The world watched with shock as Baltimore’s Francis Scott Key Bridge collapsed into the Patapsco River after the container ship Dali suffered a total power failure and struck a bridge pylon.
As recovery efforts continued and federal and state authorities grappled with the impact of the disaster, the scale of the accident is still taking shape. Six people are presumed dead. The Port of Baltimore, an important shipping hub, has been shut down indefinitely. And rebuilding the bridge could cost, according to some estimates, more than $1 billion.
Meanwhile, observers were left with questions about how a 1.6 mile bridge that carried 31,000 cars per day—11.3 million vehicles a year—could suffer such a complete collapse. The steel truss-style bridge opened in 1977 and was reportedly up to code with no known structural issues. And while the Dali was massive—measuring as long as three football fields and stacked high with containers—the extent of the wreckage was startling.
GW Today spoke with Engineering Professor Emerita W. M. Kim Roddis, a registered professional engineer with experience in bridge design, about the factors that contributed to the collapse.
Q: Can you help us understand how this accident happened? What factors in terms of bridge design played a role?
A: The container ship Dali lost power multiple times on its way out of Baltimore Harbor. The ship’s inability to steer resulted in it heading at an angle towards the southwestern major bridge pier—the pylon.
The navigation chart for Baltimore Harbor shows four protective devices called dolphins, one in front of each pier for outgoing and incoming ships. A dolphin is made up of either a group of piles tied together at the top, or circular sheet pile cells filled with material such as sand or concrete. Dolphins are located at the edge of the shipping channel ahead of the pylon. They essentially serve as bumpers to deflect or slow boats and ships that are headed toward the pylon.
The [Dali’s] angled course allowed the ship to miss the dolphin and strike the pylon. When the ship collided with the pylon it exerted a huge crushing force on the pier, bursting the pylon apart. This pylon was the only support for the bridge on that side. Once the pylon was lost, the bridge fell. In the video you can see the entire bridge drop straight down for several feet before the truss starts to fall apart. The continuity of the structure meant that all three spans came down when the southwestern pylon was lost.
Q: Was this preventable? Or was it just a one-in-a-million chance?
A: Definitely a rare event. The probability of these strikes is considered during bridge design. Ships were much smaller in 1977 when the bridge opened. The angle the ship came in at was unusual. So, yes, this was an unlikely accident. Many large vessels have gone in and out of Baltimore Harbor under that bridge since it opened. So one-in-a-million is in the right ballpark.
Q: What type of bridge was this? Was it particularly vulnerable? Is this a failure of design?
A: The bridge was a steel continuous through truss. The advantage of a continuous truss is that for the same amount of resources—money, materials, labor, time—you get a stronger, safer bridge than single span trusses. The disadvantage is that losing one pylon results in losing all three spans instead of one or two spans. This was not a bridge design failure.
If you look at design of the whole system—physical infrastructure, vehicles and humans— the ship design clearly needed more redundancy for power and steering. The tugs that take a ship from dock into the shipping channel could have stayed with the ship until it was through the harbor gate bridge. Perhaps that could have prevented the ship’s aberrant course.
Q: This bridge was built in the ’70s. Are more modern bridges less vulnerable to a collapse like this?
A: Bridges are designed with protection against ship strikes. These protections are stronger for more recent bridges. It is not clear that even modern protections would have been enough to prevent this particular accident. The Dali is very large. Its course was highly aberrant and it was moving relatively rapidly. It had a huge amount of inertia which might have let it plow over modern protections and still take out the pylon.
Q: Is every bridge in danger of this kind of accident?
A: With larger, heavier and faster ships, any bridge that is a gateway to a port that has a pylon near the shipping channel might be susceptible to this kind of accident.