Study Reveals Bridges at Highest Risk of Ship Allisions

Days after the National Transportation Safety Board took state highway agencies to task for unassessed bridge strike risks, a team at Johns Hopkins University has published a comprehensive list of the bridge spans that are most exposed to the hazard. The Francis Scott Key Bridge in Baltimore, which was struck and destroyed by the boxship Dali last year, emerged in the top-10 list for bridges most likely to be hit.

“With this investigation we wanted to know if what happened to the Key Bridge was a rare occurrence. Was it an aberration? We found it’s really not,” said Michael Shields, Johns Hopkins associate professor and lead author. “In fact, it’s something we should expect to happen every few years.”

The study suggests that the likelihood of ships alliding with bridges has been greatly underestimated, and that we should all expect to see more bridge strikes in our lifetimes. Owing to the dramatic increase in the size of ships, and the fact that most bridges are witnessing an unprecedented increase in vessel traffic, future allisions are virtually certain – though the severity will vary, and there are engineering interventions to reduce harm. For example, the group calculated that the Huey P. Long Bridge outside New Orleans and the San Francisco-Oakland Bay Bridge are likely to be hit by a ship within two decades.

Johns Hopkins is releasing the preliminary findings of the study on the first anniversary of the Key Bridge collapse after it was struck by the boxship Dali, killing six people who were carrying out roadway repairs on the bridge deck. While investigations are still ongoing on the cause of the allision, the reality is that the chances of another similar incident are much higher. Ship strikes should be extremely rare, with bridge design standards stipulating that the annual chances of a bridge collapse from ship allisions should be less than 1 in 10,000 years. But after collecting, mining and analyzing 16 years of U.S. Coast Guard data, the team concluded the next allision could be in the pipeline. Notably, even iconic bridges like San Francisco’s Golden Gate Bridge and Maryland’s Chesapeake Bay Bridge are not immune to allisions.

The team based its study on AIS tracking, port data and bridge data from the National Bridge Inventory to determine which large ships passed under which bridges. Using the traffic data, along with average mishap rates from the American Association of State Highway and Transportation Officials, the team was able to estimate the probability of very large ships hitting the piers of major bridges across the U.S. (Bridges without in-water piers were excluded from the study.)

The Key Bridge allision provided an unfortunate confirmation of their methods: the team predicted it would have likely been hit by a ship within 48 years. The bridge was 46 years old when it fell, having sustained a minor hit from a ship previously.

The study identifies the bridges facing the highest risks of allision over the next five decades or less. These include the Huey P. Long Bridge (17 years), the San Francisco-Oakland Bay Bridge (22 years), the Crescent City Connection in New Orleans (34 years) and the Beltway 8 Bridge in Houston (35 years). Others include the Hale Boggs Memorial Bridge west of New Orleans (37 years), Bayonne Bridge, N.Y./N.J (43 years) and Fred Hartman Bridge, Texas (47 years).

“There’s still a lot of uncertainty in predicting the frequency of ship collisions, even with the best data we have,” said Shields. “But the important point is not whether it will occur every 17 years or every 75 years. It’s that it’s happening way too often.”