[Baltimore Sun] Chesapeake Bay Bridge, Key Bridge among leaders for large ship traffic, Hopkins study shows
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In recent years only a handful of U.S. bridges had Dali-sized ships pass under them more frequently than Maryland’s Chesapeake Bay Bridge and Francis Scott Key Bridge, according to data released this week by Johns Hopkins University researchers.
The Bay Bridge and Key Bridge ranked 7th and 8th, respectively, for bridges with the highest traffic for “mega” ships, which are over 300 meters or roughly the size of the 984-foot Dali. Both bridges averaged nearly one of these ships a day over a roughly six-year period.
The data released on Monday, just a few days before the six-month anniversary of the Key Bridge collapse, is the first from a multi-phase study by Johns Hopkins University researchers into the likelihood of a bridge strike like the Dali’s collision with the Key Bridge. The data is integral as the team continues to explore its hypothesis, formed after the Key Bridge collapse: that the probability of the March 26 disaster, as well as future collisions, is underestimated.
The project was funded by a $200,000 Rapid Response Research grant from the National Science Foundation in May for researchers to begin an “urgent assessment of the country’s bridges, particularly the larger ones near major ports of entry.”
The data published by the Hopkins researchers, which comes from the U.S. Coast Guard, shows that large ships made 754,000 transits under 239 bridges in the U.S. from January 2018 to March 2024. These spans were chosen out of more than 623,000 in the National Bridge Inventory based on bridges near major ports. The analysis was limited to bridges that vessels longer than 150 meters passed under.
The Verrazzano-Narrows Bridge in New York led the list of 300+ meter ship traffic with an average of 4.6 vessels passing through daily in recent years.
A small fraction of the bridges studied make up the majority of the traffic, the data shows. There are 18 bridges — around 7.5% — with at least one 300+ meter ship passing under per week, and they make up around 95% of all large ship traffic in the data. Over 56% of traffic can be attributed to just four bridges.
For all large ships above 150 meters, or 492 feet, the Key Bridge and Bay Bridge, respectively, ranked 13th and 14th in the U.S. averaging 8.6 and 8.3 ships a day for the period. (The vast majority of ships into the Port of Baltimore pass underneath both spans, but some vessels transit the Chesapeake and Delaware Canal, which resulted in them passing under the Key Bridge, but not the Bay Bridge.)
Annual ship traffic data released by the researchers also provides an outlook into how ship traffic for the bridges has changed in recent years. With six full years of traffic to report, the Bay Bridge peaked in 2023, the last full year of data, for all large ships over 150 meters at 3,225 ships passing through — a 14.5% increase from 2019. This shows recovery following declines in ship traffic during the beginning of the COVID-19 pandemic. The Key Bridge also saw a 9% increase in 2023 from pre-pandemic traffic.
The researchers hope to, in their next phase, analyze data dating back to 2009.
The new data is one piece in the formula for addressing the research team’s hypothesis, said Michael Shields, the Hopkins engineer leading the team. Calculating the risk presented to bridges can be summarized by combining three primary criteria.
“The main three things we’re looking at are ship traffic, the base aberrancy rate, and then, given that a ship aberrates, what are the odds it hits a pier?” he said.
Aberrancy includes instances where ships veer off course and includes loss of steering or propulsion, like the vessel that had a steering issue near the Bay Bridge in August, prompting a temporary closure of the span. The researchers will also seek to customize aberrancy rates for certain ports, Shields said. For example, some ports have stronger currents or more stringent protocols such as requiring tugboats to escort ships.
“The team’s findings will be crucial in reassessing and potentially redefining the safety standards for transportation infrastructure,” Hopkins engineer Ben Schafer said in a statement earlier this year.
The American Association of State Highway and Transportation Officials (AASHTO), a national organization with members from each state, creates the nation’s lengthy (nearly 2,000 pages) specifications for bridge design. When old structures like the Key Bridge and the Bay Bridge were constructed, that bridge code did not contain any ship-strike guidelines; it wasn’t until the 1990s that AASHTO developed requirements regarding vessel collision.
Bridges today are required to meet certain risk thresholds for ship strikes. Probabilistically speaking, a critical bridge (such as ones that lead into a major port, for example) can have no larger than a 1 in 10,000 chance of collapsing from a vessel collision in any given year. However, the figures used to compute that probability are estimated, Shields said, and the Hopkins research could provide accurate data for engineers to use going forward. There has never been a national survey specifying how often large ships pass under various bridges, he said.
Plus, the Hopkins research could, depending upon its findings, prompt change by AASHTO. The organization meets annually to discuss potential changes to the bridge code.
The research team of three Hopkins professors, four Hopkins undergraduate students, two doctoral students and one Morgan State University student expects to publish more findings in 2025.
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