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The collapse of a portion of the upper deck of the Bay Bridge following the Loma Prieta Earthquake demonstrated the critical need for seismic safety on Bay Area bridges. California Department of Transportation (Caltrans) is conducting a seismic upgrade for the Bay Bridge, which encompasses six projects. This site provides information on the upgrade of the East Span section of the bridge, which runs from Yerba Buena Island to the Oakland shore. The formal name of this effort is the San Francisco-Oakland Bay Bridge East Span Seismic Safety Project (SFOBB), known more simply as the East Span Project.

suspen2a.jpg (17840 bytes)A Caltrans study determined that it is more cost-effective to replace the existing East Span with a new bridge than it would be to seismically retrofit the existing structure. After lengthy design and environmental reviews, Caltrans has identified a replacement bridge on an alignment north of the existing bridge as the Preferred Alternative. The Preferred Alternative would consist of two side-by-side bridge decks, each with five lanes plus shoulders. The existing bridge would be dismantled. Although Caltrans has identified a Preferred Alternative, it has not been approved and the state continues to analyze other alternatives. A replacement span would significantly improve seismic and traffic safety. A bicycle/pedestrian path would provide additional access and recreational opportunities.

The East Span Project is part of Caltrans' Seismic Retrofit Program. This multi-billion dollar program is modifying the structural design of several state-owned bridges in the Bay Area and throughout the state to make them less susceptible to damage in an earthquake.

Caltrans District 4 is responsible for the maintenance, operation, and administration of the seven state-owned toll bridges on San Francisco Bay and is overseeing bridge design, environmental review, permitting, and construction for the East Span Project.

Seismic Safety for the East Span

newlighting-lores.jpg (2761 bytes)Earthquakes are commonly reported in terms of their magnitude. The larger the magnitude, the bigger the earthquake, and the greater the potential for damage. When engineers design structures to resist earthquakes, they use "rock motions" -- the vibrations that travel through the bedrock caused by the slipping of an earthquake fault..

Seismologists develop the rock motions based on the structure’s location in relation to the location of earthquake faults and historical and geotechnical project site data. Engineers use these rock motions to calculate the maximum seismic forces that the structure will experience and then design the structure to resist the forces.

Rock motions propagate up through the soil layers to the ground surface, where they become "ground motions" -- rock motions cause ground motions. Ground motions are not the same as rock motions. Ground motions caused by the same rock motions will vary due to the soil conditions. The rock motions can be uniform along a project site, but for a long bridge like the Bay Bridge, the ground motions change along the length of the bridge as the soil changes.

For example, during the Loma Prieta earthquake, the ground motions varied at different locations, and this was reflected in the differing amounts of damage. Rock motions were amplified by the soft Bay muds and the resulting ground motions caused substantial damage in particular areas (such as the I-880/Cypress Freeway and San Francisco’s Marina District). Where the soil is more stable material, the rock motions were not amplified as much, the ground motions were not as great and there was little damage. Seismologists and geotechnical engineers use the rock motions and then translate them (by computer) into ground motions. However, rock motions are the basis of engineering calculations.

There are two methods of estimating the greatest rock motion that a particular structure will experience. In the past, Caltrans considered the motions from the Maximum Credible Earthquake, or MCE. The MCE is the largest reasonable earthquake at a fault without regard or consideration of how often the earthquake might occur (the return period). It also does not provide a consistent or rational assessment of the probability that a structure will experience the design earthquake. For the East Span Project, Caltrans estimated the greatest rock motions from the Safety Evaluation Earthquake, or SEE. This is defined as an earthquake that generates the largest motions expected to occur at the bridge site once every 1,500 years (a 1,500-year return period). The bridge’s expected life span is 150 years, so there is approximately a 10% chance that this earthquake would happen during its life span. Caltrans was aided in the development of the ground motions by the Ad Hoc Committee on Seismic Ground Motions. This ad hoc committee is comprised of four members:

  • The Chairperson is Professor Bruce Bolt, Professor Emeritus of Seismology at the University of California at Berkeley,
  • Dr. Norman Abrahamson, a world renowned geoscientist with expertise in assessing seismic ground motions,
  • Dr. Richard Borcherdt, a seismologist from the United States Geological Survey with a specialization in recording earthquake ground motions, and
  • Dr. Joseph Penzien, Professor Emeritus from the University of California at Berkeley who is recognized as a world leader in structural dynamics and probability and statistics.

The Seismic Safety Peer Review Panel and the Metropolitan Transportation Commission’s Engineering and Design Advisory Panel both accepted and agreed that the bridge should be designed for these SEE ground motions.

