Science Fair Project Encyclopedia
San Francisco-Oakland Bay Bridge
The San Francisco-Oakland Bay Bridge (locally known as the Bay Bridge) links Oakland to San Francisco and spans the San Francisco Bay. The bridge consists of two major segments connecting a central island, Yerba Buena Island, with each shore. The western segment terminating in San Francisco consists of two suspension bridges end-to-end with a central anchorage. The eastern span terminating in Oakland consists of a truss causeway, five medium span truss bridges and a double tower cantilever span. Construction began on July 9, 1933 and the bridge was opened to traffic on Thursday, November 12, 1936, at 12:30 p.m. Construction began shortly after that of the Golden Gate Bridge but the Bay Bridge was completed six months sooner. The total cost of construction for the bridge was $79.5 million.
History and construction
The concept of a bridge spanning the San Francisco Bay had been considered since the California gold rush days. Yet, the task seemed too daunting as the bay was deemed too wide and too deep. In 1921, an underwater tube was considered, but it became clear that it would be inadequate for vehicular traffic. Finally, with the increasing popularity and availability of the automobile, support for a transbay crossing grew during the 1920s. In 1926, the California Legislature established the Toll Bridge Authority with the responsibility of bridging San Francisco and Alameda County.
To make the bridge design more feasible, the path was chosen to pass through Yerba Buena Island, significantly reducing the amount of material needed to construct a transbay crossing. The U.S. Army and Navy granted permission to use the island as an anchorage.
The 1.78 mile western span of the bridge between San Francisco and Yerba Buena Island presented an enormous engineering challenge. The bay was up to 100 feet deep in places and the soil required new foundation-laying techniques. The solution was to construct a massive man-made concrete anchorage half way between San Francisco and the island and build two complete suspension bridges on either side.
The eastern span was a marvelous engineering feat as well. The crossing from Yerba Buena Island to Oakland was spanned by a 10,176 foot combination of cantilever, long span trusses, and shorter trusses, forming the longest bridge of its kind at the time, with the cantilever portion being the most massive yet constructed.
Connecting the two halves of the bridge is Yerba Buena Tunnel, which was the largest diameter bore tunnel in the world, measuring 76 feet wide, 56 feet high, and 1,700 feet long. The enormous amount of rock and dirt excavated from the tunnel was used in part to create Treasure Island.
When the bridge first opened, the upper deck consisted of three lanes of traffic in each direction. The lower deck carried three lanes of truck traffic (with the central lane initially for passing only — a very dangerous configuration) and two tracks of urban railway used by the Key Rail System. Automobile traffic increased dramatically in the ensuing decades while the Key Rail System declined and in 1957 the bridge was reconfigured with five lanes of westbound traffic on the upper deck and five lanes of eastbound traffic on the lower deck. Trucks were allowed on both decks and the railway was removed. Owing to a lack of clearance for trucks through the upper deck portion of the Yerba Buena tunnel, it was necessary to lower the upper deck where it passed through the tunnel and to correspondingly excavate the lower portion. This was done while the bridge was in use by using a movable temporary span over the portion being lowered. On the lower deck of the tunnel and its eastern viaduct extension it was necessary to remove central supports, with each transverse beam being doubled to take the load across all lanes. It was also necessary to further reinforce each beam supporting the upper deck throughout the entire span, modifications still visible to the traveler.
During the October 17, 1989, Loma Prieta earthquake, which measured 7.1 on the Richter scale, a 50-foot section of the upper deck of the eastern truss portion of the bridge collapsed onto the deck below, indirectly causing one death (actually due to misdirection of traffic by the California Highway Patrol). The bridge was closed for a month and one day as construction crews repaired the fallen section. It reopened on November 18th of that year.
Given the distance to the epicenter of the Loma Prieta (roughly 70 miles south of San Francisco), there was great surprise at the localized destruction around the Bay Area. Analysis points to the likelihood of reflected seismic waves from deep earth crust discontinuities. Failures were mostly located in areas of poor soil conditions due to building over filled-in creeks or on sand and rubble mixes from the 1906 earthquake - all of which were saturated with water and prone to liquefaction. (An exception was the Cypress Viaduct collapse, blamed on poor engineering in certain details, combined with large-structure resonances that had not been considered during design.)
