Why the Baltimore bridge collapsed so quickly

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The steel frame of the Francis Scott Key Bridge sits on top of a container ship after the bridge collapsed in Baltimore.
Enlarge / The steel frame of the Francis Scott Key Bridge sits on top of a container ship after the bridge collapsed in Baltimore on March 26.

Just shy of half past 1 in the morning, the MV Dali, a giant container ship, was sailing gently out of the port of Baltimore when something went terribly wrong. Suddenly, lights all over the 300-meter-long vessel went out. They flicked on again a moment later, but the ship then began to veer to the right, toward one of the massive pylon-like supports on the Francis Scott Key truss bridge—a huge mass of steel and concrete that spans the Patapsco River.

The Dali’s lights went out a second time. Then the impact came. The ship plowed into the support, with large sections of the bridge’s main truss section instantly snapping apart and falling into the river. It took just 20 seconds or so for the structure to come down.

Now, a major US port is in disarray, and several people who were working on the bridge at the time of its collapse are missing. A rescue operation is underway. President Biden has called the disaster a “terrible accident.” Ship traffic is currently stuck on either side of the crash site, and a major roadway through Baltimore has been cut off.

“It’s a dreadful tragedy and something you hope never to see,” says David Knight, a bridge expert and specialist adviser to the UK’s Institution of Civil Engineers. But commenting on footage of the bridge collapse, he says he is not surprised by the manner in which it crumpled.

Large steel structures may seem invulnerable, but steel, explains Knight, is relatively lightweight for its size. As soon as it is pushed or pulled the wrong way with enough force, it can fold like paper. In this case, the Francis Scott Key Bridge was a “continuous,” or unjointed, bridge that had a 366-meter-long central truss section. (Truss bridges use steel beams, arranged in triangular shapes, to support their load.) The central truss was made up of three horizontal stretches, known as spans, with two sets of supports holding these above the water. It was the third-largest structure of its kind in the world.

“When you take a support away, there is very little in the way of robustness,” says Knight. “It will drag down, as we saw, all three spans.” The separate approach spans remain standing. There is nothing in Knight’s view that immediately suggests any structural problem with the bridge. An engineering firm, Hardesty & Hanover, confirmed to WIRED that it performed an inspection of the bridge in 2019, and that other inspections have been carried out since, but did not provide any additional details on the state of the structure. WIRED has approached H&H for further comment. In June last year, the US Federal Highway Administration rated the condition of the bridge as satisfactory.

The immense force of the container ship impact should not be underestimated, adds Knight. Such vessels require a lot of power and time—perhaps many minutes—to come to a complete stop. The Francis Scott Key Bridge was completed in 1977. In more recent decades, bridge engineers have commonly incorporated defenses to reduce the potential damage by ship strikes when bridges are erected in similar locations, Knight says. These include hydraulic barriers and additional concrete around the base of bridge supports, for instance. However, even with such fortifications in place, heavy strikes can still cause devastating damage.

It is not clear why lights turned off and on again on the Dali, a Singapore-flagged ship built in 2015. “That is an indication of a massive problem,” says Salvatore Mercogliano, a maritime historian at Campbell University in North Carolina and a YouTuber who has analyzed the crash.

At the time of the accident, two pilots—mariners who board a ship to help it navigate particular stretches of water, including in and out of ports—from Baltimore were on board. The Dali was broadcasting its position publicly via the automatic identification system (AIS) and was traveling at a speed of over 8.5 knots. It then slowed to around 6 knots in the moments before the crash, according to AIS data.

Both pilots and all crew members on the Dali are accounted for. There are no reports of injuries, the ship’s management company, Synergy Group, said in a statement on March 26.

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