Video reconstruction of MV Dali and wreckage of the Francis Scott Key Bridge made with Gaussian splatting by Mike Coronna. Used with permission.
Roopinder Tara contributed to this article.
On March 26, we awoke to news reports that could have been something out of a movie. In the early hours of the day (00:39 local time in Baltimore), the container ship MV Dali set sail from the Seagirt Marine terminal on a 27-day journey that was to take her halfway around the world to Colombo, Sri Lanka. Her course would have taken her around Africa’s Cape of Good Hope to avoid the missiles being launched against ships in the Red Sea by the Houthi rebels. But another danger lay straight ahead. At 1:24 a.m., the ship suffered a catastrophic power failure. The pilot of the ship was unable to regain control and the MV Dali crashed into the Francis Scott Key Bridge, which carries I-695, part of the Baltimore beltway.
Tragically, the heavily loaded Dali, all 200,000 tons of her, made a direct strike to the pier supporting the main span over the Patapsco River, resulting in an almost immediate collapse of the bridge. An alert bridge authority officer picked up the ship’s Mayday call and quickly dispatched units that blocked vehicles from the bridge’s entrance. The last vehicle on the bridge cleared the doomed spans just 39 seconds before they fell. But on the bridge were eight construction workers doing repairs on the deck. Two were rescued. The bodies of three others have been recovered and the rest are still missing at the time of this writing.
And then there is the money.
“Insured losses could total between $2 billion and $4 billion,” said Marcos Alvarez, managing director for global insurance ratings at Morningstar DBRS quoted in a Reuter’s report, an amount that would eclipse the payout for the Costa Concordia, the cruise ship that ran aground in 2012.
A finite force
As the wreckage of the bridge lays across the channel, bringing the Port of Baltimore to a grinding halt, the nation’s attention is once again brought into sharp focus on another catastrophic infrastructure failure, just 2 years removed from the collapse of the Fern Hollow Bridge in Pittsburgh, Pa. Whereas, the concern about most bridges is their failure from age and neglect, failure from accidents like ship and vehicle strikes, such as with the collapse of the I-95 overpass in Philadelphia, must also be considered.
There are still more questions than answers, especially from non-engineers who are left wondering what, if anything, could have been done to keep the Francis Scott Key Bridge safe.
Mind the air gap
Although engineers can design a pier large enough to resist the force of a large ship, practically speaking, the better option is designing the pier to carry the loads of the bridge and place protection around it.
The first line of defense in bridge protection is simple avoidance: get the bridge decks out of harm’s way and let vessels pass by. The Golden Gate Bridge, for example, averages 220 feet above the water at midspan. Bridges like the Francis Scott Key, which span major shipping channels, are designed so that vessels, even tall ships, can pass underneath via the primary shipping channel.
Pilots of tall ships sailing under bridges air aided the National Ocean Service Administration (NOAA) Air Gap System, which gives readings of the vertical clearance (the distance between the water surface and reference points under the bridge) and a vessel’s air draft (the distance between the water surface and the highest point of the ship. Readings are given every 6 minutes to account for changes in tides, loads and currents.
Any sort of work on a bridge that reduces the vertical clearance, such as a bridge inspector dangling under a bridge [Ed: the author is a bridge inspector], raises alarms with ship pilots, mariners and bridge authorities—all of whom are mindful of the necessary clearance for their vessels and safety of their bridges.
Stay in your lane
Under normal conditions, ships do not deviate from the channel because it has been dredged specifically to the correct depth to prevent ships from running aground. The main span of the Key Bridge was 1,200 feet long—the third-longest continuous-span steel truss in the world.
The next line of defense are structures that are built specifically to protect bridge piers that are adjacent to the primary shipping channel. The most common are large concrete pylons referred to as dolphins.
Ideally, islands seen from above water are elongated, oval or racetrack shaped, maximizing water flow and vessel traffic while still giving the bridge towers strength in the flow direction.
Mind the dolphin gap
Dolphins should be sized to withstand the biggest ship that they might encounter. Judging from the photographs, the four dolphins around the towers of the Francis Scott Key Bridge appear to be around 28 feet in diameter, so small relative to the ship that in many accounts reporters did not see them at all and seemed to blame the absence of dolphins for the bridge failure. Those who looked at the scene more closely and noticed the dolphins dismissed them as puny, a mere pinhead against a massive ship. Missing from reporting altogether was the spacing of the dolphins, which left a gap more than wide enough for MV Dali to sail through untouched.
MV Dali had veered off course after a power failure. Last-minute corrections after power was restored increased the speed of the vessel and changed the point of impact, quite possibly making the situation worse.
Perhaps with an abundance of caution, 32 more dolphins were clustered around other towers. Conspicuous in their absence are dolphins guarding against ships deviating from the shipping lane and hooking into the tower through the gap.
Dolphins were credited with saving a bridge from a runaway barge on the Mississippi River in La Crescent, Minn., leaving only superficial damage to the bridge.
“These things are concrete and filled with rock,” said Mark Luther, director of the University of South Florida Center for Maritime and Port Studies, in an interview on WUSF, an NPR station. “They stick way down into the bottom, and they rise about 15 feet above the waterline.”
The Philadelphia story
As you read this, the Delaware Memorial Bridge between Delaware and New Jersey is undergoing a $93 million bridge collision protection program, which includes the installation of eight 80-foot-diameter dolphins in what may be the smartest and—in light of the Francis Scott Key Bridge disaster—the most prescient, though not perfect, dolphin configuration yet. The bridge’s previous protection system was fenders similar to those installed at the base of the Francis Scott Key Bridge.
“Installation of these cylinders at the tower structure/pier structures in the Delaware River are designed to enhance protection of the peers in the event a ship loses power and/or steering and could accidentally hit the bridge,” said Jim Salmon, public information officer for the Delaware River and Bay Authority.
