Word's Biggest Bridge

THE SAN FRANCISCO-OAKLAND BAY BRIDGE

Location: Interstate 80 between San Francisco and Alameda Counties.

Length:23,000 feet (4.5 miles), total project: structural and roadway including approaches, toll plaza, etc.,8.4 miles.

Structure: Suspension, tunnel, cantilever and truss

West Bay Suspension Bridge:Length 9260 feet (2822 meters)

Vertical clearance: 220 feet

Span Length: 2,310 feet

Tower Height:526 feet (from water level)

East Bay Cantilever Bridge:Length 10,176 feet ,Vertical clearance 191 feet

Span length: 1,400 feet

Deepest Bridge Pier: 242 feet below water level - 396 feet high

Tunnel: Largest bore tunnel in the world: 76' wide, 58' high (546 meters (1700') long)

Opened: November 12, 1936

Cost: $77 million (Including Trans bay Transit Terminal)

Traffic Lanes: Upper level: five lanes westbound

Lower level: five lane eastbound

Avg. Daily Traffic: 270,000 vehicles

CONSTRUCTING THE IMPOSSIBLE

Conceived in the Gold Rush Days, a bridge spanning the San Francisco Bay linking The cities of San Francisco and Oakland always seemed like an engineering and financial impossibility. The water separating the cities was too deep and wide. In fact, in 1921 a trans bay underwater tube crossing was recommended as the best way of crossing the bay. However this idea was soon deemed inappropriate for automobile traffic.

In 1926, the California Legislature created the Toll Bridge Authority, a policy-making body charged with the responsibility for bridging San Francisco and Alameda County.

The challenges facing the Toll Bridge Authority were monumental. California State Highway Engineer Charles C. Purcell was put in charge of organizing the design and construction of the Bay Bridge. Fortunately, between the two shorelines was a mountain of shale rock rising above the Bay: Yerba Buena Island. The island divides the Bay into two sections allowing for two crossings, which would meet at the island. Permission was granted from the Army and Navy, tenants of the island, to use it as an anchorage.

Yet spanning the 1.78 miles between the San Francisco and Yerba Buena Island required ingenuity on a grand scale. The water, 100 feet deep at some points, and the underlying soil conditions required new techniques for placing bridge foundations. The solution: build two suspension bridges.

A total of 17,464 wires, each 0.195 inches in diameter, were spun in each of the two cables supporting each bridge. A shuttle wheel took a loop of wires from one anchorage and carried it over the towers to the other anchorage, hooking it to anchored eye bars. The shuttle then picked up another loop of wire and shuttled it back, hooking this loop on an at the other end. In this manner the cables were spun, forming a cable which is 28.75 inches in diameter. Each cable exerts a pull of 37,million pounds of dead and live load on its anchorage.