Western span retrofitting
The western suspension span has also undergone extensive seismic retrofitting. Most of the beams were originally constructed of two plate I-beams joined with lattices of flat strip or angle stock. These have all been reconstructed by replacing the lattice stock with continuous steel plate. Also, the entire bridge was fabricated using hot steel rivets, which are impossible to heat treat and so remain relatively soft. Analysis showed that these could fail in shear under extreme stress and so at most locations each was removed and replaced with a high-strength bolt. Additional diagonal bracing has been added underneath each deck and may be seen under the lower deck in one of the illustrations above at the right.
Eastern span replacement
It was clear that the eastern span must be made earthquake resistant. It had been known for over thirty years that a major local earthquake on either of the two local faults (the infamous San Andreas and the lesser known but far more dangerous Hayward) would destroy the major cantilever span. Like most expensive-to-solve potential problems, there was no political will to act. The Loma Prieta earthquake was the proverbial "wakeup call" for all concerned. Estimates made in 1999 placed the probability of a major earthquake in the area within the following 30 years at 70 percent, although recent studies announced in September 2004 by the United States Geological Survey have cast doubt on the (statistical) predictability of large earthquakes based upon the duration of preceding quiet periods.
Initial retrofit proposal
The initial proposal for the eastern span involved the construction of substantial concrete pylons to replace or supplement the existing supports. There would also be modifications to the lattice beams as is now complete for the western span. The original cost estimate for this refit was 200 million US. dollars. The overall appearance would be little changed.
Initial replacement proposal
Engineering and economic analysis in 1999 suggested that a simple replacement bridge would cost only a few hundred million dollars more than a retrofit of the existing eastern span and that a replacement 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 the authorities decided to replace the entire eastern span. The design proposed was a simple elevated viaduct consisting reinforced concrete vertical pylons and post-stressed precast segment concrete spans. The design criteria was that the new bridge should resist an 8.5 magnitude earthquake on any of several faults in the region, but particularly the nearby San Andreas Fault and the Hayward Fault. This was not received well by either the public or the politicians, being characterized as a "freeway on stilts".
Signature span proposals and selection
A design contest was held for a "signature span" (a span with distinctive and dramatic appearance, unique to the site). A number of innovative proposals were examined and a winner was selected.
After more than a decade of study, construction began on a replacement for the cantilever portion of the bridge on January 29, 2002, with completion originally slated for 2007. The new eastern "signature" span was to feature a pair of side-by-side, five-lane concrete viaducts linking to a single-towered, self-supporting suspension span between the viaducts. If completed, this would have become the largest bridge of this type. The approach viaducts are currently being constructed just north of the existing span.
A price shock
The authorities were shocked when they opened the bids on the proposed tower portion, as there was only a single bid and it was considerably more expensive (US$1,400,000,000) than their estimate ($780,000,000), partially due to a recent and unexpected rise both in the cost of steel and of concrete. As both concrete and structural steel are now commodities within a world wide market the prices were much higher than expected due to the current building boom throughout China. (China is currently consuming 40 percent of worldwide concrete production.) Another qualified potential bidder did not bid owing to a number of construction uncertainties owing to the innovative design - another likely contribution to the very high bid. The entire project, which will require 100,000 tons of structural steel, is now expected to cost $5.1 billion (as of July 2004), 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.
Governor kills signature span
On September 30, 2004, the Governor's office announced that without sufficient funds authorized by the legislature that the bid must be allowed to expire. It is unclear if this will require a redesign to obtain a less expensive span. It might be possible to quickly redesign the span using a more conventional cable stayed design, for which the construction methods and costs are well understood.
On December 10, 2004, the Governor's office announced that the signature span concept has been scrapped, with the completion of the bridge now to be by the construction of the simple viaduct originally proposed. The design, having gone full circle, remains expensive due to the continued high cost of materials. Many are arguing that there will be little difference in final cost with this lesser proposal since the latest concept will require obtaining new permits, perhaps adding an additional two or three years and a viaduct may not even obtain Coast Guard approval since the maximum width of the ship channel will 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 spar of similar appearance but of lower cost.
Economic analysis questioned
The standpont of pro-"signature bridge" activists and regional politicians was reenforced by a legislative analyst's report in late January, 2005. This indicated, due to additional time delays and all new permitting requirements, that the Governor's skyway 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 has already added at least one hundred million dollars to the expected cost.
