prompting local, state, and federal officials to come together and look at specific bridges that have a higher risk. They need to decide what protective measures are appropriate for each bridge. “There is no ‘one size fits all solution’,” Shields says. “Some may require that extremely large ships be towed.” While that would take a fleet of vessels, he points out, “It’s a fleet of vessels that could potentially save your bridge.”
The 984-foot Singapore-flagged cargo vessel Dali was transiting out of Baltimore Harbor when it experienced a loss of electrical power and propulsion and struck the southern pier supporting the central truss spans of the Francis Scott Key Bridge, which subsequently collapsed. Six construction crewmembers were killed and another was injured, as well as one person onboard the vessel.
