Engineering Lessons One Year After The Baltimore Bridge Collapse

IRA FLATOW: This is Science Friday. I’m Ira Flatow. In the early morning of March 26, 2024, the container ship Dali struck the Francis Scott Key Bridge in Baltimore. Within 30 seconds, the bridge collapsed into the river below. Six maintenance workers lost their lives.

Now, almost exactly a year later, we wanted to look back on what led to this catastrophic event, and more importantly, look to the future. How to improve the safety of our nation’s bridges and prevent another tragic crash. Joining me now to discuss are my guests Dr. Abi Aghayere Professor of Civil Engineering at Drexel University, based in Philadelphia, and Dr. Thomas McKenney, Associate Professor of Engineering Practice, Naval Architecture and Marine Engineering at the University of Michigan, based in Ann Arbor. Welcome to Science Friday.

ABI AGHAYERE: Thank you.

THOMAS MCKENNEY: Thank you, Ira. Happy to be here.

IRA FLATOW: You’re welcome. All right. Let’s start looking back to exactly what led to this tremendous bridge collapse. Thomas, can you give me an overview of what went wrong aboard the Dali that led it to striking the bridge?

THOMAS MCKENNEY: Sure. So the basics of the incidents go roughly like this. So soon after the Dali left its terminal and entered the channel, one of its electrical breakers unexpectedly opened, which caused the first blackout. Now, although generators were running, most of the equipment lost power on board the vessel, and that also means that the main engine automatically shut down, which stopped the propeller as well.

And the crew did some additional reconnecting of breakers to restore power. However, there was another blackout that occurred. And then eventually roughly 30 seconds after that, they did get power back, but in a last ditch effort, the pilot ordered an anchor to be dropped overboard, and then eventually that led to the allision with the bridge.

IRA FLATOW: Let’s get in the weeds. Tell me what actually happened.

THOMAS MCKENNEY: So first of all, the cause of the incident is still under investigation. So the NTSB, however, has provided us some indications. And for me, I divide this into two main things. First of all, there was clearly some sort of electrical component fault, and that was associated with the first kind of main cause where one of those breakers was opened. Now, what this means, for me at least, is to put into question the condition of the vessel. The components of the electrical systems potentially might not be properly maintained.

The second thing was related to the operating configuration. Now, in this case, they were operating with what’s called a closed bus setup, which means that it might be good for efficient power distribution, but it’s not so good if something goes wrong. And so the blackout that occurred, in that case, the generators were still generating electricity, but the power wasn’t able to get where it needed to go.

IRA FLATOW: All right, Abi. So the ship hits the bridge. The bridge collapses quickly. Why so quickly?

ABI AGHAYERE: Well, it turns out that this particular bridge was fracture critical. Meaning that if one tension member fails, the whole bridge comes crashing down. And so it was no surprise that it happened that way, because the bridge was fracture critical. And the bridge piers were not designed for a direct strike from a ship like the Dali.

IRA FLATOW: What does it mean to be fracture critical? And is that like an Achilles heel on a bridge?

ABI AGHAYERE: It is an Achilles heel on a bridge. There are about 17,000 bridges in the United States that are fracture critical. We don’t design our bridges that way anymore. We design our bridges to have redundancies. But this bridge did not have that redundancy and the bridge piers were not protected.

IRA FLATOW: So how should the bridge piers be better protected?

ABI AGHAYERE: There are several ways you can protect bridge piers. You can have what we call dolphins. They are like huge bollards. For example, the Delaware Memorial Bridge is building about eight of those bollards, 80 feet in diameter, filled with sand or concrete. And they stop the ship by deflecting and dissipating the energy from the ship. Can have dolphins.

You can have man made rock islands. Rock islands are also commonly used in some bridges, like the Sunshine Skyway Bridge in Florida uses rock islands. Also uses dolphins. So those are the two main ways of stopping or preventing bridges from collapsing as a result of ship strikes.

IRA FLATOW: So when you say rock islands, that means the ship will run aground before it hits the bridge. Is that right?

ABI AGHAYERE: That is correct.

THOMAS MCKENNEY: Ira, can I make a comment there? Because I think what we saw from the incident with the Dali here is that the ship collided with the bridge, but the ship was, I mean, from a stability perspective, the ship was fine. The bridge even collapsed onto the bow of the vessel. And so that confirms that we are designing ships well, because we have a collision bulkhead, and we have the necessary design of the vessel to withstand groundings and collisions. So if a vessel did collide with a rock Island, the ship in most cases would survive, especially at lower speeds.

