24-03-2025
Span of disaster: US bridges face deadly risk of ship ramming, study finds
Span of disaster: US bridges face deadly risk of ship ramming, study finds
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NTSB recommends bridge risk assessments after Baltimore investigation
The NTSB said Baltimore officials who oversee the Francis Scott Key Bridge did not understand the bridge's vulnerability to collisions.
A year after Baltimore's Francis Scott Key Bridge collapsed after being struck by an errant cargo ship, a new study indicates such incidents are more likely to occur in the U.S. than commonly known, with potentially similar catastrophic consequences.
Some of the nation's busiest bridges are likely to be struck by ships within several decades, the study authors said, reflecting a dramatic increase over time in vessel size and shipping lane traffic. The two most vulnerable spans listed in the assessment – the Huey P. Long Bridge outside New Orleans and the San Francisco-Oakland Bay Bridge – face a likelihood of collisions of approximately every two decades.
'We have significantly underestimated the risk that large ships pose to existing bridges across the U.S.,' said Michael Shields, an associate professor of civil and systems engineering and lead investigator of the study conducted by Johns Hopkins University in Baltimore. 'These results verify that the Key Bridge was not an aberration…. It's something we should have better seen coming and can now react to by putting appropriate corrective measures in place.'
The study's preliminary findings echo a March 20 update from the National Transportation Safety Board on its investigation of the disaster. Board chair Jennifer L. Homendy faulted Maryland officials overseeing the Key Bridge for failing to recognize that the structure's vulnerability to such collisions was 30 times more than the federal standard.
The MV Dali, a 984-foot container ship headed for Sri Lanka, struck a pier of the Key Bridge after losing power early on the morning of March 26, 2024, causing the structure to collapse into the Patapsco River, killing six road workers and forcing temporary closure of the Port of Baltimore.
'It was a shocking reminder of the fragility of the engineering marvels we so often consider indestructible,' the Johns Hopkins project site noted.
The university's national risk assessment of bridge vulnerability was funded by the National Science Foundation's Rapid Response Program, which focuses on studies in the immediate aftermath of disaster. It focused on a central question: How likely was a disaster like this to occur?
Its list of the nation's 20 most at-risk bridges includes seven bridges with a probability of being struck by ships every 50 years or less, including the Huey P. Long Bridge (17 years), the San Francisco-Oakland Bay Bridge (22 years), the Crescent City Connection in New Orleans (34 years), the Beltway 8 Bridge in Houston (35 years) and the Hale Boggs Memorial Bridge west of New Orleans (37 years).
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The findings are unsettling to commuters like Darby Li Po Price, a professor of Asian and Asian American Studies at Oakland's Merritt College who drives across the San Francisco-Oakland Bay Bridge about once a month. He's sailed the waters underneath the bridge and knows how problematic they can be.
'We have a complex water system,' he said. 'The winds and currents are a lot more dramatic. And we have fog.'
Price wasn't too surprised, then, to learn the bridge was among the nation's most at-risk spans of being hit by a ship, though the discovery did prompt some gallows humor.
'Maybe it just means I'll drive across the bridge a little faster,' he said. 'I'd rather risk a speeding ticket than the bridge collapsing while I'm on it.'
Shields said the risk is incredibly low for the average daily commuter.
'You're talking about an event that will potentially occur over a span of decades,' he said. 'But if you compound that and say, what are the chances that this will occur in my lifetime ― the chances for an individual traveler are very small, but the risk to society and communities is very real.'
Why the likelihood of collisions has risen
Two primary factors contribute to greater than expected frequency of ship-bridge collisions, experts say: the immensity of today's ships and higher volumes of traffic prompted by the explosion of global trade.
'These are really large ships that you don't want anywhere near the piers of a bridge,' Shields said. 'They will be damaging, and it will be a consequential event.'
Many large bridges, Shields said, were built more than 50 years ago, when ship traffic was one third or less of the volume it is now; meanwhile, ships are several times larger than were at the time, 'so you're seeing a much higher volume of really large ships.'
The study focused on major bridges with high traffic including ships more than 150 meters long, or just shy of 500 feet. That narrowed the review to about 400 of the nation's roughly 80,000 bridges.
Project engineers evaluated several factors to determine the likelihood of ship collisions for each bridge, including traffic patterns combined with the probability of ships veering off course and the chances that those that do will strike bridge piers.
The researchers mined 16 years of U.S. Coast Guard data detailing the position, velocity, status and heading of ships in U.S. waterways, cross-referencing hundreds of millions of data points with port and bridge data kept by the National Bridge Inventory.
They also considered the reasons a ship might deviate from its path, whether it be pilot error, weather conditions or a loss of power, using a base aberrancy rate calculated by the American Association for State Highway and Transportation Officials.
Finally, they looked at the protections in place to prevent ships from striking piers – for example, arrays of so-called dolphins, pilings embedded into the seabed and visible above water level to aid navigation.
Abi Aghayere, a professor of structural engineering at Drexel University in Philadelphia, said the increasing size of vessels in navigable waterways makes it imperative that piers be protected from ship strikes.
Aghayere, who is unaffiliated with the Johns Hopkins study, said an examination of Coast Guard data conducted by one of his students found 2,100 reports of engine failure within a mile of the U.S. coast or its waterways in the last 14 years; about 100 of those occurred in the area between the Key Bridge and the Delaware Memorial Bridge 70 miles away, he said.
