Baltimore bridge collapse: Why did the ship crash and how did the bridge fall so easily?

Credit: AP

By Daniel Boal, ITV News Producer

A 948-foot long cargo ship struck the Francis Scott Key Bridge in Baltimore on Tuesday, causing the 1.6mile long structure to instantly collapse.

Following an 18-hour long search and rescue mission from US authorities, six construction workers have been presumed dead by officials.

The bridge was a critical link for trucking and other vehicles to pass between Washington, Baltimore, Philadelphia and New York and was used by 12 million vehicles last year.

The bridge collapsed within seconds, and the speed of it has raised questions with experts.

Ahead of an official investigation being launched by US authorities, experts have offered their opinion on how and why the boat crashed and why the bridge collapsed the way it did.

What went wrong with the ship?

The ship reportedly suffered a "power issue" and sent a 12-second warning over the radio. Police officers managed to stop traffic over the Key Bridge in both directions within 90 seconds.

But the construction workers were unable to be warned before the ship hit.

The ship's lights went out some four minutes before the collision occurred and after around 60 seconds the power was restored - it is likely this is when the ship's emergency generator kicked in.

Steering functionality and engine power should have been restored once the emergency generator started working, unless there was a mechanical failure which could explain the crash.

How did the incident unfold and what happens next? ITV News Correspondent Rachel Younger explains

Professor Jin Wang, Professor of Marine Technology at Liverpool John Moores University, said: “While the exact causes leading to this unfortunate tragedy remain to be seen, loss of propulsion of the ship seems to be a likely one.

"When the propulsion power is lost in a ship, the maneuverability is lost because propellers and thrusters would not be operational. In such cases the ship may drift randomly, particularly in harsh weather conditions."

The Dali container ship involved in this accident is less than ten years old with gross tonnage of 95,000 tonnes, Prof Wang said. He added the Dali is considered an average-sized container ship and reportedly had 22 crew members on board.

In Baltimore, it is required by law for a local pilot to steer ships out of the port. The ship's owners have confirmed there were two pilots on board.

What went wrong with the bridge and why did it collapse?

There have been a number of questions surrounding why the bridge fell so easily, after being hit by a vessel travelling at what is believed to have been just 9mph.

Built in 1977, the bridge was held up by a number of concrete pillars, one of which collapsed when the 948-foot long Dali collided with it.

The bridge was a continuous structure, which meant the entire structural arrangement was interconnected. When one of the pillars collapsed there was nothing supporting the arch, which then collapsed, taking the bridge with it.

Another point made by experts was that the steel truss road bridge was designed and built nearly 50 years ago when ships like the Dali were not using the Baltimore port.

It is likely there was no consideration given to a ship of that size crashing into the bridge.

Dr Stergios Mitoulis, Head of the Structures Research Group at the University of Birmingham, said: "Removing one support redistributes loads in the structure, akin to the way loads shift when standing on one leg.

"Thus even though it might look as a disproportionate consequence, the pylon collapse would lead to collapse of the bridge in all cases—this is not something that we can design our bridge decks for."

The Dali container ship rammed through the Key Bridge in Baltimore. Credit: AP

Given the vessel’s massive weight, it struck the bridge support with significant force, explained Roberto Leon, a Virginia Tech engineering professor.

“If a bridge pier without adequate protection is hit by a ship of this size, there is very little that the bridge could do,” Leon added.

Last June, federal inspectors rated the 47-year-old bridge in fair condition. But the structure did not appear to have pier protection to withstand the crash, experts said.

Experts believe the full forensic investigation of the incident could take between one and two years.

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