The Francis Scott Key Bridge’s vulnerability 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 Board announced Thursday.

Yet according to the agency, the Maryland Transportation Authority (MDTA) never calculated risks to the nearly 50-year-old steel arch continuous truss structure as recommended by the 1991 and 2009 AASHTO Guide Specifications, 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 the early morning hours of March 26, 2024, resulted in the collapse of the main span, killing six construction workers and blocking the channel for more than two months with approximately 50,000 tons of debris.

Had MDTA performed the assessment, NTSB Chair Jennifer Homendy told a press briefing, “They would’ve been able to proactively identify strategies to reduce the risk of collapse and loss of lives associated with a vessel collision with the bridge.”

The NTSB is also recommending vulnerability assessments for 68 other bridges crossing navigable waters in 13 states. During its Key Bridge investigation, the agency identified the other bridges as lacking a current vessel collision vulnerability assessment. The agency has issued an urgent recommendation to the bridges’ owners 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 AASHTO 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 Bridge in New York; and Florida’s Sunshine Skyway Bridge, which itself was built to replace a structure collapsed by a vessel impact.

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. While expressing the hope that the number of vulnerable bridges is small, Homenday said her agency would follow up with bridge owners and FHWA to ensure they follow through on the recommendation.

“We expect action,” she said. “Public safety depends on it.”

MDTA added that it is actively evaluating the pier protection systems, operational changes and vessel transit procedures of the Chesapeake Bay Bridge in Annapolis. 

Lacking Assessment

Homendy noted that her agency needed assistance from FHWA to develop data for performing its own AASHTO Method II vulnerability assessment of the Key Bridge, as MDTA failed or was unable to provide NTSB 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 [MDTA] shouldn’t have done it before now,” Homenday said of performing the assessment. She added that while Maryland officials have cooperated with the investigation, she was surprised that the Key Bridge’s vulnerability was so high. In addition to determining the bridge’s overall risk to collapse, NTSB found that Pier 17, which the Dali struck, was 15 times above the acceptable risk threshold, based on AASHTO guidance.

In a statement, MDTA countered that the collapse and loss of life “was the sole fault of the Dali and the gross negligence of her owners and operators who put profits above safety.” MDTA’s 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 NTSB’s investigation into the Key Bridge incident continues, Homendy said the agency has released nearly 40 documents related to the investigation to its public docket, including the bridge factual report and interviews. Additional materials will be added in the coming weeks, including the Dali’s voyage data recorder and voice transcripts; engineering, nautical operations and anchorage factual reports; and other studies. Homendy said the agency will then begin developing its findings and probable cause and safety recommendations, with an eye toward releasing 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 scheduled to be open in fall 2028.