The Ad Hoc Committee on Seismic Ground Motions evaluated the recently issued report by the Army Corps of Engineers (Corps) on retrofit versus replacement of the East Span. In particular, the Committee focused on one of the Corps' conclusions: "the performance of the replacement bridge during a Maximum Credible Earthquake cannot be determined. The bridge has not been evaluated or designed for a MCE event, which is larger than the SEE event." The Committee pointed out that this conclusion is based on an error in plotting a graph included in the Corps’ report. In fact, the MCE motions fall well below the SEE motions in all relevant data ranges, particularly during the initial shaking. In addition, the Ad Hoc Committee’s evaluation of the Corps’ report explained that responses of structures (estimated for earthquakes of various magnitudes and for various source-to-site distances) depend crucially on the measurement of ground motions in actual large earthquakes. As additional recordings become available from such earthquakes, these new ground motion recordings are incorporated into the existing international database. Professor Bolt has stated that the SEE standard takes into account a wider range of possible ground motions, some of which are much higher in impact than MCE's predicted motions. The Ad Hoc Committee concluded that Caltrans studied both standards (MCE and SEE) and designed the bridge to the higher SEE standard.

bay_bridge.jpg (8021 bytes)While seismic safety is the primary consideration for the East Span, additional traffic safety issues are also important. Built in the 1930s, the existing structure does not conform with current operational standards set by the American Association of Highway and Transportation Officials. The East Span Project would improve conformity with operational and safety standards to the greatest extent possible. The Preferred Alternative would meet current standards for roadway shoulders, lane widths, stopping sight distances, and other factors, helping to substantially improve public safety.

Emergency Lifeline Service

Caltrans has designated the Bay Bridge as an emergency lifeline route to be used in disaster response activities. Under Caltrans criteria, a lifeline transportation route:

  • allows emergency relief access to and through the affected region
  • connects major population centers within the affected region
  • serves as the most effective of several routes for emergency relief access
  • provides direct or nearby access to and from major emergency supply centers
  • links various modes of transportation
  • provides access to major traffic distribution centers

As a lifeline route, the bridge would be used to provide post-quake transportation service for emergency response and support for the region's economy. It is critical for the bridge to remain functional following an earthquake. Seismic safety, meeting modern operational standards, and the need to maintain a lifeline route are all integral factors in upgrading the East Span.

Overview of the Seismic Retrofit Program

Caltrans' design program to seismically retrofit state-owned bridges has been shaped by recent earthquakes in California. In particular, the 1971 San Fernando earthquake, the 1987 Whittier Narrows earthquake, the 1989 Loma Prieta earthquake, and the 1994 Northridge earthquake affected the direction, design, scientific research, and priorities of Caltrans' seismic retrofit program. These earthquakes prompted new research and funding for the seismic retrofit of transportation structures, which has included pioneering research and design focusing on the seismic behavior of large bridges. This has led to increased understanding of how bridges behave in earthquakes, new techniques for retrofitting existing bridges, and improved design criteria for new construction. Over the past three decades, this work has placed Caltrans at the forefront of the evolving field of seismic retrofit design.

The East Span Project

Caltrans began to design seismic safety improvements for the East Span following the Loma Prieta earthquake. Seismic safety strategies initially focused on retrofit of the existing East Span structure. A retrofit alternative was devised and initial environmental review was conducted. Consultation with regulatory agencies was initiated.

Cost comparisons of retrofit and replacement alternatives indicated that seismic retrofit of the existing span could be accomplished at a lower cost than the cost to replace the structure. However, replacement bridge alternatives would incur lower life-cycle costs and substantial advantages over retrofitting the existing East Span; therefore, in 1997 Caltrans and the Business, Transportation and Housing Agency recommended consideration of bridge replacement to then-Governor Pete Wilson.

In February 1997, then-Governor Pete Wilson adopted the recommendations of the Business Transportation and Housing Agency, and Caltrans that replacement of the East Span be considered. Caltrans and the Federal Highway Administration (FHWA) have considered and performed preliminary engineering on a range of possible project alternatives in accordance with National Environmental Policy Act (NEPA) requirements and in consultation with regulatory agencies. Five alternatives (No-Build, Retrofit Existing Structure, two northern replacement alternatives, and one southern replacement alternative) are currently under consideration for the East Span Project.

The Final Environmental Impact Statement (FEIS) was released in May 2001. The FEIS provides updated information on the alternatives, impacts, and mitigation measures, and incorporates responses to public and agency comments on the DEIS. The FEIS is posted on Caltrans District 4 Website. The Notice of Availability (NOA) was published in the Federal Register on May 18, 2001. The FHWA can file a Record of Decision (ROD) in the Federal Register after June 18, 2001. The ROD will close the NEPA environmental review process, explain the reasons for the project decision, and summarize mitigation measures that will be incorporated into the project. Construction of the East Span Project is targeted to begin in 2002.

