The Francis Scott Key Bridge in Baltimore stood no chance against the huge impact from a cargo ship Tuesday morning, engineering experts told Business Insider.
The bridge was destroyed in the early hours of Tuesday, collapsing in a violent fashion after a large cargo ship, the Dali, hit one of its support pillars.
Understanding why and how the bridge collapsed could have big implications for safety, both in the cargo shipping industry and in civil engineering.
The physics of the impact are “pretty clear,” said Leroy Gardner, a professor of structural engineering at Imperial College London.
“There’s a heavy impact from a cargo ship into one of the piers,” he said in a call with BI. “Once that collapsed, then the rest of the bridge followed soon after.”
Experts told BI it was unlikely that any defects in the bridge’s structure were relevant to the collapse, given the scale of the impact.
Engineers would probably have been unable to protect against such a huge impact. The Dali is a substantial vessel, 300 meters long and weighing some 95,000 tons, according to the website vesselfinder.com.
It’s of comparable size and weight to a Nimitz-class aircraft carrier in the US Navy. The Dali was almost as tall as the bridge it was trying to pass under.
“All bridge piers will be designed to resist impact from a vessel. And I think this will be unquestionably no exception to that,” Gardner said.
“I think it’s the magnitude of the force in this case, which is extremely unusual, which has caused the problem for this bridge,” he said.
The Francis Scott Key Bridge opened in 1977. Its design is a steel truss bridge. The bridge was resting on concrete piers before the impact, experts said based on footage shared online.
“The support is a very, relatively, flimsy structure when you look at it — it’s a kind of trestle structure with individual legs,” Ian Firth, a structural engineer and bridge consultant in the UK, told the BBC. “So, the bridge has collapsed simply as a result of this very large impact force.”
The impact from the Dali seemed to knock out one of the concrete piers, a “critical, significant part of the bridge,” Gardner told BI.
“Structures generally are typically designed to have a certain amount of robustness. So if there’s damage to a small part of it, the rest of the structure can remain intact,” Gardner said.
“I think losing such an important element, I would expect the entire bridge to collapse, which is what happens,” he said.
It wasn’t immediately clear what caused the crash. It also wasn’t clear from the footage how quickly the Dali was moving, experts said. It had only recently left the port.
Barbara Rossi, an associate professor of engineering science at the University of Oxford, told BI in an email that the impacting force “must have been immense to lead to these massive (concrete) structures to collapse, leaving the superstructure without one of its supports.”
Long investigations are likely to follow, said Mark Richards, director of NESTA Consulting Engineers.
“It’s inevitable that the engineering community will look at this and investigate what happened very carefully, to learn lessons from it — if there are lessons to be learned from it,” he told BI in a call.
“The bridges are designed for events that are considered by the wider community unit of engineering to be appropriate or probable. Maybe this event just steps outside of that,” he said.
“It may be that those events could not be foreseen, and so we can’t forget that,” he said.
In an emailed statement to BI, a Danish engineering and architecture consultancy called COWI said that bridges are not usually designed to withstand a direct impact from a ship.
Instead, engineers would create structures near the bridge supports that a ship would hit first, absorbing the impact.
Then “the failure would be linked to that and not to the bridge itself,” said Lorna Wharton, Head of Press and Public Affairs for COWI.
It was not immediately clear what defenses the Francis Scott Key bridge had, the experts said. COWI and Richards said that something may have been there.
Engineers will likely be considering not only the protections for the bridge itself, but how the broader dynamics of what routes ships took under and around the bridge, the experts said.
