Explained | How bridges can be protected from ship collisions

By Sherif El-Tawil for The Conversation

Michigan: The MV Dali, a 984-foot, 100,000-ton cargo ship, rammed into the Francis Scott Key Bridge when leaving Baltimore harbour on March 26, 2024, causing a portion of the bridge to collapse.

In an interview, University of Michigan civil engineer Sherif El-Tawil explained how often ships collide with bridges, what can be done to protect bridges from collisions, and how a similar disaster in Florida in 1980— just three years after the Key bridge opened, changed the way bridges are built.

This is not the first time a ship has taken out a bridge. What’s the history of ship-bridge collisions?

This is an extremely rare event. To my knowledge, there are about 40 or so recorded events in the past 65 years that involved similar type of damage to a bridge caused by a ship. So they seem to occur on average about once every one and a half to two years around the world. When you consider that there are millions of bridges around the world— and most of them cross waterways – you can imagine how rare this is.

The most influential case was the 1980 Sunshine Skyway Bridge collision in Florida, which prompted the federal government to take action in terms of developing guidelines for designing bridges for ship collision. By the early 1990s the provisions were developed and incorporated into the bridge design code, the AASHTO specifications. The American Association of State Highway and Transportation Officials produces the design code every bridge in the United States must conform to.

What was different about the Sunshine Skyway Bridge disaster from previous bridge collisions?

There were casualties. The fact that a crash could bring down a bridge, just like in the Baltimore situation, prompted the concern: Can we do something about it? And that something was those specifications that came out and eventually became incorporated in the national design document.

What those specifications say is that you either design the bridge for the impact force that a ship can deliver or you must protect the bridge against that impact force. So you must have a protective system. That’s why I was surprised that this bridge did not have a protective system, some type of barrier, around it. I have not examined the structural plans of this bridge. All I could see is the pictures that were published online, but protective systems would be very visible and recognizable if they were there.

What is currently mandated for new bridge construction, and is it sufficient to handle today’s massive cargo ships?

I estimate, based on the published speed and weight of the MV Dali, that the impact force was in the range of 30 million pounds. This is a massive force, and you need a massive structure to withstand that kind of force. But it is doable if you have a huge pier. That might dictate the design of the bridge and what it could look like. Most likely it could not be a truss bridge. It may be a cable stay bridge that has a very large tower that is capable of taking that load.

If you cannot design for that load, then you have to consider other alternatives. And that’s what the specifications say. They’re very clear about this. And those alternatives could be to build an island around the pier or a rock wall, or put dolphins— standalone structures set in the riverbed, adjacent to it, or put on fenders that absorb the energy so the ship doesn’t come in so fast. All of these are ways you can mitigate the impact.

Engineers design structures and bridges are no exception— for a certain probability of failure, because if we didn’t, the cost would be prohibitive. Theoretically, you could build a structure that would never fail, but you’d have to put infinite money into it. For a critical bridge of this type, we would consider an acceptable chance for failure to be 1 in 10,000 years.

Based on published information, I tried to compute what the probability of this event would be, and it turns out to be 1 in 1,00,000 years or so. The ship made a beeline directly to the pier that was vulnerable. It was just shocking to see such a rare event unfold.

The authority of the bridge must have considered protecting it, and the low probability of this occurring must have played a role in whether they would invest or not in protective measures. Because any type of construction in water or on water is very expensive.

Is it feasible to protect older bridges?

I think so. For some of them it might be lower tech like the island idea. And it could use maybe rocks or concrete components that would prevent the ship from reaching the pier at all.

It was a massive ship with a flared bow. The lower part of the ship, which extends beyond the bow, I believe struck the foundation system, but the bow reached the pier. The pier was like an A shape, so the bow snapped one side of the A. The other side could not support the weight of the bridge and so the whole thing collapsed. If somebody kicks your feet from underneath you, you’re just going to fall. That’s exactly what happened.

How many bridges are vulnerable to ship collisions?

I don’t know the number, but I know that bridges that are in this category, that are long span, major bridges like this, are probably less than 0.1 per cent of the bridges in the US. And some of them do not necessarily cross waterways, so that’s a subset that is an even lower percentage. So it’s a rare event occurring to a rare kind of bridge.

Are cargo ships getting larger, and is that a consideration for protecting bridges?

I expect so because there is an economy of scale. Bigger ships would be cheaper for transporting goods. But I cannot envision that the designer of this bridge 50 years ago or so would have thought that a ship this size could impact the bridge. I’m sure they would have taken steps to address that. It just didn’t cross their mind.

If this bridge had been designed to the current specifications, I believe it would have survived. There are two reasons a ship would deliver this kind of force: It’s moving too fast or it’s too heavy. And those two factors are taken into consideration in the impact force for which we design. So if we are taking those explicitly into consideration, then a bigger ship, yes, it’s a bigger force, and we would design for that.

But let’s go forward another 50 years and imagine you have a much larger ship that comes into being. At that time, bridges will have been designed for smaller ships, and you have the same problem all over again. It’s hard to predict how big these things will go. You can design for current ships, but as they evolve, it’s hard to predict many years into the future.

Are there other takeaways from this disaster?

The loss of this bridge, beyond the tragic loss of life, is going to be felt for many months if not years. It’s not a straightforward process to replace a bridge of this magnitude, of this span distance. It’s something that will require a lot of planning and a lot of resources to come back again to where we were before.

(Sherif El-Tawil is a Professor of Civil and Environmental Engineering, University of Michigan)