2015 OCEA Project Finalist – The San Francisco–Oakland Bay Bridge New East Span

March 3, 2015

The San Francisco–Oakland Bay Bridge New East Span is a finalist in the 2015 ASCE’s Outstanding Civil Engineering Achievement (OCEA) award. Established in 1960, the OCEA Award recognizes a project that makes a significant contribution to both the civil engineering profession and society as a whole. 

San Francisco–Oakland Bay Bridge New East Span. Photo Credit: Barrie Rokeach

San Francisco–Oakland Bay Bridge New East Span. Photo Credit: Barrie Rokeach

After the 1989 Loma Prieta earthquake caused the collapse of a section of the upper deck of the original East Span of the San Francisco–Oakland Bay Bridge, the California Department of Transportation (Caltrans) determined that replacing the seismically vulnerable bridge was the safest, most cost-effective option.

Considered the State of California’s largest public works project to date, the 2,047-foot-long, 258.33-foot-wide San Francisco–Oakland Bay Bridge New East Span (East Span) is the longest single-tower, Self-Anchored Suspension Span (SAS) in the world and the world’s widest bridge.

The East Span opened to traffic on September 2, 2013, and consists of four seamless and interrelated components: the single-tower SAS, with a main span length of 1,263 feet; the 1.2-mile-long Skyway that ascends from the Oakland shoreline to connect to the SAS; the Oakland Touchdown, a 4,229-foot-long low-rise, posttensioned concrete box girder that links the bridge to California’s Interstate 80; and the Yerba Buena Island Transition Structure, a 1,542-foot-long posttensioned concrete box girder, which connects the SAS to the island.

One of the most heavily used toll bridges in the U.S., it carries close to 300,000 drivers each day. However, the East Span is located in a high seismic zone between 2 major faults capable of producing large earthquakes. Therefore, the new $6.4 billion East Span project was engineered to meet stringent seismic criteria. This includes its designation as a critically important regional Lifeline Structure that must be able to open to emergency traffic shortly after the strongest ground motions that engineers can expect in a 1,500-year period. The East Span’s design life of 150 years was also set at 2 times the existing bridge standard.

ASCE News Associate Editor Doug Scott interviewed Marwan Nader, Ph.D., P.E., M.ASCE, lead design engineer of the SAS, and vice-president and technical director of T.Y. Lin International’s Bridge Line of Business; and Sajid Abbas, Ph.D., P.E., M.ASCE, lead design engineer for the Skyway and vice-president at T.Y. Lin International.

1. What is the most innovative or creative aspect of your project?

The East Span project represents tremendous breakthroughs in bridge design and engineering. When faced with the complex challenges of the region’s seismicity, the San Francisco Bay’s formidable geological features, and community demand for a landmark structure, the design team of T.Y. Lin International / Moffatt & Nichol, Joint Venture, working in conjunction with Caltrans, the Metropolitan Transportation Commission, and the California Transportation Commission, was willing to advance existing bridge design and engineering principles in order to provide the highest levels of seismic safety, functionality, and aesthetics.

2. What was the biggest challenge?

The most significant challenge was the region’s seismicity. The main architectural feature of the East Span is the 525-foot-tall tower of the SAS. Using leading research on shear link beams used in eccentrically braced building frames, the tower was designed to comprise four steel legs connected by shear link beams. These beams allow the legs to move independently during an earthquake, keeping the tower elastic enough to resist catastrophic damage to the main bridge structure while keeping the tower rigid enough to stay upright. The SAS is the first bridge of any kind to use fusible shear links in its tower to protect tower shafts during a seismic event.

Because the geological conditions of the bay made it unfeasible to anchor the suspension cable system in the ground at either end of the bridge, the SAS uses a single, 2.6-foot-diameter cable that anchors in the superstructure itself. The cable wraps around the western end of the bridge, ascends to the cable saddle atop the tower, and descends and splits into anchorages set only on the eastern end of the SAS. Measuring nearly a mile long, it is the largest and longest cable ever used for [an] SAS, and the longest, prebent looped suspension cable ever used for a bridge.

