Eastern Span Replacement Of The San Francisco–Oakland Bay Bridge

The eastern span replacement of the San Francisco–Oakland Bay Bridge was a construction project to replace a seismically unsound portion of the Bay Bridge with a new self-anchored suspension bridge (SAS) and a pair of viaducts. The bridge is in the U.S. state of California and crosses the San Francisco Bay between Yerba Buena Island and Oakland. The span replacement took place between 2002 and 2013, and is the most expensive public works project in California history, with a final price tag of $6.5 billion, a 2,500% cost overrun from the original estimate of $250 million. Originally scheduled to open in 2007, several problems delayed the opening until September 2, 2013. With a width of , comprising 10 general-purpose lanes, it is the world's widest bridge according to Guinness World Records.

The Bay Bridge has two major sections: the western suspension spans and their approach structures between San Francisco and Yerba Buena Island (YBI) and the structures between YBI and the eastern terminus in Oakland. The original eastern section was composed of a double balanced cantilever span, five through-truss spans, and a truss causeway. This part became the subject of concern after a section collapsed during the Loma Prieta earthquake on October 17, 1989. The replacement span is engineered to withstand the largest earthquake expected over a 1500-year period, and it is expected to last at least 150 years with proper maintenance.

Background

Bridge design experts have known for over 30 years that a major earthquake on either of two nearby faults (the San Andreas and the Hayward) could destroy the major cantilever span. Little was done to address this problem until the 1989 Loma Prieta earthquake. The earthquake measured 6.9 on the moment magnitude scale and while the epicenter was distant from the bridge, a 50-foot (15 m) section of the upper deck of the eastern truss viaduct portion of the bridge collapsed onto the deck below, indirectly resulting in one death at the point of collapse. The bridge was closed for a month as construction crews removed and reconstructed the fallen section. It reopened on November 18, 1989, with a new stronger retrofit in place. The failure was at the transition between the easternmost through-truss and the westernmost double-deck causeway segment, a location where the inertial response character of the structure makes an abrupt change. Analysis of the event completed by internal staff has shown that the bridge was close to a far more catastrophic failure in which either the through-truss or the causeway segment would have dropped from their common support structure.

It was clear that the eastern span needed to be made more earthquake resistant. Estimates made in 1999 placed the probability of a major earthquake in the area within the following 30 years at 70%, although studies announced in September 2004 by the United States Geological Survey have cast doubt on the predictability of large earthquakes based upon the duration of preceding quiet periods. A more recent (2008) analysis asserts an increased probability of a major event on the Hayward Fault.

Design proposals

Retrofit

The initial proposal for the eastern span involved the construction of substantial concrete columns to replace or supplement the existing supports. There would also be modifications to the lattice beams as are now complete for the western suspension spans. The original cost estimate for this refit was $200 million. The overall appearance would be little changed. Owing to the retention of the original structure, the bridge's ongoing maintenance costs would continue to be high. The robustness of a retrofit was called into question directly by the Army Corps of Engineers in a highly critical report and indirectly by the collapse of a retrofitted overpass in the 1994 Northridge earthquake in Los Angeles, that structure having been modified in response to the San Fernando earthquake 23 years prior.

Replacement

Engineering and economic analysis in 1996 suggested that a replacement bridge would cost a few hundred million dollars more than a retrofit of the existing eastern span, would have a far longer expected useful life (perhaps 75 to 100 years rather than 30), and would require far less maintenance. Rather than retrofit the existing bridge, CalTrans (California Department of Transportation) decided to replace the entire eastern span. The design proposed was an elevated viaduct consisting of reinforced concrete columns and precast concrete segment spans as seen in the illustration at right. The design criterion was that the new bridge should survive an 8.5 magnitude earthquake on any of several faults in the region (particularly the nearby San Andreas and Hayward faults). The aesthetics of the proposal were not well received by either the public or their politicians, being characterized as a "freeway on stilts".

After this, a design contest was held for a signature span (a span with distinctive and dramatic appearance, unique to the site) by the Engineering and Design Advisory Panel (EDAP) of the Metropolitan Transportation Commission (MTC). A number of innovative proposals were examined until all but four proposals that were submitted by members of EDAP were selected as semi-finalists, and a winner was selected from this group. This posed a serious conflict of interest, as members of the EDAP who were selecting the bridge design reviewed proposals by their own firms and rejected all proposals that did not have a representative on the EDAP. The design chosen was more expensive than the alternatives, because the primary structure cannot be self-supporting until it is structurally complete. This requires the building of two bridges, the first a falsework to support the final span, which would be removed upon completion of the final span. It has also been criticized as both a less structurally robust design and with less predictable construction costs than other modern spans.

Alignment

In 1997, there was much political bickering over whether the bridge should be built to the north or to the south of the existing bridge, with the "Mayors Brown" (San Francisco's Willie Brown and Oakland's Jerry Brown) on opposite sides of the issue. Yerba Buena Island is within the city limits of San Francisco and the proposed (and current) northern alignment would cast a shadow over certain prime development sites on the island's eastern shore. Even the U.S. Navy (at the time the controlling authority of the island) was involved at the behest of San Francisco in restricting Caltrans soil engineers' access to the proposed site . That may have caused up to a two-year delay and many hundred millions of dollars in additional costs.

Various options were determined to be worthy of consideration and were carefully examined jointly by state and federal authorities, with input from the United States Coast Guard.

