For years, civil engineers have recognized a problem with bridges: Many are not designed to withstand the pounding of the types of cargo ships that frequently pass through their waters. Those concerns came to a head on Tuesday with the devastating collapse of the Francis Scott Key Bridge in Baltimore, Maryland. Engineers have been trying to prevent this kind of failure for decades—and even now, they’re not sure the available solutions are enough.
“We were not designed to handle the lethal force of an impact like this — millions of pounds,” said Atorod Azizinamini, a professor of civil and environmental engineering at Florida International University. edge. “The collapse really had nothing to do with the type of bridge…redundancy or inspection.”
The Key Bridge is the second longest continuous truss bridge in the United States, behind the Astoria-Megle Bridge connecting Oregon and Washington. But this week, the huge MV Dali cargo ship collided with the Key Bridge’s support columns, causing its complete collapse within seconds. Two construction workers died and four others are presumed dead.
“The continuity of the three spans means that when the main span’s piers are destroyed by the ship, the two directly affected spans lose one of their primary supports,” said Douglas Schmucker, a professor of civil and environmental engineering at the University of Utah. .” edge. “When the middle span collapsed, it actually pulled the third span because it was designed as a whole to function together, not in isolation.”
This is not the first such catastrophic bridge collapse. In 1980, three years after the foundation bridge was completed, a freighter struck one of the support beams, causing much of Florida’s Sunshine Skyway to collapse, killing 35 people. The National Transportation Safety Board’s accident report noted that a factor in the bridge’s failure was the lack of a pier protection system that could “absorb some of the impact or redirect the ship.” When rebuilding the Sunshine Viaduct, engineers decided to install dolphins – concrete structures placed around the bridge’s piers – to absorb the impact of collisions while also stopping ships from hitting the bridge directly.
In response to the Sunshine Skyway tragedy, the American Association of State Highway and Transportation Officials (AASHTO) introduced new specifications for ship collision design of highway bridges in 1991. The new standards say engineers should design navigable waterways “to prevent collapse of the superstructure” taking into account the size and type of ship, available water depth, ship speed and structural response. “
However, Schmucker noted that these new rules typically take several years to be adopted. “It’s probably not until the 2000s that you actually see bridges designed specifically for it,” Schmucker said. “That’s because our process of building critical bridges over navigable waterways is very long. They’re expensive…and their integration with the environment can be a challenge.”
As bridge builders began to adopt AASHTO’s ship collision guidelines, we saw the construction of bridges like the Arthur Ravenel Jr. Bridge in South Carolina. Completed in 2005, it replaced an old bridge that was structurally unsound and not high enough for cargo ships to pass. Each pier of the Ravenel Bridge is surrounded by an acre of rocky islands, so if a cargo ship lost control near the bridge, it would run aground before colliding with the piers.
The older bridge cannot withstand a collision with a cargo ship the size of Dali. Baltimore’s Key Bridge was completed in 1977 at a cost of approximately $110 million. It straddles the Patapsco River and is adjacent to the busy Port of Baltimore, where approximately 11.3 million vehicles cross the river each year. In addition to helping divert traffic from the Baltimore Harbor Tunnel, the base bridge also serves as a critical route for vehicles carrying hazardous materials that are not allowed into the nearby tunnels.
officials told New York Times Key Bridge installs concrete barriers in the river “designed to deflect or slow down out-of-control vessels.” However, their considerable distance from the bridge’s support beams means they do not offer any real protection in the event of a direct impact.
Even if there were concrete islands around the Key Bridge’s beams, they might not be enough to mitigate the effects of a collision with a large cargo ship like the MV Dali.“I’m not sure any practical system, even bridge retrofits, would have been effective or even practical to prevent this disaster,” said Khalid M. Mosalam, a structural engineer and professor of civil engineering at the University of California, San Francisco. .” Berkeley told Baltimore flag.
The nearly 1,000-foot-long MV Dali was only half-full when it struck the Keys Bridge. It weighs approximately 95,000 tons, not including the 4,700 containers on board. The ship, chartered by Maersk Line, is almost the same length as the Eiffel Tower, and it’s far from the only ship of its size.
The average size of cargo ships has grown significantly over the past two decades as shipping lines respond to global demand for cargo. According to the Organization for Economic Co-operation and Development, more than 90% of traded goods are transported by waterways, and maritime trade volumes are expected to triple by 2050. The reason shipping companies are starting to opt for larger “mega-ships” is simple: The bigger the ship, the more cargo they can carry on a single voyage from retailers like Amazon, Target and Walmart.
But larger ships come with their own risks. In 2021, the 1,312-foot-long Ever Give ran aground in the Suez Canal and was trapped for nearly a week. Ports have had to adjust to accommodate the increase in ship size. In 2017, the Panama Canal expansion project was completed to “meet the growing demand for maritime trade using larger ships.” In 2019, the Bayonne Bridge connecting New York and New Jersey was raised 64 feet so that ships could pass under it.
At a press conference this week, U.S. Transportation Secretary Pete Buttigieg acknowledged that some modern bridges are “designed with different functions to mitigate impacts and protect the piers.” But he also pointed out that there is uncertainty about its effectiveness. “Right now, I think there’s a lot of debate going on in the engineering community about whether these features have any role to play in this situation.”
The collapse of the Key Bridge may cause some cities to reevaluate the safeguards they put in place on their bridges. Last year, New Castle, Delaware, launched a $93 million project to install dolphins around the piers of the Delaware Memorial Bridge. While the city took the initiative to shore up the bridges, federal agencies will soon force other cities to do the same.
Aziz Namini expects the National Transportation Safety Board to take a “very hard look” at current regulations to see if there are better ways to protect the nation’s bridges. “The first thing we learn as engineers is that public safety is the primary concern,” Aziz Namini said.