San Francisco-Oakland Bay Bridge

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 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 got 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 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 cantilever bridge, the longest bridge of its kind at the time.

Connecting the two halves of the bridge is Yerba Buena Tunnel, which the largest diameter bore tunnel in the world, measuring 76 feet wide, 56 feet high, and 1,700 feet long.

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.

The total cost of construction for the bridge was $79.5 million.

During the 1989 Loma Prieta earthquake which measured 7.1 on the Richter scale, a section of the upper deck of the cantilever portion of the bridge collapsed onto the deck below. The bridge was closed for months as construction crews repaired the fallen section.

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.

To cover the cost of the new span, the toll for westbound automobile traffic was raised from $1.00 to $2.00, along with other state-run bridges. Eastbound traffic remains toll free.

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

External links

• Symphonies in Steel: Bay Bridge and the Golden Gate (http://www.sfmuseum.org/hist9/mcgloin)
• San Francisco-Oakland Bay Bridge East Span Seismic Safety Project (http://www.dot.ca.gov/dist4/eastspans/index)