Engineers and bridge designers were raising questions Tuesday about structural safety in the wake of a catastrophic collapse overnight of Baltimore’s Francis Scott Key Bridge. A cargo container struck one of the bridge’s support columns, plunging most of the bridge and members of a road crew into the Patapsco River.
Many similar bridges are protected by barriers designed to prevent or reduce the impact when a vessel collides with a pier, “especially those crossing busy shipping channels where large vessels like this one come and go frequently,” Ian Firth, a British structural engineer and bridge designer, said in an email to The Baltimore Sun.
Such installations can take a number of forms, Firth said, including cable systems, pontoons, custom-destined caissons, and submerged islands. But among the most widely used are “dolphins,” circular sheet pile cells filled with material such as sand or concrete that essentially serve as bumpers.
Engineers deployed the structures in the aftermath of a similar tragedy more than 40 years ago. The Sunshine Skyway, a two-span bridge built in 1954 across Lower Tampa Bay collapsed on May 9, 1980, when a freighter called the Summit Venture collided with a support beam. The impact sent a 1,200-foot section of the road, seven cars, and a bus into the water. The collapse killed 35 people.
When engineers built its replacement, they included dolphins mounted atop a set of artificial islands in a system that’s visible to the naked eye. The new bridge was completed in 1987.
No such artificial islands were visible Tuesday near the piers of the Key Bridge, but some protection was apparently in place on a parallel system of transmission towers. ShibataFenderTeam, an international fender design and manufacturing firm, announced in a news release in December that it had recently built five transmission towers, including two with “dolphins in the water.”
The company covered the foundations of the towers and the two dolphins with 174 tons of customized sliding plates “to protect them from potential water-traffic collisions,” according to the release.
“This sort of protection is what FSK did not have, and we can see now that it may — may — have helped” had it been in place, Benjamin Schafer, a professor of civil and systems engineering at Johns Hopkins University, said in an email to The Baltimore Sun.
SFT officials at the company’s offices in Lansdowne, Virginia, did not immediately respond Tuesday to requests for comment.
Though Firth stressed that he was not familiar with all the particulars of the Key Bridge, he said it did seem to him that protection of the Key Bridge’s pillars could have been stronger.
“If properly designed, located and constructed to withstand the forces generated by such large vessels,” an effective system “should provide adequate protection to prevent such vessels reaching the critical bridge supports,” he wrote. “But the narrowness of the channel in this case might have made it quite difficult. The dolphins would need to be big and may restrict the channel too much.”
Installing a system capable of stopping vessels of this size would “depend on a number of things, such as the local topography, the water depth, the underwater ground conditions, environmental considerations and so on,” added the London-based Firth, who has worked on projects worldwide. “And it would not be cheap.”
Engineers say dolphins come in a range of shapes, sizes and levels of protectiveness, at times determined by cost.
A Baltimore Sun article that appeared the day after the Tampa disaster in 1980 quoted the director of engineers for what was then called the state Toll Facilities Administration as saying the Key Bridge had a type of “concrete dolphins” at the time.
The story cites the official, Mike Snyder, saying they were intended to deflect ships from the piers, and even if they failed to deflect a vessel entirely, they might absorb enough of a ship’s force that a collision “would be a glancing blow by the time [the ship] hit the pier.”
But, he noted, the bridge could not withstand a direct impact by a large freighter and that, as of 1980, installing such a system would not be economically feasible.
“Whatever unit got struck, that section would be knocked down,” he said.
Three months later, in August 1980, a freighter that lost power chipped a concrete piling on the bridge, according to a Sun article at the time. The piling was partially protected by a “concrete and wood collar,” but the accident did damage estimated at $500,000.
