San Francisco-Oakland Crossing Starts To Take Shape
Bart Ney gets concerned calls from people just about every day. They see the steel trusses starting to cross San Francisco Bay between Yerba Buena Island to the west and the new 1.2-mile-long precast concrete Skyway to the east, alongside the existing eastern steel truss span of the old Bay Bridge. They wonder why the trusses don’t look at all like the striking self anchored assymetrical suspension span they saw in the renderings.
That’s because it’s not really a bridge. Like much of the construction under way on the $6.3-billion San Francisco-Oakland Bay Bridge reconstruction program, it’s all temporary.
"This falsework is massive," says Ney, spokesman for the California Dept. of Transportation, the agency in charge of the Bay Bridge reconstruction program. "We have to build a truss bridge that will support the 28 steel-deck sections being fabricated in China now." Unlike a traditional suspension bridge, the truss bridge is needed "because you cannot suspend the individual decks for the cable system as they arrive," he adds.
Superstructure work on the deck and single tower for the 2,047-ft-long self-anchored suspension (SAS) span is still some months away, but this year’s planned preparatory and temporary construction are giant feats in their own right.
In addition to falsework for the SAS span, part of a $1.4-billion contract held by a joint venture of American Bridge Inc., Coroapolis, Pa., and Fluor Enterprises Inc., Aliso Viejo, Calif. Rancho Cordova, Calif.-based C.C. Myers Inc. will demolish and replace a 300-ft-long double-deck section of transition bridge over one weekend later this year. The 1.2-mile Skyway portion of the east span is just completed, and a $429-million seismic reconstruction of the west approach wrapped up this month.
The project has already featured a book’s worth of political intrigue and dramatic construction feats. From the rejection of a single $1.4-billion bid by a team including American Bridge and Fluor in 2004 to the rebidding in 2006, from allegations of faulty welds and an FBI investigation to more recent mundane weld-related delays, there’s not been many dull moments. Construction feats to date include a 1,700-ton, 150-ft megapick of a steel-tub girder in 11 hours, a 2,100-ton steel foundation box for the 525-ft-tall single tower and a Labor Day lift-out of a 6,500-ton section of roadway.
But the biggest feats are yet to come. American Bridge/Fluor is waiting for the arrival of a custom-built crane from China with a 328-ft-long boom and the capacity to lift 1,700 metric tons, says its project director, Michael Flowers. Even the barge had to be custom-built. "The barge is 400 feet long, 100 feet wide and 22 feet deep—about the size of a football field," Flowers says.
After the crane arrives this spring, it will be used to erect both the first of four lifts for the single tower and the steel-deck segments of the SAS span. The trapezoidal steel-box girders carrying the twin roadways will rest on the falsework bridge, awaiting the suspension cables.
"On steel bridges, typically one piece weighs between 50 to 150 metric tons," says Flowers. "On this bridge, many of the pieces will weigh over 1,000 metric tons." Overall, some 50,000 metric tons of structural steel will make up the SAS span. "The temporary bridge that we’re currently building to support [the segments] will weigh about 23,000 to 24,000 [metric] tons," adds Flowers.
Out of seven temporary foundations for the falsework bridge, three are in the water. The joint venture has built two of those and is preparing to start a third, says Flowers. The foundations include 275-ft-deep steel piles with diameters of 42 in. and 48 in. The last foundation will be done in late April or early May, he says.
As sections for the SAS permanent bridge begin arriving from Shanghai in the spring, crews will begin placing them on the falsework from west to east, following the falsework’s continuing erection. Knowing there would be a learning curve on orthotropic decks for the Chinese company fabricating the girders, "Our risk managers identified potential problems in construction and built in contingency funds and extra time," says Ney. This proved fortuitous when problems arose with tack weld cracks, he says.
"In the worst-case scenario, if the cracks propagated, we’d have a maintenance issue in 20 years," he says. After consulting with third-party peer-review groups and experts, "we decided to redo the welds." To make up the several months of lost time, the team is ramping up fabrication rates. The delay is not expected to push back the 2013 completion date.
Transitions
Caltrans’ team devised other ways to safeguard against potential delays. In its $300-million CM-at-risk contract, C.C. Myers will remove 300 ft of an existing steel-truss span near Yerba Buena Island, lift a new section 150 ft and place it on a skid system designed by Mammoet, a Netherlands firm specializing in heavy lifting. Then the section will be slid into place over three days.
That will complete a temporary 1,200-ft-long bypass that will assume the job of connecting the Bay Bridge to Yerba Buena Island’s road tunnel. All traffic will be shifted onto the temporary detour, supported by 200-ft-tall steel towers. Drivers will use this detour, just south of the original roadway, until traffic is moved onto the new east-span alignment three to four years from now.
Caltrans asked C.C. Myers to build 13 footings that will support the new permanent transition span. "That way they could be built off the critical path, and we could deal with unknown conditions," says Ney. The contract for the permanent transition span, estimated at $378 million, will go out to bid sometime this summer, he adds. For seismic safety, the double-deck H-shaped structure will feature separate foundations for each deck.
American Bridge/Fluor expects the first shipments of tower sections to arrive this fall. The 525-ft-tall tower is made up of quadrants connected horizontally with shear links. It sits on 13 piles almost 200 ft long and anchored into bedrock; each pile is about 8 ft in diameter and filled with concrete. Crews will use the custom-made crane to build the first 154-ft section of the main tower. Since each quadrant piece weighs 1,000 metric tons, "that first lift will be 4,000 metric tons," says Flowers. "After that, we have to build a temporary tower using strand-jack equipment to lift the other three sections." The crane doesn’t have the boom to reach the tower’s top, and "the pieces are far too heavy for any conventional tower crane to lift," he adds.
The joint venture of Kiewit/FCI/Manson completed foundation work for the SAS span 80 days early in January, earning a $5-million bonus and shaving $33.5 million off the $313-million budget. The foundation for the main tower includes 13 cast-in-drilled-hole piles with 10-ft diameters as deep as 200 ft, socketed through a layer of mud into bedrock.
The east pier by contrast sits in a deeper layer of bay mud, or "a bowl of jelly," says Marwan Nader, lead designer for local design joint venture T.Y. Lin International and Moffatt & Nichol. Cast-in-steel-shell piles go down almost 340 ft through mud to bedrock, supporting two concrete piers rising 120 ft above water. The main cable will anchor into the decks at this end. At the western end of the SAS span on Yerba Buena Island, two 66-ft- long, 66-ft-wide, 30-ft-deep piers in bedrock support 100-ft-tall columns.
With the bridge’s assymetry causing concerns about uplift, the 236-ft-wide, 34-ft-long, 18-ft-deep cap beam for that pier will be prestressed in all directions, says Nader. It’s at this end where, in lieu of typical anchorages, the single 2.6-ft-dia suspension cable will wrap around the deck in a continuous loop. But that comes in 2011 and beyond. "The next year will be exciting," says Nader.
