Specialist heavy lifting contractor Bigge Crane & Rigging, of San Leandro, California will use computer-controlled Hydrospex strand jacks to precision lift the 2,000USt (1,800t) steel span off a barge to a height of approximately 200ft within tolerances of 1/32in.

Although strand jacks are not new technology, they have never been used on this scale before in bridge construction in the US.

At the Oakland side (to the east), where the Skyway bridge deck has already been constructed, the strand jacks will be mounted on a mobile jacking platform that will be supported by a pair of 60ft girders, cantilevered 28ft off the bridge deck.

At the San Francisco side (to the west), main contracting consortium Kiewit-FCI-Manson has constructed two steel lifting towers, founded on piles driven into the bay for this purpose.

Bigge will furnish two girders that span 125ft across the top of this lifting tower and strand jacks will be mounted on top of a mobile jacking platform that sits on the girders. Both of Bigge’s jacking platforms are capable of synchronised eight-inch longitudinal and transverse movement.

At the Oakland end, Bigge is using four strand jacks, each of which has a lifting capacity of 365USt. Each jack pulls on a bundle of 31 strands. Each rope is 0.62 inches (15.7mm) in diameter.

At the San Francisco end, six jacks of 235USt lifting capacity will be used. These jacks have 19 wire rope strands, each 0.62 inches in diameter.

“No traditional crane, particularly a barge-mounted one on the water, could do this job with the required combination of lifting capacity and precision,” said Pete Ashton, Bigge vice president major projects.

This is the first major project on which Bigge has used its new strand jack equipment since becoming the exclusive representative of Hydrospex, the Dutch manufacturer that also supplied strand jack technology to raise the sunken Russian Kursk submarine in 2002.

The first transition span, for the eastbound carriageway, is scheduled to be lifted on or around 16th January. The second span, parallel to the first, to carry the westbound carriageway, is scheduled to be lifted into place in June.

Each lift will take approximately 10 hours, as the strand jacks raise the load at speeds of up to 30ft per hour. During the lifts the tubs, as these transition spans are called, will be maintained with an eighth of an inch of the required attitude. Given that the tubs are 200ft long by 85ft wide, this can only be achieved by computerised monitoring.

On each lift, once the tub is lifted to elevation, a temporary support tower will be skidded under the tub for support until the concrete joint with the existing Skyway is poured and cured, filling a gap of six feet.

The San Francisco (west) end of the tub will be supported by another temporary tower until the suspension bridge is constructed. When each of these towers is skidded into position, Bigge will lower the tub onto the towers and adjust the tub for the exact cross fall and longitudinal elevation.

After the tub is secured to the temporary support towers, the San Francisco end lift system will be dismantled. The Oakland end will be left in position for three months while the concrete closure pour cures.

Bigge’s input, however, starts long before the lift.

It is also contracted to load the tubs onto a barge at Portland, where they have been fabricated, and ship them to the site. The first one will be loaded onto the barge on 29th December using 48 axle lines of Scheuerle self-propelled hydraulic modular trailer (SPMT).

The barge will then be towed out of the mouth of the Columbia River to the Pacific Ocean where it will follow along the coastline to the San Francisco bay, travelling under the Golden Gate Bridge to the site wharf in Oakland.

However, because of bridge piers, the erection towers and other obstacles, there is not room for the barge to bring the tub in the correct orientation. Instead it has to position with the tub at 90 degrees to the bridge. Therefore Bigge will have to use its SPMT again to rotate the 200ft-long tub on top of the barge by 90 degrees.

“Transporting and lifting such a super-heavy load to such a height is a significant operation,” said Bigge president Weston Settlemier. “We are unaware,” he continued, “of such a heavy load ever being lifted so high anywhere in the US.”

Settlemier concludes: “However, we have engineered a solution, using our own equipment, which is the very latest in computer-controlled lifting technology that makes this job possible.”