Erecting New San Francisco Bay BridgeMay 2012
By Peter Crisci
Californians like to do big things, and the new San Francisco-Oakland Bay Bridge currently under construction is said to be the largest public works project in the state’s history.
SELF-ANCHORED SUSPENSION SPAN
The “signature” portion of the new bridge, being constructed by a joint venture of American Bridge and Fluor (ABF), is a self-anchored suspension span 1,263 feet in length. Unlike a traditional suspension span in which the main cables are anchored at their ends and under tension, a self-anchored suspension span utilizes a single continuous main cable which forms a loop that is anchored in (passes through) the deck at each end of the span and passes up and over the support tower. As a result, the span is in compression.
Because of the nature of a self-anchored suspension span, the deck must be completed first, and then the primary cable is placed and anchored to the deck. This requires falsework to support the deck until the primary cable and suspenders have been installed. In this case, some 18,000 tons of steel were required to construct the necessary falsework.
PLACING DECK SECTIONS
The bridge deck is being fabricated as “orthotropic box girder” (OBG) sections. The sections are barged as close as possible to their intended locations and then lifted onto the falsework truss by a mammoth barge-mounted crane with a 2,000-ton lift capacity. The deck sections are not placed directly onto the truss; rather, they are placed on special cradles. “Each cradle serves as a large skid to support the OBG while it is being pushed,” explains Dan Hester of ABF.
A deck section resting on its cradle is pushed along the falsework truss by means of a “pushing frame” designed and fabricated by ABF. The pushing frame incorporates 10-foot-stroke hydraulic jacks, which are used to inch the cradle along the truss in a clamp-and-release fashion until the deck section reaches its ultimate location. According to Nick Greco of ABF, “About a day is required to push a section into place, and it’s about a 1-week process to completely place and install a deck section.”
The time required to lift and push the massive deck sections (typically well in excess of 1,000 tons each) can vary depending upon how close to their intended location they can be barged. “We only have to push some a few meters because we are able to set them very near their final location. Because of their size, it is easier to set the OBGs away from the splice at least a few feet, and then use the hydraulic jacks on the pushing frame, as well as the Enerpac jacks, to adjust the piece to the correct orientation, slope, etc., to make the connection. The initial connection takes about 6 hours from when we make the first bolt until we have enough of the connection made that we can leave it for the night. It takes a couple weeks total to complete all the welding and bolting,” says Hester.
STRAND JACKS HOIST CROSS-BEAMS
The new span will actually have twin, parallel decks, each providing five traffic lanes. Stability and stiffness are achieved in part by periodic cross-beams.
Because of tidal restrictions near Yerba Buena Island, the huge crane cannot be used to hoist the two westernmost cross-beams into place between the parallel spans. Greco explains that ABF designed and fabricated a gantry structure to lift certain cross-beams that could not be erected by the crane into place. The gantry straddles the two span-trusses and supports a transverse-moving carriage to provide X-Y positioning.
For the lifting mechanism on the gantry, ABF selected a novel strand jack system from Enerpac Integrated Solutions.
THEN AND NOW
American Bridge is not a newcomer to the San Francisco-Oakland Bay Bridge—the same company built the original (presently still in service) bridge in 1936. However, a portion of the original bridge was seriously damaged in a 1989 earthquake. After lengthy studies and deliberations, the state decided that reworking and seismically upgrading the old bridge would be more expensive in the long run than constructing a new bridge.
The new suspension span incorporates numerous earthquake-resistant features, some quite ingenious. For example, “hinge pipe beams” are inserted between sections. During an earthquake, the beams can function like a replaceable fuse, absorbing seismic energy. The beams are designed to be readily replaced if damaged in an earthquake.
Visit http://baybridgeinfo.org/construction-cams for up-to-date construction news. Cameras from EarthCam provide live feeds of the work on the bridge. ■
About The Author:
Peter Crisci is the Enerpac Integrated Solutions Business Leader for the Americas. For more information, visit www.enerpac.com.
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