Grade alternatives included:
*Extending the sea level approach grade westward, with a steep approach to the span.
*Using a relatively constant grade, including on a portion of the span.
*Using a relatively constant grade to near the span, with the span level.
The last alternative was chosen because it was considered to have a superior visual effect and improved driving experience. The grade of the new approach (estimated possible ranges of 1.710%-1.779%; vertically-curved suspension deck with crest, slightly different from what was chosen) to the channel span is somewhat less than that of the previous structure (exactly 4% for ground-upper deck connection; exactly 2.74% for deck trusses and east 2 through trusses; transition gradient for center through truss; exactly 1.3% for cantilever arms and west 2 through trusses; vertical curve with crest between cantilever towers) and less ship clearance is provided under the span owing mostly to the depth of the deck box structures.

Alignment alternatives included (see image at right for details):
*S4: a southern alignment, slightly curved, but a shorter route than the northern alternatives.
*N2: a two-bend northern alignment close to the existing bridge.
*N4: a single bend alignment, with the main span tending northward to the curve to the eastern approach viaducts, those being parallel to the existing double-deck truss causeway approach.

The last alternative was selected, as it presents a superior view of San Francisco to the west compared to the others where views are obscured by Yerba Buena Island. Any more northerly track would encounter more difficult geotechnical circumstances.

Naming proposal

In December 2004, the San Francisco Board of Supervisors, in honor of Joshua A. Norton, passed a resolution "urging the California Department of Transportation and members of the California Assembly and Senate to name the new additions to the San Francisco Bay Bridge in honor of Emperor Norton I, Emperor of the United States and Protector of Mexico." The proposal was not supported by the Oakland City Council and the bridge does not have an official name.

Bidding and initial construction

Although it was somewhat controversial, authorities decided to allow bids to include major components and materials not made in the United States. This was partly due to the cost of materials, and especially due to the lack of suitable fabrication facilities within the United States, or even within the western hemisphere. In contrast, China, where the SAS deck components (including the massive cable, key sections of the iconic tower and deck) were built by Shanghai Zhenhua Heavy Industries Company, has low cost materials producers. Other major components were produced in Japan, owing to the availability of large steel casting, welding, and machining capabilities. Suspender saddles were made in England. As Federal highway funds generally come with "Made in America" restrictions, the bridge was built without such funds, for which it would otherwise qualify owing to its carriage of Interstate 80.

The authorities were shocked when they opened the bids on the proposed tower portion and only a single bid for US$1.4 billion was received, considerably more than their estimate of around $780 million. This was partially because of a rise in the cost of steel and concrete, particularly as a result of the concurrent building boom in China,World Steel Association
World Crude Steel Production but also due to construction uncertainties owing to the innovative design. The entire project, requiring 100,000 tons of structural steel, was expected to cost $6.2 billion as of July 2005, up from a 1997 estimate of $1.1 billion (for a simple viaduct) and a March 2003 estimate of $2.6 billion that included a tower span. Despite the increase in costs, construction began on the replacement on January 29, 2002, with completion originally slated for 2007. The span finally opened on September 2, 2013.

Removal of signature span

On September 30, 2004, the office of Governor Arnold Schwarzenegger announced that, without sufficient funds authorized by the legislature, the bid must be allowed to expire. It was, at the time, unclear if this would require a redesign to obtain a less expensive span.

File:SFOBBPropsalsOneOfSix.jpg, 1: Self Anchored Suspension (all steel)
File:SFOBBPropsalsTwoOfSix.jpg, 2: Modified SAS Concrete Tower
File:SFOBBPropsalsThreeOfSix.jpg, 3: Two-span Asymmetrical Cable-stayed (concrete)


File:SFOBBPropsalsFourOfSix.jpg, 4: Two-span Symmetrical Cable-stayed (concrete)
File:SFOBBPropsalsFiveOfSix.jpg, 5: Two-tower Three-span Cable-Stayed (concrete)
File:SFOBBPropsalsSixOfSix.png, 6: Extend the Skyway Bridge to YBI

On December 10, 2004, the governor's office announced that the signature span concept had been scrapped, with the bridge to be the simple viaduct originally proposed. The design, having gone full circle, remained expensive due to the continued high cost of materials. Many argued that there would be little difference in final cost with this lesser proposal since that concept required obtaining new permits, perhaps adding another two or three years; furthermore, a viaduct may not even be able to obtain Coast Guard approval, since the maximum width of the ship channel would be reduced by almost half. Local reaction to this announcement was intense, with most suggesting that the bridge be built to appear as proposed — either in the steel material as bid or using a reinforced concrete tower of similar appearance but of lower cost.

Reinstatement of original design

The standpoint of pro-"signature bridge" activists and regional politicians was reinforced by a legislative analyst's report in late January 2005. The report indicated, due to additional time delays and all new permitting requirements, that the governor's viaduct proposal could likely cost additional funding and take longer to complete than the proposed signature span. This view was reinforced by a further report in March 2005 indicating that the delay imposed by the governor had already added at least $100 million to the expected cost (subsequently modified to $83 million in a December 2005 report).

The design controversy continued for over six months. In essence, the governor believed that the entire state should not share in the costs of building the bridge, as he considered it to be a local problem. Northern Californians pointed out that when the southern portions of the state experienced disasters, the state supported rebuilding, especially as seen in earthquake rebuilding of freeways and the subsequent seismic retrofit of state freeway structures and bridges. Since the objective of the replacement of the eastern span is to prevent the necessity of complete rebuilding after a large earthquake, Bay Area residents felt justified in their call for state support.

A compromise was announced on June 24, 2005 by Governor Schwarzenegger. The governor said that he and State Senate President Pro Tempore Don Perata

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