ST. IGNACE, MI— The Mackinac Bridge is at a lower risk of collapse due to a vessel collision than nationally established thresholds.
That’s according to a report from consultant engineering firm Parsons that was submitted to the National Transportation Safety Board in March.
Following the March 2024 collapse of the Francis Scott Key Bridge in Baltimore after it was struck by a container ship, the NTSB called on owners of 68 bridges in the United States, including the Mackinac Bridge, to evaluate those structures for risk of collapse.
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.”
The NTSB found that the Key Bridge was almost 30 times above the acceptable risk threshold for critical or essential bridges.