The 80-foot-diameter dolphins around the towers are designed to protect the bridge against a ship roughly the same size as MV Dali carrying half the maximum load of containers (MV Dali may have been carrying nearly a full load) and going about the same speed. However, a ship the size of MV Dali at 31 degrees from the shipping channel could still slip past the dolphins and strike the tower, as shown above.
Understanding the bridge
In the aftermath of the Francis Scott Key Bridge collapse, much was made over the bridge’s designation as a “fracture critical” bridge. In layperson’s terms, a steel bridge is considered fracture critical if it contains nonredundant members in tension whose failure would result in a high probability of collapse. The term fracture critical is somewhat of an antiquated term in bridge engineering parlance due to its nebulous interpretation. But it has been revived, being used repeatedly by the popular media to explain the collapse of the Francis Scott Key Bridge. Without a structural analysis, we can’t be sure its design had anything to do with its collapse. Attempts to paint this collapse as a reminder of the failing health of American infrastructure are premature at best.
A bridge with suboptimal substructure protection in a shipping channel would sustain heavy damage and likely collapse whether or not it was fracture critical. The bridge did not collapse because of its truss design. It collapsed because of the lack of pier protection.
The nature of crossing a 1,200-foot-wide shipping channel necessitates a design heavy on Nonredundant Steel Tension Members. Although trusses have largely fallen out of favor due to their complexity and construction challenges, their replacement, cable-stayed bridges, are also in the same fracture-critical category. The challenges of spans over 1,000 feet push materials to their limits. The simple continuous beam lines that you get with 100-foot spans are not possible with 1,200-foot spans. Could the Francis Scott Key Bridge have been divided into 12 consecutive 100-foot simple spans, each with its own bearing line? The economics of bridge building, the width of shipping lanes and the size of container ships would make such a bridge impractical.
What comes next?
The bridge engineering community and transportation officials have been quick to respond to past tragedies and failures of American bridges. The collapse of the Mianus River Bridge in Connecticut in 1983 led to an increased focus on the inspection of fracture-critical bridges after major deficiencies in the state’s inspection practices were uncovered. The collapse of the I-35 bridge in Minneapolis resulted in higher scrutiny of gusset plate connections on truss bridges. As previously mentioned, after the Sunshine Skyway Bridge was struck by a ship, there was increased commitment across the country to install protection around bridge piers. However, with no strong mandate handed down at the federal level, existing bridges were grandfathered in.
In all fairness, it’s difficult to evaluate the amount of protection required for a particular bridge given the unpredictability of a 100,000-ton ship’s movement during a mechanical failure. The measures in place at the Key Bridge may have slowed the ship enough to avoid a collision had it not veered off at an angle, but that’s not how the scenario played out.
The challenges faced by infrastructure policymakers are immeasurable, even more so given their reliance on federal funding to complete projects. The previously mentioned Delaware Memorial Bridge is a privately owned and managed bridge that collects toll dollars to fund its maintenance operations. That agency can make the collective decision to spend $93 million on a protection system for its piers because its bridge generates revenue and it has limited assets to maintain. The Francis Scott Key Bridge collected only $3 for each passenger vehicle with an E-ZPass (compare that to the Golden Gate Bridge, which gets $8.75 with a FasTrak charge) and the tiny state of Maryland, which is crossed with waterways, has thousands of bridges to maintain.
It is a difficult decision to make concerning infrastructure funding. Do you spend the $93 million on a set of dolphins for a major bridge, or do you replace 20 crumbling smaller bridges? Of course, in hindsight, it looks like a much smarter investment would have been to spend nearly $100 million on pier protection, but this MV Dali was a true black swan event. Had MV Dali lost power at any other time, minutes before or after when it did, none of this may have happened.
The rate at which ships are increasing in size is the classic hockey stick curve. MV Dali, christened in 2015, is five times larger than the largest container ship that was sailing when the Francis Scott Key Bridge opened in 1977. These days, MV Dali doesn’t crack the top 20 biggest ships, the biggest of which, the MSC Irina, has a capacity of 24,346 TEU (twenty-ft equivalent units), 2.5 times the capacity of MV Dali. The average size of container ships will only continue to grow. Planning and designing protection measures is difficult, but not impossible—and is definitely a necessary task.
How well will dolphins, such as those being installed at the Delaware Memorial Bridge, withstand the impact? We can’t know in advance. Unlike guardrail and traffic barriers, which are tested against impact with all sorts of vehicles and several angles, testing of fully loaded ships is just not done. No one is offering their ships to go full kamikaze or even run aground on rocks, and no bridge authority or government agency has offered crash test dolphins.
Resiliency, a term popularly applied in conversations about climate change and the need to ensure infrastructure can withstand rising sea levels and more intense weather patterns, needs to be applied to structures built long ago but facing modern threats, both in size and number and both accidental and deliberate.
The Francis Scott Key Bridge will be rebuilt in due time. The federal government has already pledged to that. What comes next with respect to implementing standards for in-channel protection measures? Finding funding for projects at bridges without minimal levels of pier protection will be difficult, but public outcry after seeing one of the worst bridge collapses in U.S. history will be high. What is the proper level of protection given the continued upward trajectory of ship tonnage? And who pays for it?
A modest proposal—similar to how tolls for large vehicles are higher on roads and how canals (Panama and Suez) operate—is to charge for passage. With large ships paying higher fees to enter U.S. ports and crossing below major bridges, the revenue could be used for bridge protection infrastructure. This would relieve U.S. taxpayers as well as insurance companies while putting the onus on big ships, most of which are flying flags of other countries to avoid taxes and regulation.
We have to do something. Otherwise, our old bridges with little or no protection are only accidents waiting to happen.