Yet the collapse in Baltimore on Tuesday might have been avoided, some of the engineers said, if the piers had been better able to block, deflect or withstand such a collision. And some of the engineers questioned whether the bridge’s piers had adequate blocking devices that are known with a self-explanatory name: fenders.
In bridge engineering, fenders can be anything from simple pyramids of rocks piled around the pylons to major concrete rings padded with slats of wood, designed to shield the bridge’s supports from damage by water or collisions.
It was not clear whether any such protection built around the bridge’s piers was sufficient to guard against even a glancing hit from a 95,000-gross-ton container vessel.
And the U.S. secretary of transportation, Pete Buttigieg, expressed doubt on Tuesday that any bridge could have withstood such a serious collision.
“This is a unique circumstance. I do not know of a bridge that has been constructed to withstand a direct impact from a vessel of this size,” he told reporters.
Yet a different perspective emerged in initial comments by the investigators who will be sorting out what happened in the collapse.
Jennifer Homendy, the chair of the National Transportation Safety Board, said protective structures would be a part of the investigation into the collapse. “There’s some questions about the structure of the bridge — protective structure around the bridge or around the piers to make sure there isn’t a collapse,” she said, responding to a reporter’s question.
“We are aware of what a structure should have. Part of our investigation will be how was this bridge constructed? It will look at the structure itself. Should there be any sort of safety improvements? All of that will be part of our investigation.”
The Maryland Transportation Authority did not immediately respond to a request for comment on the design of the piers in Baltimore, and did not say whether any fenders were installed to protect them.
Between 1960 and 2015, there were 35 major bridge collapses worldwide because of ship or barge collisions, resulting in the deaths of 342 people, according to a 2018 report from the World Association for Waterborne Transport Infrastructure, a scientific and technical organization.
The deadliest crash took place in 1983, when a passenger ship collided with a railroad bridge on the Volga River in Russia, killing 176 people, according to the report.
It was only after “a marked increase in the frequency and severity of vessel collisions with bridges” that attempts to study and address the risks were initiated in the 1980s, said the report’s authors, Michael Knott and Mikele Winters.
A widely circulated video of the Key Bridge failure drew attention to the disastrous collapse of the upper bridge structure. But engineers who reviewed the footage said that did not appear to be the culprit in the disaster. Instead, they said, the superstructure failure was most likely a secondary effect of the pier crumbling beneath it after the collision.
Engineers who reviewed images of the bridge both before and after the collapse said no significant fender structures were visible. Only fairly small structures were visible in photos taken at the foot of the pier, and they did not appear to be substantial enough to be able to stop a large ship, some of them said. They said the structures may have served another purpose entirely — like preventing water from scouring and undermining the pier’s foundation.
Benjamin W. Schafer, a professor of engineering at Johns Hopkins University, said, after looking at images of the bridge taken before the disaster, “If you zoom further out, you can see these large cylinders that sort of define the shipping channel. They are to direct the ships and they are part of the bridge structure. Some would say those are protective structures. But I haven’t seen any evidence of fenders myself.”
In some bridges, engineers may elect, instead of fendering, “the alternative of making the pier exceptionally strong,” said Shankar Nair, a structural engineer with over half a century of experience who is a member of the National Academy of Engineering. But the visual evidence so far, he and others said, suggested that the pier was simply not strong enough to survive the collision.
The structure’s apparent vulnerability left some engineers dumbfounded.
“This is a huge shock,” Dr. Nair said. “A bridge of that size and importance should not collapse when hit by an errant vessel.”
The importance of sturdy fenders on bridge piers was backed up by a similar accident that occurred in 2013 when a 752-foot-long tanker collided with a support of the San Francisco-Oakland Bay Bridge. According to a National Transportation Safety Board report on the incident, the support stood — although $1.4 million in damage was done to the fendering system, which cushioned the impact.
In other cases when collisions lead to full or partial collapses, shortcomings in the fendering system are usually involved, said Matthys Levy, a longtime structural engineer and co-author of “Why Buildings Fall Down.”
“It’s usually an issue of fendering,” Mr. Levy said. “The fendering is not strong enough.”
According to a description of the Key Bridge by an American Society of Civil Engineers manual, the 8,636-foot-long structure in Baltimore was opened to traffic in 1977. The steel span above it, a design known as a truss, can be vulnerable to failure itself — damage to individual elements of the truss can theoretically cascade into a wider collapse. But that did not appear to be the case in Baltimore, engineers who reviewed the footage said: The truss, they said, was simply unable to remain intact when the pier was taken out beneath it.
Tuesday’s collapse raises the question “of how vulnerable are the piers and what is done or should have been done to protect them in the event of something like this,” said Donald O. Dusenberry, a consulting engineer who has investigated many bridge failures.
Mr. Dusenberry, in pointing to the issue of fender protection, said that it was impossible to make a full determination of what was installed without reviewing structural drawings of the bridge.
But images taken before the disaster, he said, suggested that small barriers that could be seen rising around the bridge’s piers, roughly at water level, would be unlikely to be able to stop a large ship. Effective fenders, he said, had to be far enough from the pier to keep the bow of a large ship from striking the pier, and large enough to absorb the energy of a collision. Assuming nothing had changed since the prior pictures were taken, he said, the visible structures did not seem up to that task.
“Maybe it would stop a ferry or something like that,” he said. “Not a massive, oceangoing cargo ship.”
One of the catastrophes prompting scrutiny of the issue of bridge collisions was the collapse of the Sunshine Skyway Bridge in Tampa, Fla., in 1980.
The structure collapsed when a cargo ship hit a pier, bringing down part of the main span and killing 35 people. Seven years later, a shrimp boat hit a bumper erected on the bridge built to replace it.
While catastrophic collisions garner the most attention, vessel collision accidents with bridges are not uncommon and regularly cause damage that, according to the 2018 report, “varies from minor to significant but does not necessarily result in collapse of the structure or loss of life.”
Mr. Schafer, the professor of engineering at Johns Hopkins, said fenders were undeniably important to preventing catastrophic collisions but that the size of the vessel that hits a bridge plays a critical role.
“When people think about fenders, they’re thinking about something that is similar in scale, in size, to the supporting concrete structure itself,” Mr. Schafer said. “So, you know, if that is 30-feet across, you might think of a fender which is like 30 feet as well. Right?”
The problem, he said, comes with trying to design protection against something so large as a container ship. “Could we design something that’s big enough to divert a runaway cargo ship? Yes. Would it be of a scale that’s practical? Probably not.”
Rather than build bigger fenders, Mr. Schafer said, the key is to divert ships before they get dangerously close to the piers and fenders. “That would be the physical answer,” he said. “The better answer is to have the people and the processes in place, so it never happens.”
