by The freighter collision that destroyed the Francis Scott Key Bridge in Baltimore on March 26, 2024 is raising questions about how much engineers can do to prevent such disasters 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 retrofitting existing ones , so they can survive extreme events.
How difficult is it to design a bridge to withstand the force that brought down the Francis Scott Key Bridge?
Once engineers understand the forces a structure will be subjected to,,, they can design a structure to withstand them. That said, we know that each force has a range of magnitudes that can occur. For example, not all trucks on the roads weigh the same, not all earthquakes are the same, and not all ships weigh the same. We incorporate this diversity of forces into the design.
Even if built according to a certain set of plans, the ultimate strength of the structure can vary. There are variations in the strength of the materials used. For example, concrete delivered on two consecutive days will have a visually different ultimate strength. This variation 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 we could build two bridges from the same set of plans and have them end up with the same strength.
Based on the weight and speed of the ship that struck the Francis Scott Key Bridge, the US bridge design code today called for the bridge to be designed to withstand a lateral force of 11,500 tons. This means that the bridge is able to withstand a lateral blow of that magnitude. That equates to the weight of about 50 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 withstand lateral loads of this magnitude resulting from wind or earthquakes. However, it is a question of how much one wants to spend on the structure, and many design goals and constraints must be balanced against each other.
What do engineers do to ensure safety in extreme events?
Our knowledge of how extreme events affect structures is constantly evolving. One area where this is very clear is earthquake engineering. After each earthquake, structural engineers learn what worked and what didn’t, and then the building and bridge design codes evolve. Infrastructure owners also try to retrofit existing structures that were designed to earlier codes.
Ship collisions and their impact on bridges is a similar area where understanding is evolving and design codes are improving. There have been over 35 major bridge collapses worldwide caused by ship collisions from 1960 to 2015. Engineers assess the failures, and update the engineering codes so they can better account for the effects of collisions ship
How has the design of the bridge evolved since the construction of the Baltimore bridge?
The Francis Scott Key Bridge was designed in the early 1970s. Construction began in 1972, and it opened to traffic in 1977. This was before the Sunshine Skyway in Florida collapsed in 1980, caused by a ship collision, similar to what happened in Baltimore. The collapse of that bridge led to the initiation of research projects that led to the development of the US guidance specification in 1991 which was updated in 2009.
Based on that guideline specification, bridge design codes were modified to include forces due to ship collisions. Francis Scott’s Key Bridge design would not need to consider the effect of ship collisions. The current US bridge design code states:
“Where collision of vessels is expected, structures should be as follows:
• Designed to combat vessel collision forces and/or
• Sufficient protection by fenders, dolphins, berms, islands, or other devices that can be sacrificed.”
Another change since the 1970s is that cargo ships have increased in size and weight. The ship that brought down the Sunshine Skyway in 1980 weighed 35,000 tons, and the ship that collided with the Francis Scott Key Bridge weighed 95,000 tons.
With the increasing weight of cargo ships, perhaps the most cost-effective design strategy to prevent bridge collapse due to vessel collisions is to protect the bridge piers from impact. This is done by building a bridge collision protection system, which is often a concrete or rock structure around the pier and prevents the ship from approaching the pier, as is done to protect many of our national monuments.
A pier protection system was installed when the Sunshine Skyway bridge was rebuilt, and has been used on many other bridges. The same approach is currently being implemented by the Delaware River and Bay Authority at a cost of US$93 million to protect the piers of the Delaware Memorial Bridge.
But what about existing bridges such as the Francis Scott Key Bridge? Bridge owners face a huge challenge in finding the financial resources needed to retrofit their bridges to meet the latest design codes and to account for the increased impact loads expected due to the heavier ships and heavier. Both things happened here. That is, design codes have changed and improved, and loads have become much larger. Infrastructure engineers and owners strive to prioritize where their limited funds can be used to increase structural safety and minimize the chance of structural failure.
What can universities do?
Job No. 1 of structural engineers is to protect the public and minimize the risk of life-threatening structural failures. To do that, engineers need to be able to calculate the forces that our structures may be subjected to. This includes situations where a large ship accidentally collides with a bridge, or when a major earthquake or hurricane strikes.
In these extreme cases, the structure will almost certainly sustain 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 structure is improving all the time.
Retrofitting structures built to previous codes is an ongoing process, and this disaster is an ongoing process leading up to it. There is a lot of infrastructure in the United States that was designed according to old codes, and we have bigger trucks crossing our bridges, and bigger ships going under them.
Engineers cannot reduce the probability of failure to zero, but they can reduce it to the point where failures occur very frequently and until many unexpected circumstances come together to bring down a structure.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by: Michael J. Chajes, University of Delaware.
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Michael J. Chajes does not work for any company or organization that would benefit from this article, does not consult with, shares in a company or organization that would benefit from this article, and has not disclosed any relevant affiliations beyond their academic appointment.
