What Went Wrong and How Do We Prevent It From Happening Again?
On March 26, 2024, at 1:28 am, a cargo (container) ship struck one of the Francis Scott Key Bridge’s main piers; with the three Northeast approach spans and the main span collapsing into the Patapsco River. The collapse ultimately resulted in six fatalities. How does this happen in 2024; can it happen again; and how may this be prevented from occurring again in the future? This is what many of us in the engineering community are now pondering.
The Francis Scott Key Bridge in Baltimore Maryland was a steel arch continuous latticed-truss bridge, the second longest in the United States and the third longest in the world; spanning 1.6 miles and opening in 1977. This bridge crosses one of the busiest shipping routes in the United States. The bridge’s continuous lattice truss relied on its overall in-tact structure to maintain integrity; in engineering terms, it was “fracture critical”. This means it had no redundancy against removal of support of any particular part of it, i.e. at a support or within the connections of the truss. The container ship collision destroyed the southwest main truss pier, causing the south and central approach spans to collapse leading to the collapse of a northern span. Each failure sequence took seconds, and within 30 seconds the entirety of the trussed spans, and three others, had fallen into the Patapsco River.
The bridge had “dolphin” and “fender” protections against ship impact, but these protection systems designed decades ago are insufficient for the mass of cargo ships operating today and were only designed to deflect tugboats in operation at the time of bridge design and opening. “Fenders” wrap around the support columns or piers to protect them from damage, much like the bumper of a car. “Dolphins” are independent structures typically made of concrete that are designed to stop ships from colliding with the support columns in the first place, or at least reduce the impact. The Key Bridge had four, (4), spaced “dolphins” placed well in front of, behind, and in line with the triangulated main piers that held up the main latticed steel arch structure, a pair at each approach side. These dolphins were inboard of the main portal bent bridge piers which were the main supports at each of the bearing ends of the trussed structure.
The bridge passed federal inspection in 2022, scoring a 6 out of 9 and ranking it in “fair condition”. The bridge was determined to be fully compliant with the current federal bridge codes.
While bridge piers are designed and capable of absorbing some impact and resulting bending stress, the weight (mass) of the 985 ft long vessel was no match for the 47-year-old bridge according to engineers. The vessel was as wide as it was tall, so although the container ship had turned off power, ultimately preventing greater loss of life, the kinetic energy associated with that mass of vessel was no match for the bridge pier. It is estimated that the Dali, the container ship that struck the pier, weighed between 300,000 and 400,000 metric tons fully laden.
Automatic Identification System (AIS) transceiver data aboard the vessel showed the ship traveling at a speed of 8.7 knots (10.0 mph or 16.1 km/h) at 1:25 am before departing the channel and slowing to 6.8 knots (7.8 mph or 12.6 km/h) at the time of the collision two minutes later, at 1:28:45 am. The average tugboat weighs approximately 200 tons, on average, and operates at an average speed of up to 15 knots depending upon hull efficiency. This is significantly less kinetic energy, (KE); KE= Mass x Velocity, than the average container ship.
According to the New York Times, the Francis Scott Key Bridge was particularly vulnerable. As long ago as 1980, engineers had warned that the bridge, because of its fracture critical design and lack of redundancy, would never be able to survive a direct hit from a container ship. An example of redundancy is wearing suspenders, in addition to wearing a belt, to keep your pants up.
The Francis Scott Key Bridge was one of many bridges nationwide that are at risk for various reasons. The NY Times reports that 309 bridges across the United States have been found to have severe foundation deterioration leaving them vulnerable to impacts, should they be hit in a similar manner.
In 2021, the Biden Administration pushed through legislation, the Bipartisan Infrastructure Law, providing $40 billion in repairs costs for bridges nationwide. Those bridges slated to receive any repair funding are only about 1/3 of the estimated 43,000 bridges nationwide reported to be in poor condition, or approximately 17,200 bridges.
While the Francis Scott Key Bridge may have been an anomaly in some respects, it can’t be assumed other bridge collapses aren’t a possibility in the future due to similar circumstances.
Some states are using this recent tragedy to reevaluate evacuation plans and update tugboat protocols vs. immediately addressing their bridge protection systems from a structural standpoint. The cost of analysis and construction of dolphin and fender protection systems for today’s container ships would be tremendous but may need to be considered in initial budgets considering the cost of the Francis Scott Key Bridge replacement which has been estimated to be at $400 million dollars and take over 5 years. This figure does not include related direct economic impacts on labor and supply chain.
Now, 2 1/2 weeks after the collapse, much has been learned and much remains unknown. What is certain is that the condition of bridges and their support systems in the United States are getting more attention. The solution is a multi-pronged approach addressing structural redundancy issues, inspection criteria for support protection systems, and emergency response protocols when a bridge is struck to prevent loss of life.
Ultimately, accidents will happen and all we can do is learn from each tragic event as we try to anticipate the future.
Written by Dana R. Pezzuti and Ernest G. Heinrichs PE; BE Structural PC
Sources:
CBS News – What we know about the condition of Baltimore’s Francis Scott Key Bridge and how this sort of collapse could happen
https://www.nytimes.com/interactive/2024/04/06/us/bridge-collapse-protections-baltimore.html
https://En.wikipedia/wiki/Francis_Scott_key_Bridge_Collapse
https://apnews.com/article/baltimore-key-bridge-collapse-rebuilding-a126a3b732af95e86d1fbef01e9fb487
https://www.thebaltimorebanner.com/key-bridge-dolphins-fender-protection-3LZVAL7HDVHBXAEF4G5LBFWZHE