IRA FLATOW: So Abi, there was nothing wrong with the bridge that led to this collapse or the structure itself.

ABI AGHAYERE: Nothing wrong with the bridge as far as we know. I mean, the bridge was old. So I mean, it would not be in tip top shape like it was when it was built in 1977. But there was no way a bridge could sustain 34 to 35 million pounds of impact force that was generated when the Dali hit those bridge piers.

IRA FLATOW: As you just said, the Key Bridge was built in the ’70s, and we know back then cargo ships were not nearly as big as they are now, right? I mean, how has bridge construction changed? Are different standards now in effect?

ABI AGHAYERE: One of the major bridge collapses that occurred in the United States occurred in 1980, the Sunshine Skyway Bridge. 35 people died. And as a result of that, the American Association of State Highway and Transportation Officials, AASHTO, came up with guidelines in 1991 to protect bridge piers that are in navigable waterways.

And what happened, though, was that it only applied to new bridges. They did not mandate existing bridges or bridges that were under design at the time the guidelines came into being to have this protection. So new bridges from, let’s say, 1994 onwards had bridge protection, but existing bridges like the Key Bridge, like the Bay Bridge, which is not far away from the Key Bridge, do not have this protection. They don’t have the pier protection and therefore they are vulnerable.

And as NTSB just reported, there are many bridges, about 68 bridges, with unknown vulnerability that they’ve asked the owners, the 30 owners of these 68 bridges, to calculate the vulnerability and compare it to the threshold that AASHTO provides to see how vulnerable their bridges are currently.

IRA FLATOW: Who pays for the protection? I mean, if you have a collapse of a bridge like this, are the ships insured enough to pay for the repairs that go on, or are taxpayers, or who’s going to pay for that protection?

ABI AGHAYERE: I doubt that the ships are insured enough. I mean, the Key Bridge is estimated to cost $2 billion to replace. Now, you can’t even put a cost on the loss of six lives as a result of this collapse. Then you have business disruption. That’s another cost, a huge cost. So, I mean, I think it’s going to fall on the taxpayers at the end. The bulk of it, that is. I don’t know that the ship is insured enough to pay billions and billions of dollars.

IRA FLATOW: Thomas, let’s talk about that ship a bit. Let’s go back to the issue of the power failures aboard it for a moment. Should there, could there, can there be more redundant systems aboard these huge shipping containers?

THOMAS MCKENNEY: Yes, there can be. And there are vessels already out there that have more redundant propulsion systems. I designed cruise ships for over eight years. If you look at a cruise ship configuration, they have at least two or even three redundant propulsion or propulsors. So think multiple shafts and propellers. And they’re also all electric configurations, where you have multiple engines producing electricity that provides to the propulsion and auxiliary.

The reason why basic cargo vessels are single direct drive propulsion engines is because of the efficiency of them. They’re very, very efficient. And what that means is that you’re able to reduce your cost to transport as well as your fuel consumption and also emissions. So the configuration is driven by the efficiency of it. But there are a number of ways that you can do to improve redundancy, including multiple propulsors.

You can also have some additional emergency systems. So if you had more like local power sources for steering and larger emergency generator power, you would also have the ability to do more during an emergency situation.

IRA FLATOW: So we’re talking money here.

THOMAS MCKENNEY: It’s all about the cost in the end. There’s an increase in complexity, but there’s also a trade off. Everything is a trade off when we start looking at this. So as we move to more of these more complicated, costly configurations, they’re also in some cases, less efficient, which means we’re emitting more climate pollutants, air pollutants, and other things as well.

IRA FLATOW: Abi.

ABI AGHAYERE: Yeah, I’d like to jump in here, too, and ask Thomas about tugboats. Because I understand that for the Key Bridge, the Dali was being escorted by the tugboats turned back right before the strike, the ship strike. I was wondering if tugboats are another solution to escort these ships under these bridges. And then once they clear the bridge, the tugboats can come back rather than coming back before they cross the bridge.

THOMAS MCKENNEY: So that’s possible. The challenge is that if you look at how fast these vessels typically move through the channel, it’s roughly six knots plus or minus a little bit. Tugboats are not that efficient at changing the trajectory or the direction of vessels that large traveling at that speed. So of course, one solution is to go slower and to have more support. But of course, that also affects the efficiency of port operations as well.

