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Bay bridges safe, but only to a point

In the aftermath of Hurricane Ivan, the twin spans of Interstate 10 across Escambia Bay near Pensacola resembled a mouthful of bad teeth, all gaps and snaggles.

Would that have happened here if early forecasts had come true, and Hurricanes Charley or Ivan had chewed their way up Tampa Bay as a Category 3 or 4 storm, ripping through the Sunshine Skyway, the Gandy and Howard Frankland bridges and the Courtney Campbell Parkway?

While experts can't answer the question with absolute certainty, Dwayne Kile, design engineer for this region of the Florida Department of Transportation, painted a vivid bottom line.

"If we ever had one of those surges coming up here," Kile said, "there'd better be nobody on those bridges."

For the most part, Florida's bridges are like Timex watches. They're built to take a licking. Summer thundershowers and the occasional tropical storm can hammer them for years with no serious ramifications. But when bridges and roaring walls of water try to occupy the same space at the same time, as happened in Pensacola, the bridges will lose.

"It's not so much about storms as about physics," said John Bolles, vice president of Meisner Marine Construction Co. in Tampa, one of the largest builders of bridges and other marine structures in the country.

"Water is not compressible," he said. "It will push everything out of its way. Bridges are built to take the lateral stresses of water pressing against them. But in this (I-10) case, a wall of water got up under the bridge and lifted it.

"We can build a bridge that can withstand a Category 5 hurricane, but the taxpayers couldn't afford it."

Although most bridges built by FDOT are about 12 feet above mean high tide, a wall of water 20 to 40 feet high, which is what experts say tore the I-10 bridges apart, inevitably will do some damage.

But experts say there are factors of geography that might help the fortunes of Tampa Bay's bridges.

The relatively narrow opening of Tampa Bay means the water would "tend to jam up at the mouth of the bay, and that slows it down," Kile said.

In addition, it's a long trip up the bay. The shipping channel, from the mouth of the bay to the Port of Tampa, is 58.4 miles, and distance tends to counteract surge. Moreover, each bridge would act as a sort of breakwater for the next as the water moved north.

"The Skyway was built for especially heavy lateral loads because it was built to withstand barge impacts," Kile said. "The Skyway is also higher than most, so a lot of surge can pass under it."

One of the consequences of the damage to I-10 was the havoc created for commercial truck traffic. The interstate is an economic lifeline in the Panhandle. But the interstates in the Tampa Bay area are better protected.

"I-275 is on the east side of Pinellas County, so it doesn't have the exposure to the Gulf that I-10 has," Kile said. "And before a big surge took out the Howard Frankland, it would have to get past the Gandy."

One force all bridges have to contend with is the "scour factor," the impact of fast-moving water on the soils around bridge pilings.

"Part of what happened at Pensacola was all that water moving so fast eroded bridge supports, which allowed the surge and the waves to tip the spans just enough for the lift to move them or take them down," said Manjriker Gunaratne, a professor of civil engineering at the University of South Florida.

Bridges are designed with the scour factor in mind and are inspected regularly to make sure the soils around the supports are acting as expected, Kile said.

"When you're planning a bridge, you always ask how deep my foundation has to go, so when I lose material, the bridge will still stand up," he said. "You don't design for where the bottom is now."

The design never takes events such as Ivan into account because it would be too expensive.

"There is a cost-benefit point you can't really go beyond," Gunaratne said. "Let's say a bridge column needs to go down 50 feet below ground for adequate support. Your computer models tell you that over time, you might lose 5 feet of soil to erosion. So you drive the pilings down 55 feet to compensate.

"And then Ivan comes, and scours out 10 feet all at once, and your bridge doesn't have even the minimal support it needs. But you can't design for that event and go down an extra 10 or 20 feet in the beginning because nobody would have the budget for it."

The most vulnerable part of a bridge to such water action is the middle, because water moves fastest down the middle of its channel.

"The middle piers should be the strongest," Gunaratne said. "But aesthetically, it doesn't look as nice to have some piers of different diameters than others. So we design for the middle ground, and that leaves the ones in the middle of the channel the most vulnerable. Going for uniform isn't the best way to design for catastrophe."

According to Bolles, of Meisner Marine, one alternative is to build bridges low to the water, so surges will wash over them instead of getting under and lifting. But that has other drawbacks.

"You want to keep the main members of the bridge as far away from the saltwater as you can because steel and salt don't mix very well," Kile said.

The bottom line for motorists is that when a storm approaches, bridges are the last places you want to be. And that goes for the causeway portions _ the roadways built on elevated spits of land _ as well as the over-water sections. All the bridges that cross Tampa Bay have causeway components, Kile said, and they are lower in elevation than the bridges themselves.

Usually the seawalls are at 5 feet and the roadways at 8 feet above mean high tide, Kile said. During the No-Name Storm of 1993, the tides were at 5.4 feet, and they were topped by waves 2 to 3 feet above that, he said.

"That's why we closed the bridges."

The "scour factor"

Bridge designers know water action will wash away some soil from around pilings over the life of the span and compensate for it by driving the support columns deeper than they really need to be. If a piling needs to go into the soil 50 feet for adequate support, the engineers may drive it down 55 feet to provide leeway. But a catastrophic storm, such as Hurricane Ivan, can scour out excessive amounts of soil, and leave pilings vulnerable to the deadly twin pressures of waves and storm surge, which can push them over or lift whole sections of roadway and move them.

Source: ESRI, GDT

SANTA ROSA Pensacola Santa Rosa Island 87 5 miles ESCAMBIA 399 Damaged bridge Area shown Escambia Bay 30 Pensacola Bay East Bay Backwater Bay 98 10 90 The " scour factor" Bridge designers know water action will wash away some soil from around pilings over the life of the span and compensate for it by driving the support columns deeper than they really need to be. If a piling needs to go into the soil 50 feet for adequate support, the engineers may drive it down 55 feet to provide leeway. But a catastrophic storm, such as Hurricane Ivan, can scour out excessive amounts of soil, and leave pilings vulnerable to the deadly twin pressures of waves and storm surge, which can push them over or lift whole sections of roadway and move them. Roadway Piling Water Soil Normal conditions Normal soil level Wave motion Catastrophic water conditions sweep away excessive amounts of soil from piling Potential hurricane aftermath Times graphic STEVE MADDEN Source: ESRI, GDT SANTA ROSA Pensacola Santa Rosa Island 87 5 miles ESCAMBIA 399 Area shown Escambia Bay 30 Pensacola Bay East Bay Backwater Bay 98 10 90 Damaged bridge 75 19 Alt. 275 92 175 41 92 Petersburg Ruskin Apollo Beach Tampa 60 45 Tampa Bay Hillsborough Bay 10 miles 19 Alt. St. Petersbu Pinellas Park HILLSBOROUGH PINELLAS Courtney Campbell Parkway Howard Frankland Bridge Gandy Bridge Sunshine Skyway

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