INSIDE THE SUNSHINE SKYWAY
Before breakfast, I padded down to the beach andscanned Tampa Bay. In the morning haze I glimpsed two majestic sailboats cresting a huge wave more than a mile offshore. I waited for the boats to dip back into the water, but they didn't move. Behind them, I suddenly noticed what looked like twin railroad bridges, one of them missing a large section. It was then that I realized how different the new Sunshine Skyway is from its predecessor--aesthetically as well as tructurally.
The old Tampa Bay crossing, crippled by a phosphatefreighter in a storm seven years ago, is a steel-truss bridge. Florida officials wanted to repair its damaged southbound span, opened in 1971, but the Federal Highway Administration pronounced the bridge obsolete. The state's Department of Transportation considered replacing it with another steel bridge or tunnel, but a pioneering design for a concrete bridge won out in the bidding.
The high-level portion of the new bridge--conceived byFigg and Muller Engineers, Inc., of Tallahassee, Fla.--has a 1,200-foot main span, the longest cable-stayed concrete span in the Western Hemisphere. It's also one of the first American bridges built with cables attached to the center of the deck rather than the edges. This design costs less than dual towers, and the cables don't obstruct a spectacular view of the bay. But holding together the bridge's hollow concrete segments--some of them the heaviest ever precast--requires an intricate web of cables below the bridge's deck as well as above it. As the bridge shifts and settles, scientists will study the distribution of stresses with electronic equipment embedded in the concrete. To find out how the pieces of this giant puzzle are assembled and monitored, I walked the backbone of the new Sunshine Skyway--and even descended into its belly.
Rob Kay, safety director for the project's general contractor,Paschen Contractors of Chicago, Ill., took me to the main span. At the northern pier, workers hanging in baskets were spraying cable stays with the bright yellow paint that has earned the bridge its title as "Florida's Golden Gate." I walked to the southern pier and followed Kay down a ladder into a dimly lit concrete tunnel. There, I had a closer look at the bridge's anatomy. The anchor heads that grip the cable stays poked through the ceiling, and large black tubes containing tensioning cables streched along the floor. "You can't see most of the tendons," Kay explained. "They're embedded in the segments." I tried to picture the enormous cables straining in every direction. "Sometimes it seems like the bridge is alive," Kay confided.
The steel cables--bundles of individual wires woven intoseven-wire strands--support precast concrete segments that weigh up to 220 tons. Frenchman Jean Muller designed the suspension system for the bridge's high-level portion. He patterned it after the Brotonne Bridge, which he designed to span France's Seine River. Europeans have been building hollow-tube concrete bridges since World War II, when steel was scarce. Now that concrete has become an economical alternative in the United States, builders are turning to European cable-stayed designs.
The cable-stayed span is jsut one section of the $250million Sunshine Skyway. The longest part of the 4.1-mile crossing is composed of low-level twin-trestle approaches. To build the higher portions of the bridge, three different construction methods were used (see drawings).
The competing forces of concrete and cables at work inthe structure are what make it stable in heavy winds over the bay. The new bridge has a number of safety advantages over its predecessor, which is slated to become the world's longest fishing pier. The new skyway's two main piers are farther apart and its main span is higher than the older bridge's, so ships aren't as likely to crash into it. Thick concrete walls along the deck will keep vehicles from plunging over the edge but won't block the view.
Around the piers, 36 special bumpers protect the bridgefrom wayward ships. Called "dolphins" because of the animal's reputation for aiding helmsmen, they can stop a 747 jet at a landing-approach speed of 120 mph. The dolphins are made from steel sheet piling filled with crushed stones and capped with concrete. A timber fence surrounds each barrier. If trouble developes, the bridge has television cameras and electronic message boards to warn drivers.
Bridge with a brain
The skyway has another monitoring system that mayalert officials to problems: Hundreds of strain gauges and temperature sensors installed in the bridge's roadway segments, piers, and pylons are wired to a computer embedded in one of the main piers, making the bridge a giant laboratory, says Antonio Garcia, skyway probject manager for the Florida Department of Transportation.
Outside Garcia's office is an array of concrete cylinders. "Thoseare made from the same concrete as the bridge," Garcia says. Wires connected to the blocks measure their "shrinkage" (how much the concrete contracts as water evaporates from it) and "creep" (how much the concrete contracts when it is compressed by springs). Data from the cylinders will be compared with data from the bridge to find out how laboratory measurements compare with actual behavior. Data will be collected for five years. "By then, the bridge will have settled down," Garcia says.
The information should help designers confirm the assumptionsthey have relied on for years. Figg and Muller are already using the experience they gained designing the skyway to create new bridges. For a span now under construction in Virgnia, they invented a "delta truss" design, in whichcable stays anchor in a triangular segment that supports the load of two box girders. Together, the three segments form the same shape as one of the large segments used in the Tampa Bay project, but the smaller pieces are easier to cast. The designers say that the combined technology of cable-stayed suspension and precast segmental construction that they introduced in the skyway project can now be used for shorter spans.
Gale Document Number:A4968694