It’s a race against time to get the $6-billion San Francisco-Oakland Bay Bridge built before the next Big One. From spiral reinforcement column cages to steel hinges, the bridge showcases the latest seismic-resistance techniques.
The 1989 Loma Prieta earthquake heavily damaged the eastern span of the old Bay bridge, a double-deck truss. A portion of the upper deck collapsed, killing one person and closing the bridge for a month (ENR 10/18/90 p. 36). Under performance-based criteria, a “lifeline” bridge is to be serviceable after a major quake.
The 1.5-mile-long segmental concrete box girder Skyway section comprises 60% of the new Bay bridge and sits in soft mud. Twenty fabricated steel hinge pipe beams, up to 60 ft long, 4 in. thick and 6 ft in diameter, are placed in pairs at expansion joints to allow for deck movement, says Dan McElhinney, Bay Area District 4 chief deputy director for the California Dept. of Transportation.
|Spinal cages (center left) and hinge pipe beams (center right) augment Skyway columns. (Images courtesy of Ty Lin International)|
The design used for the Skyway columns was first used on the Bay Area’s new Benicia-Martinez bridge. Skyway columns, supporting the 28-ft-wide soffits of the box girders, include reinforcement concentrated in circular cages at the corners, where seismic demand is heaviest.
Battered steel tube piles, about 300 ft long, are used on the Skyway instead of vertical piles to allow for smaller pile caps, about 49 ft x 62 ft, says Tim Ingham, project engineer with San Francisco-based T.Y. Lin International, which designed the bridge with Moffat & Nichol, Los Angeles. This reduces weight and bulk, increasing seismic performance.
The signature self-anchored suspension (SAS) section has a 1,263-ft-long main span. “We were lucky with the SAS because the main tower is in rock,” says Ingham. But the eastern end requires up to 8-ft-dia piles through unstable mud to as deep as 300 ft.
For the 524-ft-tall main tower, designers developed a special shape of four columns, connected horizontally with shear links. “As the columns are closer together and are placed between the two decks, we are able to put more horizontal members, or shear links, between the columns,” says T.Y. Lin Chairman Man-Chung Tang. This increases the capacity of the tower against seismic movement, he says.