For years, civil engineers have understood that bridges have a problem: many of them are not designed to withstand a blow from the type of cargo ships that usually pass through their waters. Those concerns came to a head Tuesday with the devastating collapse of the Francis Scott Key Bridge in Baltimore, Maryland. It's the kind of failure that engineers have been trying to prevent for decades, and even now they're not sure if the available solutions are enough.
“We didn't design for the lethal force that comes with an impact like this: millions of pounds,” says Atorod Azizinamini, a professor of civil and environmental engineering at Florida International University. The edge. “The collapse really has nothing to do with the type of bridge…redundancy or inspection.”
The Key Bridge was the second-longest continuous truss bridge in the U.S., behind the Astoria-Megler Bridge connecting Oregon to Washington. But this week, the massive freighter MV Dali collided with the Key Bridge's support column, causing a total collapse that occurred in a matter of seconds. Two construction workers were killed and four others are presumed dead.
“The continuous nature of three spans means that when the ship destroyed the main span dock, the two spans directly affected lost one of their main supports,” says Douglas Schmucker, a professor of civil and environmental engineering at the University of Utah. The edge. “When the middle leg collapsed, it essentially took the third leg with it because it was integrally designed to work together, not in isolation.”
This is not the first catastrophic bridge collapse of this type. In 1980, three years after the completion of the Key Bridge, a large portion of Florida's Sunshine Skyway Bridge It collapsed when a freighter crashed into one of its support beams, killing 35 people. The National Transportation Safety Board accident report identified the lack of a dock protection system that could have “absorbed some of the force of the impact or redirected the vessel” as a factor in the bridge's failure. When rebuilding Sunshine Skyway, engineers decided to install dolphins (concrete structures placed around the bridge piers) to absorb the impact of a collision while preventing the ship from directly hitting the bridge.
In response to the Sunshine Skyway tragedy, the American Association of State Highway and Transportation Officials (AASHTO) introduced new specifications for the design of vessel collisions on highway bridges in 1991. new standards They say engineers should design navigable waterways “to prevent collapse of the superstructure taking into account the size and type of vessel, the depth of water available, the speed of the vessel, and the response of the structure.”
However, Schmucker notes that these new rules typically take a few years to be adopted. “It could easily have been until the 2000s that you actually saw a bridge designed for it,” Schmucker says. “That's because of the long process we use for major bridges over waterways. “They are expensive…and integrating them into the environment can be a challenge.”
As bridge builders began to adopt AASHTO vessel collision guidelines, we saw bridges like the Arthur Ravenel Jr. Bridge being built in South Carolina. It was completed in 2005 to replace an older bridge that was considered structurally unsound and not high enough for cargo ships to pass under. Ravenel Bridge has one-acre rocky islands surrounding each of its docks, so if a freighter lost control near the bridge, it would run aground before hitting the dock.
Older bridges were not built to withstand collisions with Dali-sized cargo ships. Baltimore's Key Bridge was completed in 1977 and cost around $110 million to build at the time. It extended over the Patapsco River, adjacent to the Busy Port of Baltimorewith around 11.3 million vehicles cross it every year. In addition to helping divert traffic from the Baltimore Harbor Tunnel, the Key Bridge also served as a critical route for vehicles transporting hazardous materials, which are not permitted inside the nearby tunnels.
Officials say The New York Times that the Key Bridge had concrete barriers installed in the river that were “intended to divert or slow an out-of-control vessel.” However, they are located far from the bridge's support beams, meaning they offered no real protection in the event of a direct impact.
Even if the Key Bridge had concrete islands surrounding its girders, they may not have been enough to lessen the effects of a collision from a cargo ship as massive as the MV Dali. “I'm not sure if any practical system (or even a modernization of the bridge) would have been efficient or even practical to prevent this disaster,” said Khalid M. Mosalam, a structural engineer and professor of civil engineering at the University of California. Berkeley, account The Baltimore Flag.
The nearly 1,000-foot-long MV Dali was only half full when it crashed into the Key Bridge. She weighs about 95,000 tons, not counting the 4,700 containers she carried on board. The ship, chartered by the Maersk shipping company, is almost the same same length like the Eiffel Tower, and it is far from the only one of that size.
Over the past two decades, the average size of cargo ships has increased grown enormously as shipping companies face global demand for goods. According to the Organization for economic cooperation and developmentMore than 90 percent of traded goods travel by water, and maritime trade volumes are expected to triple by 2050. The reason Shipping companies have begun to choose The search for larger “mega ships” is simple: the larger the ship, the more goods it can carry in a single trip from retailers like Amazon, Target and Walmart.
But larger ships also carry their own risks. In 2021, the 1,312-foot-long Ever Given ran aground in the Suez Canal and was trapped there for nearly a week. Ports have had to make adjustments to accommodate ships increased sizes. In 2017, the Panama Canal Expansion project It was completed to “meet the growing demand of maritime trade using larger vessels.” In 2019, the Bayonne Bridge connecting New York and New Jersey it was raised 64 feet so that ships could fit underneath.
During a press conference this weekUS Transportation Secretary Pete Buttigieg acknowledged that some modern bridges are “designed with different features to mitigate impacts and protect their piers.” But he also notes that there is uncertainty surrounding its effectiveness. “Right now, I think there's a lot of debate among the engineering community about whether any of those features could have played any role in a situation like this.”
The collapse of the Key Bridge will likely lead some cities to reevaluate the safeguards their bridges have in place. Last year, New Castle, Delaware, started a 93 million dollar project install dolphins surrounding the piers of the Delaware Memorial Bridge. While the city took the initiative to proactively reinforce the bridge, federal agencies could soon force other cities to do the same.
Azizinamini hopes the National Transportation Safety Board will “take a hard look” at current regulations to see if there are better ways to protect the country's bridges. “The first thing we learn as an engineer is that public safety is the number one issue,” Azizinamini says.