Big troubles indicated
On April 6, 2005, the FBI announced an investigation into charges by fifteen former welders and inspectors on the new eastern span that welders were rushed to an extent affecting their performance on up to one third of the welds and that workers were ordered to cover up defective welds by re-welding in a superficial manner. As these welds are now embedded in concrete and some are deeply underwater this brought a shock to the entire bay area. Investigation is continuing.
A Caltrans spokesperson responded with a public assertion that it was not possible that defective welds could be hidden from Caltrans inspectors. This assertion is expected to be tested by radiological and ultrasonic inspection of some of the welds that are currently accessible.
Some history of the eastern span
This eastern portion of the bridge has an interesting history. It has long been thought by East Bay residents that Oakland got the ugly bridge while San Francisco got the pretty one. The original Caltrans replacement design was for a simple parallel lane viaduct (resembling portions of a freeway which could be found anywhere), and which was considered to be another insult to the East Bay residents and commuters. 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. This argument may have caused up to a two year delay and billions of dollars in additional costs. While a number of proposals were submitted for a "signature" bridge, and the design chosen is considered by most critics to be acceptable from an aesthetic viewpoint, many questions have been raised by engineering experts as to its survivability under various scenarios - both natural and otherwise. The proposed tower is a self-supporting suspension bridge without cable anchorages. What is amazing is that an innovative design both beautiful and practical, produced by the architect Frank Lloyd Wright for another bay area location was not even considered. That design was for a "Butterfly" bridge intended to replace another span crossing the bay, the Hayward-San Mateo Bridge. There was a similar fight over the design of that bridge, with Caltrans initially proposing what was called by critics a "Rip Van Winkle" design, a two deck truss similar to the obsolete eastern span of the Bay Bridge or portions of the "roller coaster" Richmond-San Rafael Bridge.
Eastern span naming proposal
- 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. 
Emperor Norton, a celebrated San Franciscan, issued several edicts ordering the construction of both a bridge and a tunnel across the bay. Both were eventually constructed although not until well after his 1880 death.
When it opened in 1936, the toll was 65 cents, collected in each direction. Within months, it was lowered to 50 cents in order to compete with the ferry system, and finally to 25 cents as this was shown sufficient to pay off the original revenue bonds on schedule. As with other bridges of the era, passage was to be free after completion of the repayment of the original bonds.
In the interest of reducing the cost of collecting tolls and of building additional toll booths, all bridges in the bay area were converted to collect tolls in only one direction, with the amount collected doubled.
Tolls were subsequently raised to finance improvements to the bridge approaches, required to connect with new freeways, and to subsidize public transit in order to reduce traffic over the bridge. To cover the cost of the new span and other retrofit and various transit projects in the region, the toll for westbound automobile traffic rose from $1.00 to $2.00 and again in July 2004 to $3.00, along with other state-run bridges. A proposal for a $4.00 toll is expected soon. Eastbound traffic remains toll free.
In fiction and film
- In William Gibson's futuristic Bridge Trilogy of novels, the Bay Bridge, closed to traffic after an earthquake, has been taken over by the homeless and turned into a miniature city of the dispossessed.
- In the movie The Graduate, Dustin Hoffman can be seen driving in traffic over the bridge. To make the scene more photogenic, all the traffic is going the wrong way on the top level of the bridge. The shot was filmed very early on a weekend morning while traffic was stopped to prevent any head-on collisions.
The Bay Bridge at a glance
- Entire span
- Location: Interstate 80 between San Francisco and Alameda Counties.
- Length: 23,000 feet (4.35 miles, 7 kilometers)
- Western Suspension Bridges
- Length 9,260 feet (2,822 meters)
- Vertical clearance 220 feet (67 meters)
- Tower Height 526 feet (160 meters) (from water level)
- Eastern Cantilever Bridge, truss bridges and truss causeway approroachs:
- Length: 10,176 feet (3,101 meters)
- Vertical clearance 191 feet (58 meters)
- Deepest Bridge Pier: 242 feet below water level - 396 feet high (120 meters)
- Avg. Daily Traffic: 280,000 vehicles
- Oakland Berkeley & Eastern: Interurbans that used to run on the Bay Bridge.
- Symphonies in Steel: Bay Bridge and the Golden Gate
- San Francisco-Oakland Bay Bridge East Span Seismic Safety Project
- A Bridge Too Weak?
- CalTrans site
- CalTrans Alternatives report December 10, 2004 PDF download (not a web page)
- San Francisco Gate story on renaming the bridge
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