Vulnerability of the Francis Scott Key Bridge in Baltimore to a vessel collision was nearly 30 times above the acceptable risk threshold for critical or essential bridges when it was struck last year by the disabled container vessel M/V Dali, the National Transportation Safety Board announced on March 20.

Yet according to the agency, the Maryland Transportation Authority never calculated risks to the nearly 50-year-old steel arch continuous truss structure as recommended by 1991 and 2009 specifications of the American Association of State Highway and Transportation Officials (AASHTO), even as container vessels using the underlying deep draft channel to the Port of Baltimore continued to grow in both size and number.

The Dali’s impact with one of the bridge’s main support piers in early morning hours of March 26, 2024, resulted in collapse of the main span, killing six construction workers and blocking the channel for more than two months with nearly 50,000 tons of debris.

Had the Maryland agency performed the assessment, it "would have been able to proactively identify strategies to reduce the risk of collapse and loss of lives associated with a vessel collision with the bridge,” NTSB Chair Jennifer Homendy told a press briefing,

The board also recommends vessel collision vulnerability assessments for 68 other bridges crossing navigable waters in 13 states, which were identified as lacking the evaluation during the Key Bridge investigation. It has issued an urgent recommendation to owners of those bridges to calculate the annual frequency of collapse for these structures, using AASHTO’s Method II calculation.

The bridges, all built before the initial version of the guidance was issued in 1991, also include the Golden Gate and Coronado Bridges in California; the Huey Long Bridge in Louisiana; the Brooklyn and Verrazano Narrows Bridges in New York; and Florida’s Sunshine Skyway Bridge, which itself was built to replace a structure collapsed by a vessel impact in 1980 that killed 35 motorists. 

Should the calculations indicate a bridge has a risk level above the AASHTO threshold, NTSB recommends owners develop and implement a comprehensive risk reduction plan for their structures and has asked them to provide an initial reply within 30 days.

While expressing the hope that the number of vulnerable bridges is small, NTSB's Homendy the agency would follow up with bridge owners and the U.S. Federal Highway Administration to ensure they follow through on the recommendation. “We expect action,” she said. “Public safety depends on it.”

The Maryland transportation agency added that it is actively evaluating pier protection systems, operational changes and vessel transit procedures for the Chesapeake Bay Bridge in Annapolis.

Studies of bridge risk due to high levels of ship traffic also have been underway since the collision by the Johns Hopkins University Whiting School of Engineering in Baltimore.

Lacking Assessment

Homendy noted that her agency needed assistance from the federal agency to develop data for performing its own AASHTO Method II vulnerability assessment of the Key Bridge, as Maryland officials failed or were unable to provide the board with key information on vessel traffic, channel geometry and depth, pier protection systems and ultimate lateral capacity for bridge piers.

“There’s no reason why [the Maryland agency] shouldn’t have done it before now,” Homendy said of performing the assessment. She added that while state officials have cooperated with the investigation, she was surprised that Key Bridge vulnerability was so high. In addition to determining the bridge’s overall risk to collapse, the board found that Pier 17, which the Dali struck, was 15 times above the acceptable risk threshold, based on AASHTO guidance.

In a statement, the Maryland agency countered that the collapse and loss of life “was the sole fault of the Dali and the gross negligence of [its] owners and operators who put profits above safety.” The statement noted that as with other U.S. bridges, the Key Bridge “was approved and permitted by the federal government and in compliance with those permits,” and that over the past 50 years, “hundreds of thousands of vessels transited under the Key Bridge without incident.”

As the board investigation into the Key Bridge collapse continues, Homendy said it has released to its public docket nearly 40 documents related to the investigation, including the bridge factual report and interviews. Materials will be added in coming weeks, including the Dali’s voyage data recorder and voice transcripts; engineering, nautical operations and anchorage factual reports; and other studies. She said the agency will then begin developing its findings and probable cause and safety recommendations, with intent to release the final report this fall.

Meanwhile, Kiewit Infrastructure Co. is in the $73-million first phase of a progressive design-build contract for a 2-mile-long replacement to the Key Bridge, with the estimated $1.7-billion replacement structure currently set to open in fall 2028.