Report confirms Mackinac Bridge below threshold risk of collapse due to theoretical vessel collision

Fast facts:

• Following the March 2024 collapse of the Francis Scott Key Bridge in Baltimore after it was struck by a container ship, the National Transportation Safety Board called on owners of 68 bridges in the United State, including the Mackinac Bridge, to evaluate those structures for risk of collapse.
• The Mackinac Bridge Authority contracted with consultant engineering firm Parsons to evaluate the bridge against the American Association of State Highway and Transportation Officials (AASHTO) design guidelines.
• The evaluation found that the risk of the Mackinac Bridge collapsing due to a vessel collision was below the annual frequency of collapse threshold.

ST. IGNACE, Mich. - The Mackinac Bridge Authority (MBA) today reviewed a report from its consultant engineering firm that confirms the bridge is at a lower risk of collapse due to a vessel collision than nationally established thresholds, following an evaluation requested by the National Transportation Safety Board (NTSB).

The MBA reviewed the study conclusions during its regular meeting today on Mackinac Island. The study was submitted to NTSB in March.

“This report confirms what we’ve long believed to be true, that the risk of the Mackinac Bridge collapsing due to being struck by a vessel is extremely low,” said Bridge Director Kim Nowack. “We hope this gives greater assurance to everyone who crosses the bridge that there is very little danger of such a tragedy here.”

The report is in response to a request from the NTSB to 30 owners of 68 bridges in 19 states, recommending that each conduct a vulnerability assessment to determine the risk of bridge collapse from a vessel collision. The request came following the March 2024 collapse of the Francis Scott Key Bridge in Baltimore after it was struck by the ocean-going container ship, the Dali.

The NTSB recommendations were issued to bridge owners to calculate the annual frequency of collapse for their bridges using the American Association of State Highway and Transportation Officials (AASHTO) Guide Specifications and Commentary for Vessel Collision Design of Highway Bridges.

The analysis took into account the bridge design and structural capacity, the characteristics of the Straits of Mackinac, and the size, type and frequency of vessels navigating near the bridge.

The Mackinac Bridge review found: “The AASHTO Method II vessel collision analysis determines a total annual frequency of collapse of 0.000097, corresponding to a return period of approximately 10,350 years, which is below the AASHTO limit of 0.0001 for critical bridges. For one-way vessel traffic, the calculated annual frequency was 0.000045. Both results lie comfortably within the acceptable probability thresholds established by AASHTO for critical infrastructure, confirming that the Mackinac Bridge meets the prescribed reliability criteria.”