One image broadcast viewers are sure to see when the Tampa Bay area hosts major televised events, like a Super Bowl, is of the Sunshine Skyway Bridge spanning the water between Manatee and Pinellas counties.
Officially called the Bob Graham Sunshine Skyway Bridge (after the former Florida Governor and U.S. Senator), its 4.14-mile, four-lane roadbed rises to 193 feet above Tampa Bay, providing expansive views and for some drivers, a white-knuckle experience while approaching the bridge’s highest point, which can look like it stops abruptly at cloud level during the ascent. But one thing travelers and the people who maintain the bridge won’t have to worry about is damage or bridge failure caused by wind-induced vibrations of the 84 steel cables holding up the bridge.
That’s because researchers at the University of South Florida College of Engineering have taken on the task of testing samples of the 504 heavy-duty shock absorbers that are part of a system designed to counter vibration of the cables which, under certain conditions, could lead to harmonic forces strong enough to damage the steel and concrete bridge.
Civil and Environmental Engineering Professor Gray Mullins, PhD, says the shock absorbers used inside the Skyway are not much different than the ones used by some of the vehicles traveling on it.
“They’re something that would be on a semi or concrete mixing truck,” Mullins says. “They’re about as heavy duty as you’ll find in any application.”
And whether a shock absorber is smoothing out the ride for an 18-wheeler or keeping stay-cable vibrations in check, the same engineering principle applies, according to Mullins.
“The faster the movement a shock absorber experiences the more restriction it applies, and they call that damping,” he says. “If the vibrations get crazy, these will calm it down.”
With the four groups of cables fanning out from its two towers, the 30-year-old Skyway is what is known as a harp-design cable-stayed bridge. Mullins says the musical instrument analogy is appropriate in terms of safety as well as design.
“When the wind blows on these cables, they’re like long guitar strings - very, very thick and heavy guitar strings. When the wind hits them, and if the wind speed is just right, they can start to vibrate,” says Mullins.
According to the Federal Highway Administration, other factors such as wind direction and rainfall intensity also affect the cables, which are connected to the shock absorbers by struts extending beneath the road into the bridge’s hollow interior.
The testing takes place in a structural engineering lab at the college, using one of the computer-controlled high- force axial/torsional test systems capable of applying 550,000 pounds of force.
“Our charge was to put these shocks into our machine and apply a representative velocity of stroke and then measure the resulting force,” Mullins says. Each shock absorber is tested at four progressively higher stroke velocities.
One of the first things he and a team of graduate students had to do was find a way to increase the capabilities of their equipment in order to perform the testing. That included building and installing a displacement multiplier, essentially two 4” I-beams, to increase by five times the amount of stroke velocity that could be applied to the shock absorbers. Mullins expresses appreciation to the college’s on-site machine shop for its help in the exacting process of producing the hardware needed to upgrade the test system.
“We have probably the best machine shop in a 50-mile radius right next to us, our machine shop is phenomenal,” he says.
In the first round of tests, the Florida Department of Transportation (FDOT) provided 15, or three percent of the Skyway’s shock absorbers as well as testing parameters. Test results were provided to FDOT for review. Mullins says he expects testing of the shock absorbers to continue as part of the bridge’s overall preventive maintenance program.
“Periodic testing is important,” he says. “Just like the inspections for cracks, rust and other things, making sure these shocks function correctly is our primary objective.”
Mullins adds that conducting the tests is a good opportunity for the College to help out.
“This is a landmark structure; Tampa Bay is known because of the Skyway Bridge, so for us it’s an honor to provide this service to the community.”
Constructing the Bob Graham Sunshine Skyway bridge cost $244 million and was opened to traffic on April 20, 1987. It replaces the original bridge that was damaged when the MV Summit Venture collided with one of its piers on May 9, 1980, killing 35 people.
Story by Brad Stager, College of Engineering
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