 |
|
| ||||||||||||||||||||||||||
|
|
| ||||||||||||||||||||||||||
|
|
|
|
|
| ||||||||
The San Francisco-Oakland Bay Bridge is a double-decker half-suspension bridge and half-cantilever bridge connecting San Francisco to Oakland and passing through Yerba Buena Island. 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.
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.
On February 20, 1931, after a great deal of lobbying, California received the go-ahead by the United States Congress to build the Bay Bridge.
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 and two tracks of urban railway. Automobile traffic increased dramatically in the ensuing decades while the urban 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 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. Also, on the east side of the tunnel 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.
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.)
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 not the 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.
Engineering and economic analysis at the around 1999 showed 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 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 - essentially reinforced and post-stressed concrete spans supported by vertical pylons. The design criteria was that the new bridge should resist an 8.5 magnitude earthquake on either of the two nearby faults.
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 slated for 2007. The new eastern span will feature a pair of side-by-side, five-lane concrete viaducts linking to a single-towered suspension span between the viaducts. It is currently being constructed just north of the existing span. The project has been designed to withstand an 8.5 magnitude earthquake. The authorities were shocked when they opened the bids on the new tower portion, as there was only a single bid and it was considerably more expensive than their estimate, largely due to a recent and unexpected rise both in the cost of steel and of concrete. The project, which will require 100,000 tons of structural steel, is expected to cost $5.1 billion (as of July 2004), up from a 1997 estimate of $1.1 billion and a March 2003 estimate of $2.6 billion.
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. 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. 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.
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.
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.
|
||||
| View Live Article | This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License | |
||
