Seismic Safety Retrofit

The San Francisco-Oakland Bay Bridge (SFOBB) (Fig. 1), which connects Oakland, California to Yerba Buena Island in the San Francisco Bay, desperately needs to be replaced or retrofitted because it does not currently meet operation and safety design standards.

In fact, it is unlikely that it would survive a maximum credible earthquake (MCE), and it would likely be incapable of supporting post traumatic relief access following an MCE (Caltrans 99). Bridges like the SFOBB that are located in high-risk areas are especially vulnerable to earthquake damage (Fig. 2).

Since it has now been determined that the current bridge does not satisfy the safety requirements, the decision was made that either the existing bridge should be retrofitted, or an entirely new bridge should be built. Upon further deliberation, it was decided that a completely new bridge was the most economical, as well as the safest, choice.

Once the method of repair was determined, many contract firms were called upon to propose bids for the work to be done. Of these proposals, several contenders were chosen to compete for the final decision. The final three contenders included three separate bridge designs. Two of the designs were variations of a cable-stayed design, and the third design was a suspension bridge (Caltrans 99).

A cable-stayed bridge (Fig. 3) is a modification of the cantilever bridge, which has come into modern use relatively recently. This particular bridge design closely resembles a suspension bridge (Hardesty, Fischer, Christie, and Haber 83). In a cantilevered bridge, the steel trusses go out over the water, and an equal amount of steel is erected from the piers in the direction of the shoreline (Jacobs and Neville 68). The cable-stayed bridge consists of trusses spanning both ways from a central tower and supported by inclined cables attached to the tower at the top or sometimes at several levels. Usually two of these assemblies are placed end to end to provide a bridge with a long center span (Hardesty, Fischer, Christie, and Haber 83).

The second bridge design (Fig. 4), which is also a variation of the cable-stayed design, is different because its supporting cables are all attached at the top of the tower and support the bridge at varying angles. The other cable-stayed variation has its cables attached to the tower at different levels, so they are parallel to each other.

The suspension bridge is the design that is about the most recognizable. Several famous suspension bridges include the Brooklyn Bridge and the Golden Gate Bridge. The composition formed by the high supporting towers and the narrow horizontal deck, joined by the sweeping curve of the suspension cable, creates an awe-inspiring structure (Schodek 87).

The suspension bridge is a structure consisting of a roadway suspended from two cables that pass over two towers and are anchored by backstays to a firm foundation. If the roadway is attached directly to the cables by suspenders, the structure lacks rigidity, with the result that wind loads and moving live loads distort the cables and produce a wave motion on the roadway. When a truss hung from the cable supports the roadway, the structure is called a stiffened suspension bridge. This is the design that was chosen for the SFOBB (Fig. 5). The stiffening truss distributes the concentrated live loads over a considerable length of the cable (Hardesty, Fischer, Christie, and Haber 83). Suspension bridges are quite tolerant of earthquakes. Suspended-span designs are suitable for longer span bridges, but appropriate allowance must be made for horizontal movements to take place without danger and for transverse tilts to be accommodated. Considerable attention should be paid to the provision of bearings and expansion joints to ensure continued safe functioning under the various movements imposed (Attewell and Taylor 84).

At a May 1998 advisory panel meeting, and a future Metropolitan Transportation Commission meeting, the suspension design was finally agreed upon (Rosenbaum 98). Now the California Department of Transportation (Caltrans) is in the process of obtaining permits and meeting environmental demands. Construction is expected to begin in the year 2002.

The upgrading and retrofitting of public transportation systems is a very complicated and time consuming undertaking. Current estimations conclude that three years will be spent on design and environmental compliance, one year will be spent on gaining building permits, receiving right of way, and completing the contracting process, and finally, three years will be spent to actually build the span (Caltrans 99).

After the project is complete, the SFOBB will meet all safety and environmental requirements. Also, in the case of an MCE, the bridge will not only withstand the impact, but it will be capable of continued use immediately after the quake so that emergency supplies and personnel can be transported back and forth between Oakland and Yerba Buena Island (YBI). Ultimately, the project will help to save lives and speed social recovery in the event of an MCE.

Caltrans assembled a Project Development Team (PDT) consisting of 30 separate agencies to serve as a technical advisory committee to Caltrans decision-makers. The PDT is made up of the U.S. Navy, the Federal Highway Administration, the Department of Defense, and many more. Through the help of these and many other organizations, the project will be guided along much more smoothly (Caltrans 99). Independent contractors will do the actual construction of the bridge, with limited work being done by Caltrans and other state agencies.

Before all other work can begin, the foundation needs to be put in place. The first several months will be spent installing reinforced concrete footings in the bedrock deep below the floor of the bay. It is imperative that these foundations be secured and flawless in every way. Once the footings have been secured, the main tower structure will be erected. This tower is the structural mainframe of the entire bridge. It is this tower that the cables will be suspended from, and the very road itself will be hung from. During this time, the other base supports leading up to the tower will be constructed (Rosenbaum 98). This process is expected to take one year to complete.

The next year will be spent constructing the road portions, transporting them to their respective areas, and aligning them according to the predetermined design. At this point, specially designed earthquake resistant hinges will be installed between each section of road (Fig. 6). The hinge consists of an elastic pad that supports the longer section of the road. Cable restrainers tie the two sections together (Caltrans 99).

The last year of construction will include a variety of tasks. The spanning cables will be secured, and the suspension cables that the actual road hangs from, will be fitted and installed. At this point, the bridge is almost entirely complete, but there are still many smaller tasks to be completed. Various guy wires and guardrails will be installed, the pedestrian/bike path will be laid and finished, and final surface finishing will be completed (Caltrans 99). Upon completion of the new span, the old bridge must be demolished and removed.

A proposed start date has not yet been set. The proposed project must first work its way through the state legislature, but estimates seem to suggest that construction could begin as early as 2002 if all goes smoothly. It these estimates are correct, that would place completion early in the year 2009 (Rosenbaum 98).

Liberal estimates of the entire project range from 1.3 to 1.42 billion dollars. The project will be funded by a combination of sources. State fuel tax revenues that were collected for seismic upgrade projects will fund 36.5 percent of the costs. 27 percent of funds will come from state seismic retrofit revenue bonds, which were issued by the state of California after voter approval in 1996. The last 36.5 percent of the funds will come from the toll surcharges applied on bay area toll bridges for the last eight years (Caltrans 99).

Building earthquake-safe bridges is a grand undertaking. In the case of the SFOBB, more than 3 years are being spent determining that there is a problem and deciding what course of action should be taken, and 7 years will be spent building the bridge. Once the bridge is finally complete, it will greatly benefit those who use it, and in the case of an MCE, it will save lives.

D. B. Rosenbaum, "Bay Bridge Design Pick Debated," ENR, vol. 27, no. 6, pp. 12-13, 1998.

D. B. Rosenbaum, "Bay Area Rushes to Judgment," ENR, vol. 26, no. 5, pp. 10-11, 1997.

D. L. Schodek, Landmarks in American Civil Engineering. Cambridge, MA: MIT Press, 1987.

P. Yanev, Peace of Mind in Earthquake Country. San Francisco, CA: Chronicle Books, 1974.