The San Francisco-Oakland Bay Bridge (37°49'5?N, 122°20'48?W; known locally as the Bay Bridge) is a toll bridge which spans San Francisco Bay and links the California cities of Oakland and San Francisco in the United States, as part of Interstate 80. It is one of the busiest bridges in the United States, carrying approximately 280,000 vehicles per day.[1]
The original bridge designs were by Ralph Modjeski. The Bay Bridge opened for traffic on November 12, 1936,
Construction began on July 9, 1933. The 1.78 mile (2.8 km) western span of the bridge between San Francisco and Yerba Buena Island presented an enormous engineering challenge. The bay was up to 100 feet (30 m) deep in places and the soil required new foundation-laying techniques. At the time of construction suspension bridges could not be made with more than a pair of towers owing to stability considerations, and a two tower span would be longer than practical. The solution was to construct a massive concrete anchorage halfway between San Francisco and the island and to build two complete suspension bridges, one on either side of the central anchorage. (Modern cable-stayed bridges may have any number of towers. The design for the now cancelled Chacao Channel bridge included an innovative suspension bridge with two main spans connected by a rigid central tower composed of two A frames instead of a concrete anchorage.)
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 (3.1 km) combination of double cantilever, five long-span through trusses and a truss causeway, forming the longest bridge of its kind at the time, with the cantilever portion being the most massive yet constructed.
Much of the original eastern span is actually founded upon treated wood. Owing to very deep muds on the bay bottom it was not practical to reach bedrock, although lower levels of mud are quite firm. Long wooden pilings were crafted from entire old growth Douglas Fir trees and were driven through the soft mud to firmer bottom.
Connecting the two halves of the bridge is Yerba Buena Tunnel measuring 76 feet (23 m) wide, 56 feet (18 m) high, and 539 feet (164.4 m) long. It was the largest diameter tunnel in the world. The enormous amount of rock and dirt excavated from the tunnel was used in part to create Treasure Island.
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