To further protect the tower, a “floating” deck was designed to minimize undesirable torsional moments and shears into the tower. Consisting of 2 steel box girders running along each side of the tower, connected by 32.8-foot-wide cross beams, the SAS deck has no lateral or horizontal connection to the tower. Instead, only the suspenders connect the box girders and the tower, making the SAS the first suspension bridge with no connection between the tower and the deck.

3. Did your project have any technical issues that you had to overcome? If so, what were they and how did you overcome them?

Bay Area geology also presented immense challenges for the design of the East Span foundations. Foundation piers were anchored in bedrock at the western end of the bridge. However, as bridge alignment progresses east toward Oakland, the bedrock drops steeply and the piers overlie deep deposits of young and old bay mud, which are in turn underlain by interlayered clays and sands of the lower Alameda formation. The tower foundation was anchored using the “rock socketing” process. The lower, heavily reinforced concrete portion of each pile was placed within shafts and drilled into the bedrock. The upper, permanent steel shell was filled with more heavily reinforced concrete and welded to the concrete-encased footing box of the tower foundations at water level. The design of the Skyway foundations was inspired by technology used by the oil industry to secure deep sea oil rigs. The piles were driven up to 300 feet below the water’s surface to reach stable soils, and at a 1:10 batter (angle) to increase lateral resistance of the structure under seismic loads.

The twin viaducts of the Skyway are precast segmental bridges, erected in balanced cantilever, with a typical span of 525 feet. The segments were precast in a specialized casting yard in Stockton, California, floated on barges about 70 miles to the bridge site, and lifted into position using self-launching winch lifters and [then] posttensioned. Weighing up to 750 tons each, they are the largest segments of their kind ever cast. The Skyway viaducts consist of 4 massive frames connected by expansion joints, which resist seismic motion by allowing the frames to move and slide while maintaining overall structural rigidity. While the expansion joints on the original bridge could only accommodate a few inches of movement during an earthquake, the expansion joints on the new East Span can accommodate several feet of movement. In addition, 60-foot-long hinge-pipe beams located throughout in the decks of the Skyway and SAS are designed to absorb seismic energy by deforming in their middle or “fuse” section, minimizing damage to the main structures.

4. What time and budget challenges did your project have and what did you do to overcome them?

The success of the East Span project marks the culmination of more than 20 years of intensive planning, design, engineering research and testing, and construction processes. Following the Loma Prieta earthquake, the California state legislature passed Senate Bill 60 in 1997 to provide funding for a simple, $1.3 billion viaduct bridge, slated to open in 2006. When the new East Span opened in 2013, the approved budget was $6.4 billion. The years-long evolution of the final design in response to Bay Area community demands, project financing, and rising manufacturing and materials costs all affected the budget and delivery schedule.

To construct a bridge of this magnitude, Caltrans divided the project into 4 components, with the existing bridge to be dismantled after the new bridge opened. This enabled intensive, large-scale construction planning and required minimal lane closures on the original bridge. The success of the project was accomplished through the design and application of bridge design and seismic engineering innovations that addressed critical technical challenges; the integration of construction requirements and long-term operations, maintenance, and lifecycle rehabilitation requirements into the design process; the consideration of unique geological and climatic conditions of the bridge site; and the incorporation of architectural-quality elements into the design of each component.

5. Sustainability is one the 3 strategic initiatives here at ASCE. Describe how your project adheres to being sustainable.

The San Francisco Bay is known worldwide for some of the planet’s most diverse communities of wildlife and plants. Working with environmental groups to protect the marine mammals, birds, and fish that live in and around the Bay, the project team developed a comprehensive program to protect the Bay Area’s fragile environment, especially during construction activities. Innovative solutions for bridge repair after an earthquake were also built into the design, which saves on costs, minimizes disruptions to traffic, and enhances the safety of maintenance workers. As a notable example, the shear link beams in the SAS tower and the hinge-pipe beams in the bridge deck will permit simpler, faster, and more economical repair procedures. After a seismic event, and if needed, damaged beams can be removed and replaced, enabling the bridge to remain operational during repair procedures.

 

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