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03/27/2024 | News release | Distributed by Public on 03/28/2024 09:43

Bridge Collapse Creates Conversation in BU Structural Mechanics Class

Bridge Collapse Creates Conversation in BU Structural Mechanics Class

ENG's Doug Holmes says video presented unique opportunity to show how structural engineering works-and fails

The collapse of the Francis Scott Key Bridge in Baltimore provided a newsworthy conversation to the students in ME309-Structural Mechanics, a course at BU's College of Engineering. Photo by Tasos Katopodis/Getty Images

Engineering

Bridge Collapse Creates Conversation in BU Structural Mechanics Class

College of Engineering's Doug Holmes says raw video presented a unique opportunity to show his students how structural engineering works-and also how it fails

March 27, 2024
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When you teach a college course that's a mechanical engineering advanced elective called ME309-Structural Mechanics, the shocking collapse of a 47-year-old bridge makes for some pretty interesting classroom discussion.

Photo courtesy of Douglas Holmes

That was the case this week for Douglas Holmes, a College of Engineering associate professor of mechanics and mechanical engineering, who also leads ENG's Mechanics of Slender Structures lab.

The incredible footage of a cargo ship in Baltimore striking a support of the Francis Scott Key Bridge on Tuesday, causing the bridge to collapse in seconds like a house of cards, is what Holmes began his classroom discussion with later that day. He showed the students the video and then turned it into a conversation and a lesson around structural mechanics, engineering, and why a steel structure designed to support hundreds of thousands of vehicles every day would collapse so easily.

He replied to some questions from BU Today about what lessons could be taken from the catastrophe and how the conversation in his class unfolded.

Q&A

With Douglas Holmes

BU Today:In watching the dramatic video, what struck you the most about the collision and the rapidly ensuing bridge collapse?

Holmes:I was not surprised at how quickly this bridge came down. No bridge can withstand losing one of its primary support columns. I suppose I was most surprised that shipping vessels could get that close to the piers. It's striking to watch the bridge collapse because it is a reminder that the structures around us are built from structures that can bend, twist, buckle, and break-even the really stiff structures made of steel and reinforced concrete.

BU Today:How did you talk about this with your class? What issues came up with the students?

Holmes:I had recently assigned my class some work to understand the "load path" within a structure. For instance, a bird lands on the roof of a house, its weight pushes on the shingles, which in turn pushes on the plywood roof, which is supported by the studs in the walls that carry the load to the ground, where it is "resolved." When designing a structure, we talk about having redundancy-if one load-bearing member breaks, what remaining parts of the structure can carry the load? Often, structures are designed to have multiple ways that a load can be resolved to minimize the chance of catastrophic failure. We talked about how it's difficult to imagine how a structural engineer could have prevented such a failure with a different bridge design, and how it's likely that preventing future disasters like this will involve further minimizing the chances of a large ship taking out one of the structure's primary sources of support.

BU Today:What is your background, beyond the classroom?

Holmes:I've been a professor of mechanics and mechanical engineering for 13 years, 10 at BU, and I lead the Mechanics of Slender Structures lab here at BU. Slender structures are rods, beams, plates, shells, and we study how they fail, which is often by buckling or snapping. Our research tries to understand these instabilities to explain the shapes of things around us, and to utilize these instabilities to create advanced, functional structures, like this.

"My students commented on how the bridge looked almost like a toy as it collapsed-it's really hard to imagine steel beams and concrete columns bending and warping and breaking in such extreme ways," Holmes says. Photo via AP Photo/Mark Schiefelbein

BU Today:So do you think it's possible a structural engineer could have anticipated this kind of catastrophe and thought about the bridge design differently?

We don't always know exactly what loads a structure might experience-an example I have my students watch is this video of the 11'8″ bridge-surely no structural engineer could have anticipated this bridge being struck so many times by so many vehicles. And so we incorporate what is known as a "factor of safety" into our design. We estimate the largest loads we can imagine the structure experiencing-even if these are rare occurrences-and design the structure to withstand these extreme loads. But, incorporating factors of safety and redundancy still can't protect against extreme outliers.

BU Today:So, what did your students think watching the Francis Scott Key Bridge collapse?

My students commented on how the bridge looked almost like a toy as it collapsed-it's really hard to imagine steel beams and concrete columns bending and warping and breaking in such extreme ways. These are structural elements that in our everyday experience almost seem perfectly rigid, but of course they are not.

BU Today:Can you elaborate a little more on that conversation with students? What questions did they have? Did anything surprise you or surprise them watching the video?

The students watched in dismay-the video of the bridge collapse is striking. It almost seemed surreal. One of the students remarked on how the truss collapsed like toppling candlesticks, or popsicle sticks. I think the suddenness of the complete collapse was unexpected. The only question asked was the question everyone is asking: how do you prevent this from happening? We talked about load path, and redundancy in structural design, but again, avoiding disasters like this won't happen by building better bridges, but by building protective barriers and fenders that don't let ships that big get that close.

BU Today:From a structural perspective, do you think an accident like this can lead to changes in how things are built? Or have those changes already happened in more recent bridges and this one collapsed because it was nearly 50 years old?

Structural engineers usually learn something new from each structural failure, and adjust their future designs accordingly. It is unlikely that this catastrophe could have been avoided if this was a brand-new bridge-that pier was one of the main support columns for the entire bridge, and there's simply no way to safely redistribute those loads to a different portion of the structure.

The way to prevent a catastrophe like this is by protecting those piers to ensure that a ship cannot hit them-this can be done by adding structures around the pier that redirect vessels away from it. This bridge had protective objects in front of the piers that may have prevented this tragedy had the ship been headed straight for them. However, this ship approached the column at an angle, and those protective objects were inadequate. Changes to prevent disasters like this in the future will likely be focused on protecting the most critical structural elements of the bridge-the piers-rather than strengthening the bridge or improving the bridge design.

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