Meanwhile, just 2,360 of the more than 4,200 bridges with piers listed in the Federal Highway Administration's national bridge inventory, Aghayere said, feature protective dolphins, fenders or islands.
'There are at least 1,800 bridges without any functional pier protection,' he said. 'What is even more worrisome is that many of these unprotected bridges are fracture critical, which means that if one member of the bridge fails in tension, the whole structure could suddenly come crashing down.'
1980 Florida bridge collapse 'a wake-up call'
That's what happened in the case of the Key Bridge, which the Johns Hopkins study determined would have been among the nation's 10 most vulnerable bridges, with a 48-year likelihood of experiencing a collision. A pair of dolphins were placed too distantly from bridge piers to be effective, Shields said.
According to the study, vessel collisions caused 17 major bridge collapses in the U.S. between 1960 and 2011 – or every three years – with varying consequences.
In 1980, 35 people died when the Sunshine Skyway Bridge connecting St. Petersburg and Bradenton, Florida, collapsed after being hit by a freighter during a sudden thunderstorm; in 1993, an Amtrak passenger train derailed, killing 47, after barges pushed by a towboat lost in thick fog struck the Big Bayou Canot Bridge near Mobile, Alabama.
In 2002, a freight barge struck a pier of Oklahoma's Interstate 40 bridge after the towboat's captain lost consciousness, collapsing a section of the bridge and killing 14.
'The important point is not whether it will occur every 17 years or every 75 years,' Shields said. 'It's that it's happening way too often.'
The Sunshine Skyway's collapse was a wake-up call, Shields said, prompting new federal standards issued in 1994 requiring bridge designs to have no more than a 1-in-10,000 probability of collapse due to ship collision. However, no provision mandated that existing bridges needed to be retroactively protected.
'Most of these bridges were never designed with ship collision in mind to begin with,' Shields said, noting that just one of the assessment's 20 most at-risk bridges was built after 1994; most were built between the 1930s and 1970s.
That's evident, he said, in some of the most at-risk bridges, some of which are shorter spanned bridges with piers nearer shipping channels challenged by the length of the vessels themselves. In the case of the Key Bridge, the width of the span was only slightly longer than the ship passing through it, Shields said.
Meanwhile, bridges with lower risks – for instance, the Verrazano-Narrows Bridge in Staten Island, New York, for which the assessment predicts a ship collision every 362 years – have longer spans and piers farther removed from shipping channels. Some busy bridges such as Minnesota's Deluth Lift Bridge and California's Vincent Thomas Bridge escaped the list altogether because their supports are on land.
That's in contrast to the two most at-risk bridges, the Huey P. Long and San Francisco-Oakland Bay Bridge, each of which has at least four piers in the water where shipping traffic passes through.
'For every pier you put in the water, that's an additional risk,' Shields said.
'The physics take over'
While the Johns Hopkins study examined the probabilities of collision, it stopped short of considering collapse because of the unique factors underlying individual bridges; analyzing even one bridge for that result, Shields said, is a mammoth undertaking.
As part of last week's update, the NTSB urged bridge owners and operators nationwide to conduct risk assessments, identifying 68 U.S. spans for which it said evaluations had not been done.
Shields said he hopes his team's findings, along with the NTSB recommendations, spur port authorities and bridge owners to follow up and act to minimize such collisions. Local authorities, he said, could also employ more extreme measures such as requiring very large vessels to be towed under certain bridges.
Christopher Higgins, a professor of structural engineering at Oregon State University in Corvallis, said a lack of protective measures are likely more to blame than structural deficiencies.
'The main way you avoid ship impact, especially with large container ships, is with tugboats,' said Higgins, also unaffiliated with the Johns Hopkins study. 'If you have a ship with electrical difficulties and you lose power there isn't much you can do. The physics take over at that point and that ship's going to go where it's going to go.'
Aghayere, of Drexel University, said the cost of protecting bridge supports pales when compared to the consequences of collapse. For instance, he said, while it would cost $100 million to install dolphins protecting the Delaware Memorial Bridge against ship strikes, the cost of replacing the Key Bridge is estimated at nearly $2 billion.
'I believe these incidents are more likely than we think,' Aghayere said. Given the scale of tragedy that could have happened had the Key Bridge been struck during rush hour and that so many bridges are susceptible to collapse if struck, 'it's all the more imperative that something be done,' he said.
Ryan Zamarripa, who manages grants under the U.S. Economic Development Administration's Build Back Better Regional Challenge, fears a rash of recent federal cutbacks could affect the ability of ports to prevent such tragedies from happening again.
'Given the Trump administration's overt hostility toward federal agencies and civil servants responsible for keeping infrastructure like our ports running smoothly, catastrophes like what happened at the Key Bridge are at risk of increasing in frequency over the near term,' he said. The resulting chaos, he said, is 'severely disrupting the highly complex logistics required by modern-day commerce and transportation.'
Shields said there's no one-size-fits-all solution. Each bridge will require its own set of protective measures and protocols.
'You're never going to build a bridge that can withstand the impact of the Empire State Building, in all reality,' Shields said. 'What you really need to do is put protections in place to ensure that ship never gets anywhere near the piers to begin with.'
