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Why Baltimore’s Francis Scott Key Bridge Collapsed after Strike by Container Ship

As an enormous container ship was leaving Maryland’s Helen Delich Bentley Port of Baltimore just before 1:30 A.M. on Tuesday, it crashed into a pier supporting the Francis Scott Key Bridge. In less than a minute, the midsection of the 1.6-mile-long truss bridge plunged into the cold river below. Nothing indicates the collision was intentional, said James Wallace, chief of the Baltimore City Fire Department, at a recent press conference. The Singapore-flagged cargo ship appears to have lost propulsion—and therefore the crew’s control—while it was exiting through the harbor channel. According to the Associate Press, Maryland governor Wes Moore, who issued a state of emergency, said the crew of the ship, called the Dali, issued a mayday call with enough time to stop bridge traffic before the catastrophe.

None of the ship’s crew members were injured, but eight construction workers were on the bridge when it snapped and dropped. Maryland transportation secretary Paul Wiedefeld said that two of those workers had been rescued, according to CNN, and six are still missing as of this article’s publication. The U.S. Coast Guard and other emergency responders searched the harbor with boats and divers as helicopters circled above.

“It’s a huge tragedy,” says Benjamin Schafer, a Johns Hopkins University professor of civil and systems engineering. The collapse, he says, leaves a “stark hole” in Baltimore. Each year more than 11 million vehicles traveling along Interstate 695 crossed the Patapsco River via this bridge, according to the Maryland Transportation Authority. Some traffic could be routed to a four-lane tunnel that passes below the river—but not all of it, such as vehicles trucking hazardous materials. And until the wreckage of the bridge can be removed from the 50-foot-deep channel, it could cause shipping delays while cargo is routed away from this major mid-Atlantic hub.

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Schafer spoke with Scientific American about the destruction of what he called Baltimore’s “elegant bridge in the skyline.”

[An edited transcript of the interview follows.]

Would any bridge survive this kind of impact?

I don’t think so. Any bridge supported at two points like that and having one support just totally removed is going to be in the water as the next step.

What was the bridge made of?

The main span, which is the part seen [collapsing in video footage], is the steel superstructure. Each of the elements that you see are steel supported on concrete bridge piers, which connect it into the water.

Construction on this bridge began in 1972, and it opened five years later. If we were building this bridge today, what would we do differently?

The economics of bridges at that span [their pier-to-pier length] have changed since the middle of the 20th century, [as have] the aesthetics of what cities are looking for when they build a signature bridge like that. Almost for sure, if it was built today, it would be in a different form—probably a cable-stayed bridge, if you look around the country at what we’ve been building in the past 20-plus years. [Editor’s Note: In a cable-stayed bridge, cables run from high towers to the road, in contrast with the boxy metal construction of a truss bridge.]

But that reflects changing labor and materials costs. There are a lot of small pieces to make a metal truss bridge.

Did aging infrastructure contribute in any way to this collapse?

There’s nothing I’ve heard that indicates this particular structure had a critical aging issue or that such an issue was a critical piece to how this might fail. When I first saw the video the first thing this morning, I thought, “Oh, well, you know, maybe this or that [aspect of the bridge contributed to its collapse].” But then I sat down and watched it frame by frame. And, literally, the pier’s gone, and the bridge falls vertically down. So I was like, ‘Oh, maybe bridge scour had weakened the pier or something like that over the years.’ [Editor’s Note: Bridge scour results when water erodes the sediment surrounding a pier’s foundation.] But anything like that will be very second-order against the energy that was imparted into the end of the pier from this massive container ship.

Would a smaller cargo ship have destroyed the pier if it collided where the Dali did?

It’s hard to tell. It seems the pylon was completely obliterated. It wasn’t tilted, canted, bent or whatever—it was just gone. If you and I are out in a little sailboat, and we hit the bridge pier, the bridge pier is stronger than we are. But in this case [either because the bridge column lacked sufficient protections or because the ship was big enough to get through them], the entire pier is just gone.

A bridge is a carefully calculated thing. One of the major assumptions is that where I put the support points, I’m going to give it support. If I take that away, there’s not a design that’s going to allow the bridge to span anyway.

You literally just see it all erased, and then the bridge is moving vertically down as a rigid body…. It’s gravity. Its support has been removed. The places that you see it fail and then finally fail into the water are directly correlated to the loss of the support. It’s not some propagation of: one thing happens, and then that fails this member, and then that member that failed fails this member…. The whole bridge, every piece of steel, goes vertically down. There’s not a twist or bend or anything like that. Support’s gone; down it goes.

Are there any engineering lessons from this disaster?

So maybe we treat it as a full infrastructure problem, as opposed to a bridge problem, right? We need the Port of Baltimore. It’s a major shipping port for the entire Northeast Corridor. It has significant shipping traffic. If you look at the size of the ships from the 1970s, when the bridge was built, to now, it’s radically changed. When you look at the failure in the video, the container ship is as wide as the bridge is tall. It’s hard to get your mind around how big it really is! A lot has changed in terms of the environment in which that bridge operates and what we need it to do.

There are going to be lessons learned on how we manage our bridge support structures with respect to protecting it from shipping traffic…. I’m just speculating, but I can’t imagine that all of the processes that led to the accident would be allowed to be the same. So there are going to be regulations and processes for what it means to come into the port or to leave the port.

On a personal, emotional level, we know driving over bridges gives people some pause. The structural engineering community that works on these structures takes that very seriously and designs and cares for bridges with utmost seriousness. If there are lessons to be learned about actual structure, those lessons will be learned.


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