Some accidents are simple: two cars collide on a freeway, a tree falls on a jogger, lightning hits a golfer. Others require a chain of events, each of which is unlikely, and so are much rarer than the simple kind. The sequence of occurrences, each one fairly harmless by itself, which led to the collapse of the Francis Scott Key Bridge in Baltimore Harbor on last Tuesday, March 26 included things that by themselves would cause few if any major problems. But on that fateful night, they all aligned to end four lives and cause what will eventually turn out to be billions of dollars in direct and indirect damage.
About forty-five minutes after midnight, the Singapore-registered container ship Dali left its berth at the Port of Baltimore bound for Sri Lanka. Its route lay beneath the second-longest continuous-truss bridge in the U. S., the Key bridge, which carried traffic around the southern part of the Interstate 635 loop around Baltimore, and was the last bridge to pass under before the Dali reached the open sea. On the bridge, a construction crew was repairing potholes resulting from the previous winter.
Some time after leaving port, the Dali began to experience engine trouble. Security-camera videos recovered afterwards show that the ship's lights flickered on and off several times and black smoke began to come from the stacks. As a recent NBC News report shows, contaminated bunker fuel is a serious problem in marine shipping circles. As the giant propulsion engines are separate from the smaller diesel engines that run a ship's generators, and both were affected, some experts have speculated that contaminated fuel may have caused the ship's power problems, as that is a common factor that would account for both difficulties.
Whatever the cause, the ship's crew was alert enough to issue a mayday call by radio, which was picked up by first-responder officials, and they issued orders for traffic to be blocked on the bridge.
Less than two minutes later, the now-adrift ship plowed into one of the two main "bents" supporting the highest span of the bridge. A cloud of masonry dust can be seen on the video of the collision, shortly before the support fails and the entire truss structure breaks up and falls into the ocean. Two construction workers on the bridge were rescued, but six others remain missing and were presumably killed. While our prayers go out to those who lost loved ones and friends, the accident could have caused many more fatalities if it had happened during an afternoon rush hour with hundreds of vehicles on the bridge.
Several relatively unlikely things had to happen together for this tragedy to take place.
As it turns out, engine problems and failures due to contaminated fuel are not that uncommon. Propulsion engines of the type used on the Dali can have up to fourteen cylinders, each over a yard (~1 meter) in diameter and with a stroke of eight feet (2. 5 meters), and the amount of fuel used is phenomenal. So shippers are motivated to find the cheapest fuel around that will run the engines, and so-called bunker fuel is used. It is thicker and heavier than the type of diesel fuel used in passenger cars, and because suppliers sometimes cut corners and mix adulterants in the fuel to cheapen it, it's not uncommon for ships to lose power from bad fuel.
We still don't know the exact cause of the power loss, but the vast majority of such events occur in the open sea where a drifting ship represents lost time and money, but is not otherwise dangerous. The Dali had the bad fortune to lose its power in the worst possible place: a few hundred yards away from a vulnerable bridge.
Why wasn't the bridge structure protected from such collisions? It doesn't take a genius to guess that sooner or later, a ship might lose control and bang into a support. Not being a maritime bridge architect, I can't answer that question. As the bridge was built in 1977, it withstood nearly half a century of wear and tear from weather, traffic use, and shipping. It's an open question whether the builders tried to estimate what it would take to fend off a straying ship colliding with the supports. As there are so many variables—the size and weight of the ship, its velocity, exactly where it would hit—the designers may have just tried to make sure smaller ships couldn't damage it and hoped that the pilots of the larger ships would be equipped with radar and other navigational aids to miss the bridge supports.
And until last Tuesday, every ship capable of knocking down the bridge succeeded in missing them. But the unlikely event of a ship losing power and having just the right combination of weight, ocean currents, and momentum to wreck the bridge finally came up, and the result is as we see.
Again, we can be thankful that the main losses are monetary rather than in the form of injuries and many fatalities. This accident could have been much worse, and the ship's crew and first responders deserve credit for issuing maydays and blocking traffic as promptly as they did.
But for the foreseeable future, anyone wanting to drive around Baltimore is going to have to do it without the benefit of the Key Bridge, and until the wreckage is cleared, the Port of Baltimore is shut down. As disasters go, this one was spectacular, but way down the list of fatal accidents compared to other mechanical failures that killed many more people.
We can be sure that the next bridge will have more protection from ship collisions, whatever else is taken into consideration. And this accident may shine a light on the dubious quality of bunker fuel used in maritime service, an international market with poor quality-enforcement mechanisms, according to the NBC News report. These are good things, and while any accident is regrettable, the lessons we learn from them benefit the next generation of designers and users of the built environment.
Sources: I referred to an NBC News report on bunker fuel quality as a possible cause of the accident at https://www.nbcnews.com/news/us-news/dali-contaminated-fuel-scrutiny-baltimore-key-bridge-collapse-rcna145478, a CBS News timeline at https://www.cbsnews.com/news/francis-scott-key-bridge-collapse-timeline-911-call-dali-cargo-ship-mayday-maps-construction-worker-recovery/, and the Wikipedia articles on the Francis Scott Key bridge collapse and the maritime Wärtsilä-Sulzer RTA96-C engine, the largest reciprocating engine in the world.
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