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Bigge Crane & Rigging, San Leandro, Calif., made lifting history in February by using computers to perform the synchronized lifting and placement of a 2,000-ton steel span at San Francisco's Oakland Bay Bridge. |
February 22, 2006 — Computer-controlled strand jacks were used to link the concrete skyway bridge deck section of the San Francisco Oakland Bay Bridge with the suspension bridge this month. Alternative lifting technology never before used in U.S. bridge construction is attributed to the project's success.
Bigge Crane & Rigging, based in San Leandro, Calif., used computer-controlled Hydrospex strand jacks to precision-lift the 2,000-ton steel span off a barge to a height of about 200 feet, according to the company. At the Oakland side's skyway bridge deck, the strand jacks were mounted on a mobile jacking platform supported by two 60-foot girders cantilevered 28 feet off the bridge deck.
“Transporting and lifting such a super-heavy load to such a height is a significant operation,” said Weston Settlemier, president of Bigge Crane & Rigging. “We are unaware of such a heavy load ever being lifted so high anywhere in the United States.”
Two steel lifting towers, founded on piles driven into the bay, sit on the San Francisco side. Bigge furnished two girders that span 125 feet across the top of this lifting tower. Strand jacks were mounted on top of a mobile jacking platform that sits on the girders. Both of Bigge's jacking platforms are capable of synchronized 8-inch longitudinal and transverse movement.
At the Oakland end, Bigge used four strand jacks, each of which has a lifting capacity of 365 tons. All of the jacks were synchronized for simultaneous operation and load control within 1/32 inch. “No traditional crane, particularly a barge-mounted one on the water, could do this job with the required combination of capacity and precision,” said Pete Ashton, Bigge's vice president of major projects.
The first transition span, for the eastbound lanes, was lifted in early February. The second span, parallel to the first to carry the westbound lanes, is scheduled to be lifted into place in June. Each lift takes about 10 hours, as the strand jacks raise the load at 30 feet per hour. During the lifts, the tubs, as these transition spans are called, are maintained with 1/8 inch of the required attitude via computerized monitoring.
Once the tub is lifted to elevation, a temporary support tower is skid under the tub for support until the concrete joint with the existing skyway is poured and cured, filling a gap of some 6 feet. The San Francisco (west) end of the tub is supported with another tower until the suspension bridge is constructed. When each of these towers is skidded into position, Bigge lowers the tub onto the towers and adjusts it for exact crossfall and longitudinal elevation.
Once the tub was secured to the temporary support towers, the San Francisco end lift system was dismantled. The Oakland end will be left in position for three months while the concrete closure pour cures.
