With a ship that large, bridge was bound to collapse, engineers say
Protective shields, if any, could not save structure
To bridge experts, the collapse of Baltimore’s Francis Scott Key Bridge after being hit by a heavy cargo ship was as unsurprising as it was devastating.
When a vessel as heavy as the Singapore-flagged Dali strikes with such force one of the span’s supercolumns, or piers, the result is the type of catastrophic chain reaction that took place early Tuesday.
“If the pier was destroyed like it was, the bridge has to collapse,” said Dan Frangopol, a bridge engineering and risk professor at Lehigh University and president of the International Association for Bridge Maintenance and Safety.
“If the column is destroyed, basically the structure will fall down,” he said. “It’s not possible to redistribute the loads. It was not designed for these things.”
Maryland officials did not immediately answer questions about what protective devices may have been place in the water near the bridge and whether they were sufficient for any strike of such an important span.
Ian Firth, a British structural engineer and bridge designer, reviewed video footage and said there appeared to be at least two protective objects in the water next to the Key Bridge. The objects, known as “dolphins,” are supposed to protect maritime structures from being hit by vessels. But Dali, the container ship that struck the bridge, appeared to have come in “at an angle,” Firth said, which means the devices were unable to prevent the ship from striking the bridge.
Firth said in a telephone interview he was not surprised at how quickly the bridge came down. He noted the support structure that was struck, which would have been made of reinforced concrete, was one of two main supports responsible for doing “all the work” to hold up the bridge.
He said the ship appeared to have strayed to one side before striking the bridge, which appeared to have a lightweight support structure.
If the Dali had been traveling straight on instead of at an angle, Firth said, it probably would have hit the protective objects. And if there had been three or four vessel-protection objects around the bridge, the outcome might have been different, Firth said, adding that he expects lessons will be learned from Tuesday’s tragedy.
Firth noted that the bridge was erected, in 1977, at a time when ships were not as big as they are now and the flow of traffic was not as busy. These days, structures are designed with better protective measures in place, he said, though he noted that even a brand-new bridge would have “come down in the same way” if it were hit by such a large vessel traveling at speed.
Firth called the incident tragic and “very rare indeed.” He said the large container ship would not have had to be traveling very fast to have had such an impact, one that the bridge was simply not engineered to withstand.
In video imagery, the ship can be seen losing electrical power, then briefly regaining it before going completely dark. The ship then veers to the right, directly toward the bridge’s structural support.
The rudder may have gotten stuck in a position that caused the ship to turn, said a senior retired maritime official, who spoke on background while waiting for more details on the incident. It’s also possible that an incoming tide could have been a factor, he said.
“Obviously, they could not control the ship. They could not stop the ship,” he said.