San Francisco–Oakland Bay Bridge: Difference between revisions
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[[image:sfbaybridge_at_night.jpg|thumb|The Bay Bridge, with the skyline of San Francisco in the background.]] |
[[image:sfbaybridge_at_night.jpg|thumb|300px|The Bay Bridge, with the skyline of San Francisco in the background.]] |
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The '''San Francisco-Oakland Bay Bridge''' is a double-decker half-[[suspension bridge]] and half-[[cantilever]] [[bridge]] connecting [[San Francisco, California|San Francisco]] to [[Oakland, California|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 '''San Francisco-Oakland Bay Bridge''' is a double-decker half-[[suspension bridge]] and half-[[cantilever]] [[bridge]] connecting [[San Francisco, California|San Francisco]] to [[Oakland, California|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. |
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Revision as of 05:26, 29 July 2004
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 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 reenforce 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 cantilever portion of the bridge collapsed onto the deck below, causing one death. 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.
After more than a decade of study, construction began on a replacement for the cantilever portion of the bridge on January 29, 2002. 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 and its 100,000 tons of structural steel will cost an estimated $2.6 billion and is slated for opening in 2007. It has been designed to withstand an 8.5 magnitude earthquake.
The western suspension span has also undergone extensive seismic retrofitting. Most of the beams were originally constructed of two plate I-beams joined with diagonal crossings of angle stock. These have all been reconstructed by replacing the angle 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 each was removed and replaced with a high strength bolt. Additional diagonal bracing has been added underneath each deck.
To cover the cost of the new span and other retrofit projects around California, the toll for westbound automobile traffic was raised from $1.00 to $2.00 and again in July 2004 to $3.00, along with other state-run bridges. Eastbound traffic remains toll free. 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.
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 Bridge
- 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:
- 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