Detailed design and engineering of the Preferred Alternative for the East Span is underway and is approximately 85% complete. Caltrans is coordinating with appropriate regulatory agencies regarding necessary permits. Caltrans is also working with various resource agencies, communities, and others to develop mitigation measures for potential environmental impacts of the project. Information is being made available to contractors for future bidding on project contracts.

Many other federal, state, and local government agencies are involved in the East Span Project.

MTC's Role

bike_presentation-lores.jpg (2761 bytes)The Metropolitan Transportation Commission (MTC) is a regional governmental agency that provides regional transportation planning and coordination of transportation activities for the nine-county Bay Area. MTC functions as both the Regional Transportation Planning Agency (RTPA), a state designation, and for federal purposes under 23 CFR 134, as the region's Metropolitan Planning Organization (MPO). As such, MTC is responsible for implementing the Regional Transportation Plan (RTP), which is a comprehensive blueprint for the development of mass transit, highway, airport, seaport, railroad, bicycle, and pedestrian facilities. Requests from local agencies for state and federal grants for transportation projects are screened by MTC to determine their compatibility with the RTP. MTC is also the Bay Area Toll Authority, under Section 30950 of the California Streets and Highway Code. In this role, it administers the base $1 toll on the Bay Area's seven state-owned toll bridges.

MTC organized the San Francisco-Oakland Bay Bridge Design Task Force (Task Force) to consider replacement bridge alternatives following then-Governor Pete Wilson's February 1997 decision that replacement alternatives should be considered. All the members of the Task Force are MTC Commissioners. The Task Force mandate was to develop a regional consensus recommendation on a design option for a new eastern span for the SFOBB and recommend any additional features that might be included in the design of the bridge that would not be borne by funding allocated by the State of California. The additional features would be paid by Bay Area bridge users through an extension of the $1 toll surcharge on the state-owned toll bridges.

The Task Force formed an Engineering and Design Advisory Panel (EDAP) to advise the Task Force on issues of costs, engineering feasibility, design factors, and seismic safety. The EDAP is comprised of technical experts in structural and civil engineering, geology, seismology, and architectural bridge design. EDAP deliberations included meetings and workshops open to the public for presentation of design concepts from interested parties. Beginning with the first of four formal public hearings on March 27, 1997, the Task Force has considered replacement bridge options (e.g., different types of replacement bridge structures) and the cost and feasibility of including design features such as "signature" bridge structures and a pedestrian/bicycle path.

The Task Force made its summary recommendations to Caltrans on July 30, 1997, concerning replacement bridge types, alignment, and a request for additional analysis needed to determine cost and feasibility of design components and features. Recommendations of the Task Force were transmitted to Caltrans and the State Legislature to assist in the determination of potential funding needs for the project.

In response to Task Force recommendations, Caltrans initiated the requested preliminary design studies. The design studies were used to determine the seismic performance, cost, and aesthetics of the bridge types recommended for further study by the Task Force. The EDAP reviewed results of design studies in a series of public meetings and made specific recommendations to the Task Force. On June 24, 1998, following extensive public comment and interagency participation, the Task Force forwarded an advisory recommendation to MTC that the replacement structure be a concrete skyway structure north of the existing bridge with an asymmetrical self-anchored suspension main span supported by a single steel tower. A 4.7-meter (15.5-foot) wide pedestrian/bicycle path 0.3 meter (one foot) higher than the traffic lanes located on the south side of the eastbound structure was also recommended and accepted by MTC. The Task Force recommendation is considered advisory and represents the locally preferred option.

Opportunities for Public Involvement

pubmeet.jpg (8162 bytes)Developing a seismically safe bridge across the Bay is vitally important to Bay Area communities and the region's economy. Since work began on the East Span Project, the public has played an important role. Close to 50 public meetings and hearings have been held by Caltrans and MTC to provide information and gather input on a variety of issues.

Scoping meetings were held to obtain public input for the project's environmental review process. Informational open houses have been hosted to explain the project to people and answer questions. Numerous meetings have been held to gather public input on the selection of a design for the new bridge. Four public hearings were held to solicit comments of the public and agencies on the Draft Environmental Impact Statement.

meeting1.jpg (7979 bytes)MTC's Bay Bridge Design Task Force and Engineering and Design Advisory Panel meetings have provided the public with opportunities to be involved in refinements to the bridge design.

Public comments or questions about the East Span Project are welcome via e-mail.

The East Span News is distributed periodically to provide the public with regular project information. The site archives include past issues of the newsletter.

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FALL 1997
SPRING 1998
FALL 1998
WINTER 2001
SPRING 2002


If you have comments or questions about the East Span Seismic Safety Project, mail them to:

Public Information Office
Caltrans District 4
PO Box 23660
Oakland, CA 94623-0660

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