IRA FLATOW: So how much of this disaster can we say was a result of the bridge not having enough protection versus an issue with the ship? Let me begin with you, Abi. Is it, 50/50, 60/40, or what?

ABI AGHAYERE: I call it 50/50, because one of my students did some research recently and found out that there were about 2,100 engine failures according to the Coast Guard data within a number of years. And so engine failures will happen. But if the bridge is protected, if the bridge piers are protected, then even when those engine failures happen, what will be damaged will be the dolphins, which can be replaced. So I put it at 50/50.

IRA FLATOW: Thomas?

THOMAS MCKENNEY: For me, the main message here, the main takeaway is that we have this just basic difference in bridge lifetimes and ship lifetimes. So if we’re talking about bridges, they’re designed to last 50, 70 years maybe, whereas ships, these basic cargo vessels, are operating 10, 20 years usually. And what that means is that these newer and larger vessels have kind of outpaced the bridges in terms of the ability to change and to be updated.

And so these larger vessels, while maybe there’s been continuous improvement on reliability and redundancy, kind of the consequence of these larger ships impacting the bridges are higher now because of this discrepancy. So I think it’s very difficult to place blame. I think the most important thing is that we look at the interface between ports and ships more. Closer collaboration, more communication, engagement with ship owners and operators and ports going forward.

ABI AGHAYERE: So let me jump in here regarding the apportionment of blame. We have bridges where we load ray the bridge and say that these bridges can only take this much load. And so we don’t allow these heavy trucks to go on those bridges. So I believe the owner of these bridges, if they allow these ships to come into their port, have a responsibility to make sure that their bridge piers are protected enough to allow the ships, these large ships that are larger than maybe they were when the bridge was designed. If they allow these ships to come into the port, then they have the responsibility to make sure that those bridge piers are protected to allow those ships come in.

IRA FLATOW: So you say to the cities who own the bridges, it’s on your head if you allow them to come in and there’s an accident?

ABI AGHAYERE: Yeah, I mean, they’re receiving fees and monies from these ships, these large ships that come in. And so if you allow them to come into your port and they are maneuvering near those bridge piers that some of them totally unprotected, then you owe some responsibility to the people that are using those bridges in case that bridge goes down.

IRA FLATOW: Thomas, container ships have gotten so much bigger and bigger. And I’m recalling this is not the first time that a big container ship caused some serious issues. I’m thinking of the ship that got stuck in the Panama Canal a few years ago. I mean, could smaller ships be a solution here or is that too expensive?

THOMAS MCKENNEY: I believe that the vessel is stuck in the Suez Canal. That was an Evergreen vessel, which did cause some major disruptions. If we look at the growth of container ships, look back to the early 2000s. The largest container ship had about 8 to 10,000 20 foot equivalent units. So think about half of the trailer that you see on the highway. Today, the largest container ship is 24,000 TEUs.

Now, this gives us the economy of scale, cost per container transported. So larger vessels are clearly part of this overall improvement and efficiency of maritime transportation, which is already really efficient if you look at that compared to other forms like rail, trucking, and air. So smaller vessels, of course, can be done, but you’re going to lose that efficiency. There’s other things going on here. We have a climate crisis. We have to reduce greenhouse gas emissions. And that would be counter to some of those other objectives.

IRA FLATOW: So has the Baltimore bridge collapse made these projects to improve bridge safety against strikes a bigger priority?

ABI AGHAYERE: I believe so. There are two reports that came out just in the past few days. You have the NTSB report, talking about 68 bridges with unknown vulnerabilities, asking the owners, recommending to the owners to do a vulnerability assessment, to know what risk their bridges are under. And then you have the Johns Hopkins University report that just came out that also talks about the probabilities of ship strikes on many bridges. And so I think there’s a focus, there’s a renewed focus, I believe, on our bridge piers that are in navigable waterways that need to be protected.

IRA FLATOW: Well, we’ve run out of time. I’d like to thank both of you for letting us know more about this. Dr. Abi Aghayere, Professor of Civil Engineering at Drexel University in Philadelphia. Dr. Thomas McKenney, Associate Professor of Engineering Practice of Naval Architecture and Marine Engineering at the University of Michigan in Ann Arbor. Thank you both for taking time to be with us.

THOMAS MCKENNEY: Thank you.

ABI AGHAYERE: Pleasure.

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