National Transportation Safety Board report finds Francis Scott Key bridge was at high risk of collapse

The National Transportation Safety Board (NTSB) in the US has published an interim report into bridge safety that calculates that the risk level to the Francis Scott Key bridge, which was knocked down in 2024, was 30 times higher than the safety threshold established by the American Association of State Highway and Transportation Officials (AASHTO). 

The NTSB report also finds that 68 other bridges in the US, frequented by ocean-going vessels and built before the AAHSTO guidance was issued in 1991, need assessment for vulnerability of catastrophic collapse. Of those bridges, 34 are classified as critical/essential.

The Francis Scott Key bridge spanned the Fort McHenry Federal Channel in the Patapsco River outside the US port of Baltimore in Maryland and collapsed immediately when the Dali container vessel collided with Pier 17, which supported the central span of the bridge, on March 26 last year. The channel is one of the busiest for cargo traffic in the US and the port of Baltimore has long been the busiest finished vehicle handling port in the country.

Calculating the risk 
Following the collapse of the bridge the NTSB carried out a vulnerability assessment of the bridge using a method of calculation drawn up by AASHTO to understand the level of risk at the time of collapse. The AASHTO Method II vulnerability assessment calculation is used to determine the annual frequency of collapse (AF), which is the probability of a bridge collapse within a year because of a vessel collision. The calculation gathers data specific to each bridge and waterway, including characteristics of the vessel traffic passing under the bridge, vessel transit speeds, loading characteristics, bridge geometry and pier protection systems, among other criteria.

The report notes that the Francis Scott Key bridge, which was opened in 1977, was designed according to the 1969 AASHTO Standard Specifications for Highway Bridges. Though that guidance did not include the risk of vessel collisions or need for bridge protection, the bridge was built with protection systems. They included four 8.5 diameter dolphin piles with rubber and timber fenders around the central piers 17 and 18. The Dali did not connect with the dolphin structures in its powerless careen off course toward the bridge support.

In its 2009 Guide Specifications AASHTO recommended that bridge owners use the vulnerability assessment calculations to assess bridges built before 1991 to identify bridges at risk of a catastrophic collapse in the event of a vessel collision. However, AASHTO cannot require a bridge owner to complete a vulnerability assessment for a bridge designed before the release of the 1991 guidelines.

Recommendations on risk
The NTSB report states that the Maryland Transportation Authority (MDTA) had not and was not required to carry out a vulnerability assessment of the Francis Scott Key bridge to evaluate its risk of catastrophic collapse from a vessel collision. However, AASHTO recommended that states including Maryland performed those assessments and evaluated the risk of collapse.

Another factor playing into the risk assessment is the increase in the size of cargo vessels since the Francis Scott Key bridge was opened and the increase in vessel traffic to and from the port of Baltimore. Incorporating traffic parameters into the vulnerability assessment, the NTSB determined that if the MDTA had calculated the probability of collapse from a collision it would have identified that the bridge’s risk level was almost 30 times greater than the AASHTO risk threshold for critical and essential bridges.

The report also finds that with a vulnerability assessment carried out “the MDTA would have had information to proactively identify strategies to reduce the risk of a collapse and loss of lives associated with a vessel collision with the bridge”. Six bridge construction workers were killed when the bridge collapsed.

The NTSB report also finds that the 30 owners of the 68 bridges frequented by ocean-going vessels in the US are likely unaware of the risk of catastrophic collapse their bridges face from a vessel collision and the potential need to implement countermeasures to protect them.

Designs for the new bridge to replace the Francis Scott Key were unveiled in February this year and the bridge is estimated to be complete by the autumn of 2028 with a cost estimate upward of $1.7 billion.

Read the full report with a list of the 68 bridges also at potential risk.