A bridge strike accident that caused $145,000 of damage to a Michigan suspension bridge and $665,000 of damage to a crane was the result of poor planning, according to the USA’s National Transport Safety Board (NTSB).
The incident in May 2023 happened during the transit under the 8km main span of the Mackinac Bridge of a crawler crane with a 49m boom on a deck barge under tow. NTSB’s report describes how the managers from the towing company visually estimated the angle and length of the boom and assessed that it was safe to pass under the bridge.
Investigators determined the boom was about 50m above the water – 3m too high.
News reports from late May 2023 describe how paint was knocked off, but that inspections did not give rise to any structural concerns.
Towing vessel operators in the US should carry out a towing management system, including voyage planning. But the owners of the tow vessel could not show investigators any documentation of this, and the captain appears not have verified the safety of the transit.
The boom of the crane after the strike. Credit: US Coast Guard
The NTSB determined that the strike was caused by the captain’s and barge company managers’ ineffective voyage planning, which did not identify the crane being towed was too high to pass safely under the bridge.
No one was hurt in the incident, and there was no pollution.
In 1959, a US airman flew a nuclear bomber under the bridge, which is also known as Mighty Mac.
