Lift Off: Bay Bridge East Span Clears Last Major Egineering Challenge

The innovative suspension system for the new East Span of the San Francisco-Oakland Bay Bridge has now been fully activated, with the 35,200-ton weight of the twin road decks supported entirely by span’s tower and cable system as of mid-November, the project team announced at a press event today. The bridge is now officially the world’s largest self-anchored suspension (SAS) bridge.

Workers this month completed the complex, three-month process of transferring the bridge’s decks from the temporary underlying supports that had held them up since 2010 to the 525-foot-high tower, the nearly mile-long main cable and the 200 steel suspender ropes. Completion of the load transfer process from the falsework to the self-anchored suspension system marks the last major engineering milestone for the estimated $6.4 billion East Span replacement project, which is slated to open to vehicle traffic in September 2013.

The defining feature of a self-anchored suspension bridge is a main cable that is anchored into the road decks. This contrasts with traditional suspension bridges where the cables are anchored into the ground. “The main cable acts like a giant sling,” explained Bay Bridge project spokesman Bart Ney. “On a traditional suspension bridge the cables act like a hammock.”

“The Bay Bridge has a lot of unusual design features,” noted Steve Heminger, who appeared at the press event in his roles as executive director of MTC and its Bay Area Toll Authority offshoot as well as chair of the three-member state Toll Bridge Program Oversight Committee, which is directing the East Span project. “One of these is the absence of earth anchorages. On the new East Span, the anchorages are in the deck system — at the east end of the decks, to be exact, in what are known as deck sections 13 and 14.”

The load transfer operation began in mid-August 2012 with crews using hydraulic jacks (which exert up to 400 tons of force) to gradually tension the 200 suspender ropes that connect the main cable to the decks. Once 104 of the 200 ropes were tensioned, the bridge was essentially self-anchored and self-supporting. As the suspender ropes were tensioned, they pulled the main cable toward the deck, causing it to move down about 16 feet and out about 30 feet. This caused the decks to lift approximately 1.6 feet off their temporary supports.

Among the workers’ challenges was to balance the various forces that were at play throughout the load transfer operation. While tensioning the suspender ropes, crews engaged a jacking saddle at the western end of the bridge to maintain the superstructure’s equilibrium while simultaneously releasing the tower from its 1.5-foot westward pull, allowing it to regain its vertical stance after being loaded with weight from the cable and the 2,047-foot-long roadway decks. Since the SAS cable is anchored into the eastern end of the roadways, the cable would have naturally pulled the tower to the east, so crews pulled the tower west toward Yerba Buena Island using steel strands that anchored into the island’s bedrock to hold the tower’s vertical position.

Heminger continued, “Completion of the load transfer led to a big collective sigh of relief from all the engineers on the project. There was never any question about the cable system picking up the weight of the bridge. But there was uncertainty over whether it would get picked up right the first time or if the engineers would have to go back and forth with the tensioning to fine-tune the system. It turned out that they picked it up right the first time. This is a time to say, ‘Congratulations’ to the contractor, American Bridge/Fluor for the excellent work they have done.”

Crews have begun the next step of wrapping the cable, which involves slathering the cable with a zinc paste then encasing it with interlocking galvanized steel wires, or S-wire. Then the cable and suspender ropes will be painted with a highly elastic noxide paint to further protect the bridge from the elements. Once the cable is painted, crews will no longer need access to the cable and the catwalk that is suspended just beneath the cable will be removed.

For more information, visit: BayBridgeInfo.org/projects/sas