The Francis Scott Key Bridge failure provides an opportunity for future engineers to learn how to better protect the public

The cargo ship collision that destroyed Baltimore’s Francis Scott Key Bridge on March 26, 2024, is raising questions about how much engineers can do to prevent such catastrophes in the future. Here, Michael J. Chajes, professor of civil and environmental engineering at the University of Delaware, discusses how bridge design codes have changed over the years and the challenges of building new structures and adapting existing structures so they can withstand extreme events to survive.

How difficult is it to design a bridge that can withstand the force that brought down the Francis Scott Key Bridge?

Once engineers understand the forces a structure is exposed to, they can design a structure that can withstand those forces. That said, we know that every force has a set of magnitudes that can occur. For example, not all trucks on the roads weigh the same, not all earthquakes are of the same magnitude and not all ships have the same weight. We incorporate this variation in forces into the design.

Even if built according to a certain set of plans, the ultimate strength of the structure may vary. The materials used have variations in strength. For example, concrete delivered on two consecutive days will have a significantly different final strength. This variability in the strength of the final structure is also taken into account in the design process to ensure that the bridge or building is safe. There is no way that we can build two bridges with the same set of plans and have them end up having exactly the same strength.

Based on the weight and speed of the ship that struck the Francis Scott Key Bridge, the current U.S. bridge design code would require the bridge to be designed to withstand a lateral force of 11,500 tons. This means that the bridge can withstand a side impact of that magnitude. That corresponds to the weight of about fifty loaded Boeing 777s or the weight of the Eiffel Tower. Although this is a very large lateral force, structures can be designed to resist such forces. Tall buildings are routinely designed to resist lateral loads of this magnitude resulting from wind or earthquakes. However, it is a matter of how much one wants to spend on the structure, and many design goals and constraints must be weighed against each other.

What do engineers do to ensure safety during extreme events?

Our knowledge of how extreme events affect structures is constantly evolving. One area where this is very apparent is earthquake engineering. After each earthquake, structural engineers learn what worked and what didn’t work, and then the design codes for buildings and bridges evolve. Infrastructure owners are also trying to modify existing structures designed according to previous codes.

Ship collisions and their impact on bridges represent a similar area of ​​evolving understanding and improved design codes. More than 35 major bridges worldwide collapsed due to collisions with ships between 1960 and 2015. Engineers are evaluating the failures and updating technical codes to better account for the consequences of ship collisions.

How has bridge design evolved since the construction of the Baltimore Bridge?

The Francis Scott Key Bridge was designed in the early 1970s. Construction began in 1972 and was opened to traffic in 1977. This preceded the 1980 collapse of the Sunshine Skyway in Florida, which was caused by a ship collision, similar to what happened in Baltimore. The bridge collapse led to the initiation of research projects that culminated in the development of a US guide specification in 1991, which was updated in 2009.

Based on that guide specification, the bridge design codes were changed to include forces resulting from ship collisions. The design of the Francis Scott Key Bridge did not take into account the effects of ship collisions. The current US bridge design code says the following:

“Where collisions with ships are expected, structures should:

• Designed to resist collision forces and/or

• Adequately protected by fenders, posts, berms, islands or other sacrificial options.”

Other changes since the 1970s are that cargo ships have increased in size and weight. The ship that downed the Sunshine Skyway in 1980 weighed 35,000 tons, while the ship that collided with the Francis Scott Key Bridge weighed 95,000 tons.

With the increasing weight of cargo ships, the most cost-effective design strategy to prevent bridge collapse due to ship strikes may be to protect the bridge piers from the impact. This is done by building a bridge collision protection system, which is often a concrete or rock structure that surrounds the pier and prevents the ship from reaching the pier, as is done to protect many of our national monuments.

The rebuild of the Sunshine Skyway Bridge installed a pier protection system, which has been used on numerous other bridges. The same approach is currently being used by the Delaware River and Bay Authority, at a cost of $93 million, to protect the piers of the Delaware Memorial Bridge.

But what about existing bridges like the Francis Scott Key Bridge? Bridge owners face a huge challenge in finding the financial resources needed to adapt their bridges to meet the latest design standards and to take into account the expected increased impact loads due to increasingly heavier ships. Both things happened here. That is, the design codes have changed and improved, and the loads have become much larger. Engineers and infrastructure owners are doing their best to prioritize where their limited resources can be used to increase structural safety and minimize the chance of structural failure.

What can universities do?

The main task of structural engineers is to protect the public and minimize the risk of structural failures that pose a threat to human life. To do that, engineers must be able to calculate the forces to which our structures can be exposed. This includes cases where a large ship accidentally crashes into a bridge, or a major earthquake or hurricane strikes.

In these extreme cases the structure will almost certainly suffer damage, but if at all possible it should be resilient enough not to collapse. The design codes are continuously updated to take into account new knowledge, new materials and new design techniques. The reliability of our constructions continues to improve.

Retrofitting structures built to previous codes is an ongoing process, and this disaster brings it to the forefront. The US has a lot of infrastructure designed according to old codes, and we have bigger trucks crossing our bridges, and bigger ships passing under them.

Engineers can never reduce the probability of failure to zero, but they can reduce it to the point where failure occurs very rarely and only in cases where numerous unforeseen circumstances combine to make a structure vulnerable to collapse.

This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.

It was written by: Michael J. Chajes, University of Delaware.

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Michael J. Chajes does not work for, consult with, own stock in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond his academic appointment.

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