Around two in the morning a tug is pushing north through the Straits of Mackinac with a deck barge on a long wire behind it.
On the barge sits a crawler crane with its crane boom raised at an angle, headed for a job further north.
Ahead of them a suspension span crosses the whole channel.
Nobody aboard, and nobody who sent them, knew how tall that boom was.
They had a figure. It was not a measurement.
Why you cannot look at a boom and know its height
The height of a raised boom is not one property. It is two numbers multiplied together, the length of the boom and the angle it is pitched at, and the tip goes wherever that arithmetic sends it.
Which makes eyeballing it far worse than it feels. On a boom this long, being wrong about the angle by ten degrees shifts the tip by something like twenty feet vertically.
Get the length wrong as well and the two errors do not cancel. They stack.
The instrument for the angle half is an inclinometer, it costs almost nothing, and reading one takes about a minute. The length half is written on the equipment.
So the height was available to anybody who wanted it that night, in the way that a fact is available and a guess is not.
What they believed against what was true
Managers at the company looked at the crane before it left and put the boom angle somewhere between 50 and 60 degrees. They also worked from a boom length of 140 feet.
The boom was 160 feet, with a sheave attachment on top of that, and it was sitting at roughly 62 degrees.
Both estimates were wrong, and both were wrong in the direction that makes a boom shorter than it is.
Run the real figures and the tip stood about 162 feet above the water. Investigators later put the angle that would have cleared the span at around 55 degrees, which the crane was nowhere near.
Even the steepest end of the range the managers guessed at would not have fitted under the bridge that night.
The night itself
It happened on the seventh of May in 2023. The tug ran 103 feet and the barge trailed well astern on its towing gear as they came up on the main span.
The clearance was 153 feet. The bridge is published at 155 at midspan, and the water was running high, which quietly removed the last two feet before anybody arrived.
Securing wires caught first. Then the boom went into the stiffening truss under the roadway, and the contact pushed it backward, which raised the angle further and drove the tip higher into the structure above.
The boom bent and folded down across the aft end of the barge.
No one was hurt. Nothing spilled into the straits.
Where the failure actually sat
The interesting part is that this was not a lapse in the moment. Nobody put an inclinometer on the boom, and the air draft of the tow was never checked against the bridge’s published clearance.
There was no documented assessment before departure, and after managers told the captain the tow was ready, he took no further steps of his own to confirm it.
Vessels like this are required to run a towing safety management system, a written set of procedures covering exactly this kind of planning. The paperwork existed. The measurement did not.
That gap between a permit or a procedure and an actual verified number is a recurring shape. A flatbed running an inch under its permitted height still took out an Alaska overpass, and the state responded by going out to remeasure every bridge on that highway.
What it cost and what the board said
The safety board published its findings on the tenth of February in 2025, and named the probable cause as ineffective voyage planning that failed to identify that the crane was too tall to pass.
The damage split is worth reading twice. The bridge came to about 145,000 dollars. The crane came to 665,000, which is to say the equipment that hit the structure was hurt roughly four times as badly as the structure it hit.
Bridges are built to be enormous and dumb. Booms are built to be light enough to swing a load, which is the same reason they crumple.
The board’s stated lesson is almost aggressively plain. Get the most accurate and objective data on the crane and the bridge before getting underway, and check air draft against published clearances using proper navigational resources along the route.
Which is another way of saying a number rather than a look, a lesson a railroad bridge in Durham keeps teaching one truck roof at a time.
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