Weighty Challenge

Bay Bridge’s SAS will be the longest such structure in the world

By Greg Aragon

It may not appear so to passersby, but work on the new $1.4 billion self-anchored suspension span of the San Francisco-Oakland Bay Bridge is nearly half complete.

“This is a global project; it is not being erected in place,” says Michael D. Flowers, project director for the joint venture of American Bridge of Coroapolis, Pa. and Fluor Enterprises of Aliso Viejo, which is leading construction on the bridge. “Right now, all the elements are under way all over the world, with materials coming to us from the U.S., China, Japan, Korea and Great Britain.”

Flowers says that people currently see the construction of a temporary truss bridge with seven 15,000-ton towers in the middle of San Francisco Bay. When this false work is complete, it will support the new, 1-mi-long self-anchored suspension span, or SAS, until it is assembled and readied for load transfer onto three new foundations.  

“We are building a 25,000-ton temporary bridge to support a 50,000-ton permanent bridge,” Flowers says.

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Begun in 2006, the SAS project is one of a series of seismic safety projects that will completely reinforce the historic 8-mi-long Bay Bridge. The SAS, designed by T.Y. Lin of San Francisco and Moffatt & Nichol Engineers of Long Beach in a joint venture, is part of the 2.2-mi-long replacement of the East Span between Yerba Buena Island and Oakland.

The double-deck East Span will stay in operation until work on the SAS and the new 1.2-mi-long elevated Skyway viaduct project is finished. 

A seismic retrofit of the bridge’s West Span was completed in 2004.

When complete in late 2013, the SAS, with a main span length of 1,263 ft, will be the longest single-tower, self-anchored suspension bridge in the world. It will feature a 525-ft-tall steel tower, two parallel roadways with five lanes each and a 15-ft-wide cantilevered bicycle/pedestrian path on the eastbound side.

The new bridge is designed to echo the existing towers of the West Span, as well as the towers on the Golden Gate Bridge. Its vertical elements – the tower, piers and lights – have been designed to emphasize clean modern lines.

Adding to the structure’s distinctiveness is an asymmetrical design. The bridge will have a longer forward span (east of the tower), which will provide a more gradual transition from the gently sloping Skyway and will give the new East Span a unique silhouette.

By switching from the existing East Span’s double-decks to side-by-side roadways, the designers want drivers to enjoy unencumbered and expansive views of the Bay Area as they pass beneath an angled canopy of suspension cables.

The graceful and futuristic white-colored span will have three support foundations and utilize a unique self-anchored, single-cable suspension system, which is quite different from traditional main cable suspension bridges.

A typical suspension bridge has twin cables with smaller suspender cables connected to them that hold up the roadbed and are anchored to separate structures in the ground. With the SAS, there will appear to be two main cables, but there is actually only one.

The cable is 31in. in diameter and made up of a 137 smaller strands, which are compacted together and then splayed out like the bottom of a broom at each end. Each strand is individually anchored into a structural-steel rod within the eastern end of the roadway. The entire main cable is then carried over the tower, wrapped around the two side-by-side decks at the western end and then anchored a second time at the eastern end.

“The bridge does not have concrete anchors at either end, so as a result that big over-carry force that the cable would normally impart into the anchorage has to be resisted by the bridge deck itself,” Flowers says. “The bridge deck is a big box girder that takes compressive loads similar to a column in a building.”

The project’s foundations, which are located in an active seismic zone, are embedded deep into rock (from 30 ft on the western end to 340 ft at the eastern terminus) and are designed to withstand a massive earthquake.

“The single tower is designed to have shear links that interconnect the four shafts,” says Marwan Nader, Ph.D., PE, lead designer and project manager for the American Bridge/Fluor Enterprises JV. “These shear links will act as fuses in a major event to protect the shafts and dissipate energy.”

But before the bridge can withstand an earthquake, its massive segments must be assembled, which is no easy task, Flowers says.

“The pieces on this bridge are about 10 times the size and weight of pieces we would typically encounter when building a bridge,” he says. He adds that the biggest tower piece weighs 1,000 tons and the biggest roadway pieces weigh about 1,300 tons.

“And there isn’t any conventional construction equipment that is capable of raising that kind of load, so what we had to design a [special] crane,” Flowers says.

To meet the challenge, the team created the mammoth “Left Coast Lifter.” Costing about $50 million, the mechanical marvel consists of a 300-ft-long Shanghai-built crane sitting atop an Oregon-built barge, which measures 400 ft long, 100 ft wide and 22 ft deep. The crane has been safety-rated to lift weights up to 1,700 tons.