Sherif El-Tawil is a professor of civil and environmental engineering at the University of Michigan.
The MV Dali, a 984-foot, 100,000-ton cargo ship, rammed into the Francis Scott Key Bridge when leaving Baltimore harbor 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 100,000 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% of the bridges in the U.S. 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?
It is highly likely that cargo ships are indeed getting larger due to the economy of scale. Larger ships offer cost-effective transportation of goods. However, it is doubtful that the designers of bridges constructed 50 years ago anticipated ships of such size potentially impacting their structures. It is probable that they would have implemented measures to address this issue, but it may not have been a primary concern at the time.
If these bridges were designed according to current standards, they would likely withstand the impact of larger ships. The force exerted by a ship is influenced by its speed and weight, both of which are considered when designing for impact force. Therefore, if these factors are carefully accounted for, the bridge should be able to withstand the force of a larger ship.
Looking ahead another 50 years, if even larger ships are introduced, bridges may once again face the challenge of being designed for vessels of smaller sizes. It is challenging to predict the extent to which ship sizes will increase in the future. While current designs can accommodate present-day ships, future advancements may pose unforeseen challenges for bridge protection.
Reflections on the aftermath of the tragedy
Aside from the heartbreaking loss of life, the impact of this bridge collapse will reverberate for months, if not years. The task of replacing a structure of this scale and span is no easy feat. It will demand extensive planning and a significant allocation of resources to restore the infrastructure to its former state.
This content has been adapted from The Conversation under a Creative Commons license.
