For years, civil engineers have understood that bridges have a problem: Many of them aren't designed to withstand the pounding of cargo ships that routinely pass through their waters. Those concerns came to a head on Tuesday with the catastrophic 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 adequate.
“We didn't design for the lethal force generated by that kind of impact — millions of pounds,” said Adorod Azizinamini, a professor of civil and environmental engineering at Florida International University. on the edge. “The collapse and the type of bridge … really had nothing to do with the decommissioning or the inspection.”
The Key Bridge was the second longest continuous via truss bridge in the United States, behind the Astoria-Meckler Bridge connecting Oregon to Washington. But this week, the massive MV Tally cargo ship crashed into the Key Bridge's support column, causing a complete collapse in seconds. Two construction workers were killed and four others are presumed dead.
“The 3-span continuity means that when the main span is destroyed by the ship, the two directly affected spans lose one of their primary supports,” says Douglas Schmucker, a professor of civil and environmental engineering at the University of Utah. on the edge. “When the middle space collapsed, it dragged the third space with it because it was designed to work together, not in isolation.”
This is not the first disaster of this type of bridge collapse. In 1980 – three years after the Key Bridge was completed – A large portion of Florida's Sunshine Skyway Bridge A cargo ship collided with one of its support beams, killing 35 people. National Transportation Safety Board Accident report It identified the lack of a ship protection system that could “absorb some of the impact force or divert the ship” as a factor in the bridge's failure. When rebuilding the Sunshine Skyway, engineers decided to install dolphins — concrete structures around the bridge's piers — to absorb the impact of a collision while preventing a boat from crashing directly into the bridge.
In response to the Sunshine Skyway tragedy, the American Association of State Highway and Transportation Officials (AASHTO) introduced new specifications for the ship collision design of highway bridges in 1991. New standards Engineers say that navigable waterways must be designed “taking into account the depth of water, speed of the ship and reflection of the structure to prevent collapse of the superstructure, taking into account the size and type of vessel available”.
However, Schmucker notes that it will be a few years before these new rules are generally adopted. “It was probably easily in the 2000s before you actually saw a bridge designed for that,” Schmucker says. “Because of that long process, we use significant bridges over navigable waterways. They're expensive…and they can be challenging to integrate with the environment.
As bridge builders began to follow AASHTO's vessel collision guidelines, we saw bridges like the Arthur Ravenel Jr. Bridge in South Carolina being built. The old bridge was replaced in 2005 after it was deemed structurally weak and not high enough for cargo ships to pass under. There is the Ravenel Bridge Acres of rocky islands all around At each of its piers, if a freighter loses control near the bridge, it will dissolve before colliding with the ship.
Older bridges were not built to withstand collisions with dally-sized freighters. Baltimore's Key Bridge was completed in 1977, costing about $110 million to build at the time. It straddles the Patapsco River and adjoins it Busy Baltimore Harborcarry around 11.3 million vehicles Every year passes it. In addition to helping divert traffic from the Baltimore Harbor Tunnel, the Key Bridge also served as an important route for vehicles carrying hazardous materials that were not allowed into nearby tunnels.
Officials say The New York Times The main 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 offer no real protection in the event of a direct impact.
Although the drawbridge has concrete islands around its beams, they may not be sufficient to mitigate the effects of a collision from a large cargo ship such as the MV Dali. “I don't know if any practical system (or even a redesign of the bridge) would have been efficient or practical to prevent this disaster,” Khalid M. Mosalam, a structural engineer and professor of civil engineering at the University of California, Berkeley Baltimore Banner.
The nearly 1,000-foot-long MV Daly was half-full when it hit the drawbridge. It weighs about 95,000 tonnes, not including the 4,700 containers on board. Chartered by the shipping company Maersk, the vessel is approximately Same length As the Eiffel Tower – it is far from the only tower of its size.
Over the past two decades, the average size of cargo ships has It grew enormously Shipping companies struggle with global demand for cargo. As stated therein Organization for Economic Co-operation and DevelopmentMore than 90 percent of trade goods travel by water, and maritime trade volumes are expected to triple by 2050. The reason Shipping companies are starting to choose Simpler for large “mega-ships”: A larger ship can carry more products in one trip from retailers like Amazon, Target and Walmart.
But big ships also have their own risks. In 2021, the 1,312-foot-long Ever Given ran aground in the Suez Canal and got stuck there for nearly a week. Ports had to make changes to accommodate the ships. increased doses. In 2017, the Panama Canal Expansion plan It was completed to “meet the growing demand of maritime trade using larger ships”. In 2019, the The Bayonne Bridge connects New York and New Jersey Raised 64 feet, so ships were docked below.
when A press conference this week, U.S. 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 their effectiveness. “Right now, I think there's a lot of debate going on in the engineering community about whether any of those features would have played a role in a situation like this.”
The collapse of the force bridge will lead some cities to re-evaluate the safeguards on their bridges. Last year, New Castle, Delaware, Launched a $93 million project To install dolphins around the piers of the Delaware Memorial Bridge. Even if the city took the initiative to preemptively reinforce the bridge, federal agencies may soon force other cities to do the same.
Azizinamini expects the National Transportation Safety Board to take a “very hard look” at existing regulations to see if there are better ways to protect the nation's bridges. “The first thing we learn as an engineer is that public safety is the primary issue,” says Ajizinamini.