TAMPA, Fla. – For longtime Florida residents, the collapse of the Francis Scott Key Bridge in Maryland triggered memories of the collapse of Tampa’s Sunshine Skyway Bridge.
In 1980, a freighter ship collided with a support column during a severe storm while rush hour traffic was on the bridge. The bridge’s collapse killed 35 people.
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However, the new Sunshine Skyway Bridge, which opened in 1987, was built with safety in mind, with features meant to make navigating in and out of Tampa Bay easier for large ships and to lower the risk that the bridge will get hit or suffer catastrophic damage.
“These measures include elevating the bridge, widening the channel, and incorporating two layers of protection for bridge piers. Most notably, features such as ‘dolphins,’ which serve as physical barriers as well as rock islands that completely surround the main channel supports and go all the way to the sea floor,” said Kristen Carson, a spokesperson for the Florida Department of Transportation.
‘Gateway to Tampa Bay’
The 4.1-mile-long Bob Graham Sunshine Skyway Bridge, which opened in 1987, spans the mouth of Tampa Bay and connects St. Petersburg with Manatee County via Interstate 275. It’s a gateway to Port Tampa Bay, which is the largest cargo port in Florida.
The bridge’s design is iconic, with its bright yellow steel cables. It is the first cable-stayed bridge in the United States. Those cables help hold the main span of the bridge, which was built wider and higher to allow more ships to clear the bridge.
FDOT says the bridge has an air draft clearance of 175 to 180 feet in height, depending on the tide. The clearance space between the two main pillars is more than 1,000 feet. This gives harbor pilots more room when moving in and out of the bay.
Surrounding those center pillars are large rock islands, made of various-sized rocks, that go down to the sea floor. The main pillars themselves also have steel-reinforced concrete collars around the foundations that are 12 feet thick, FDOT says.
“What is obvious here was the protection system around the bridge pier,” said Dr. Hussam Mahmoud, a structural engineer with Colorado State University who specializes in load impacts on structures, including bridges. “The main pillars or columns are sitting on footings that are very wide, so that provides protection. So there’s a lot of (material) that if a collision happened, you’d have to crush all this concrete to get to the columns.”
Before a ship even gets to the pillars, it would have to pass the dolphins. These are large concrete barrier columns that rise about 17 feet above the water level. There are 36 of them around the six pillars closest to the center of the bridge.
FDOT says these dolphins can handle the impact of an 87,000-ton ship traveling at 10 knots (11.5 mph).
‘You calculate the risk’
But ships are bigger now.
In 2019, Port Tampa Bay welcomed the CMA CGM Dalila, which has a gross tonnage of 89,787 tonnes, making it the largest container ship to ever sail into the port. The shipping company uses the port as a weekly stop.
Mahmoud said whether the dolphins would stop a bigger ship depends on the ship’s weight and the speed a ship is coming in at impact, and the likelihood is the dolphins will sustain damage. However, in that situation, Mahmoud said the goal is not for the dolphins to survive impact.
Even if a dolphin takes a direct hit from a ship and is unable to stop it, FDOT says the rock island is there around the main pillars as an extra layer of protection.
“The amount of (material) they have, essentially, and how big the island is, would be such that if an 87,000-ton ship hits it, with crush, crush, crush it, and it will bring the ship to a complete stop before you get to the column,” Mahmoud said.
However, Mahmoud said that if a ship comes in sideways, it’s a different matter.
“What if a ship comes from an angle, right, so you’re more exposed if come between the two main spans and come in at an angle, you’re much more exposed,” Mahmoud said. “But the likelihood of a scenario like this is really low when you have done your job in protecting the towers. You also want to make sure that the navigation through the main span is smooth.”
The likelihood, that risk, is something engineers have to take into account, and Mahmoud says sometimes another factor stands in the way – the size of the project’s budget.
“The probability of a ship hitting the main bridge piers, and the probability of the bridge collapsing, the probability of having an unusual situation, how would we even calculate that?” Mahmoud said. “Ultimately, you calculate the risk, and you put the risk into different categories, low, medium and high.”
“There’s no such thing as zero probability, we know that. But the likelihood is incredibly small,” he continued.
The U.S. Coast Guard and Port Tampa Bay have a “Cooperative Vessel Traffic Service” to manage traffic in and out of the port, and harbor pilots that can guide ships.
“Florida maintains continuous coordination with various stakeholders, including local law enforcement, the U.S. Coast Guard, and seaport authorities, to ensure comprehensive safety measures are in place,” said Carson with FDOT.
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