THE SAN FRANCISCO/OAKLAND BAY BRIDGE

Night photo of Bay Bridge

FACTS AT A GLANCE

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 Transbay 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 transbay underwater tube crossing was recommended as the best way of crossing the bay. However this idea was soon deemed inappropriate for automobile traffic.

Practical and economic concerns would make the bridge a reality. Oakland streetcar lines were laid out to feed passengers to a fleet of ferry boats traversing the bay. In 1928, ferries carried over 46 million passengers between the two shorelines. Finally, with the popularity and mass production of the automobile, it was determined that a bridge was necessary and such a structure could support itself with tolls.

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.

Using plans conceived by Daniel E. Moran of New York, the nation's top expert on deep-water foundations, Purcell decided to build a center anchorage between the shoreline and Yerba Buena Island. The anchorage would be a monolithic concrete pier supporting one end of each of the two suspension bridges connecting Yerba Buena Island with San Francisco.

New techniques were implemented in the construction of the center anchorage. Fifty-five steel tubes, each 15 feet in diameter were filled with compressed air, bound within a caisson and towed via tugboat to the middle of the channel. Anchors were installed on the bay bottom and cables from the anchors were used to guide the caisson into place. The steel pipes were sunk through the water into the Bay mud. Clamshells, a digging apparatus lowered by chain, were dropped through the huge pipes to excavate the bay mud. Water jets were used to clear the mud in the spaces between the pipes. As one pipe was lowered to a desired depth it was capped and filled with compressed air, while the next pipe was lowered. Through this process the caisson was lowered over 100 feet through the bay mud and clay until it sat on bedrock 220 feet below the water at low tide.

The west bay's first suspension tower was installed using coffer dams to provide a dry work area for the foundation. Steel sheet piles were driven into the bay floor, eventually forming a water tight coffer dam. The sea water was then pumped out and the suspension bridge tower's foundation was laid. Hammerhead cranes, rising from atop the tower itself, were used to raise the steel structure. Four suspension towers were constructed in this manner, two on each side of the center anchorage.

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 eyebars. The shuttle then picked up another loop of wire and shuttled it back, hooking this loop on an eyebar 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.

THE EAST BAY CROSSING

The crossing from the Oakland shoreline to Yerba Buena Island was an immense feat of engineering, although less difficult than the deep water crossing on the other side. It was spanned by a 10,176 foot cantilever bridge, the longest bridge of its kind at the time. This bridge employs the world's deepest bridge pier, sunk 242 feet below the water level.

The cantilever and suspension bridges meet at Yerba Buena Island via a tunnel through the shale hill on the island. The Yerba Buena Tunnel is listed in the Guinness book of World Records as the largest diameter bore tunnel in the world, measuring 76 feet wide by 56 feet high.

Construction took three years, and was completed six months ahead of schedule. The bridge had consumed over six percent of the total steel output of the nation in 1933. Total costs were $77 million, including the construction of the Transbay Transit Terminal.

UNFORSEEN EXPANSION

Almost as soon as the bridge was opened in 1936, traffic on the Bay Bridge exceeded levels predicted for 1950. This was partly due to the lack of other bridges crossing the bay, but also because passengers abandoned the ferry services and chose to cross via the bridge. The bridge operators lowered tolls in an attempt to lure ferry users. The strategy was successful.

In the early years, the bridge carried three lanes of auto traffic in each direction on the upper deck. The lower deck was reserved for truck traffic and the inter-urban railway, including the Key System street cars that ran through the East Bay.

Auto traffic increased greatly. In 1958, $49 million was allocated to re-configure the bridge. The railway system was removed and the upper deck was re-aligned to carry five lanes of westbound truck and auto traffic. The lower deck carried five lanes of eastbound traffic. The road deck through Yerba Buena Island had to be lowered to accommodate the large trucks that would now be allowed on the upper lanes. This work was done while traffic continued to use the bridge.

IMPROVEMENTS/INNOVATIONS

Increasing traffic volumes have made additional innovations necessary.

In 1971, tolls were reduced in priority lanes for high occupancy vehicles (HOV), encouraging bus and carpool use. By 1973, more than half the 50,000 commuters entering the toll plaza between the commute hours of 6 a.m. and 9 a.m. used the HOV lanes. In March of 1975 the tolls for these lanes were dropped entirely.

One of the most important innovations on the Bay Bridge was the installation of a signal system to regulate traffic on the bridge. The traffic metering system functions through roadway sensors linked to a main computer which activates signals that rhythmically merge 15 lanes into five and allows carpools and buses to bypass any backup before entering the bridge itself.

Traffic accidents were reduced 15 percent after the metering system was installed . As many as 500 more vehicles per hour can cross the bridge during peak periods due to metering.

EARTHQUAKE RETROFITTING

A section of the bridge was damaged in the 1989 Loma Prieta earthquake which measured 7.1 on the Richter scale. Bolts holding a section of the upper deck on the truss section sheared causing a portion of the deck to unhinge and fall onto the lower deck. The earthquake demonstrated that despite the Bay Bridge's behemoth stature and deep piers, it was vulnerable to damage during strong quakes.

Retrofit work to prevent any future failures has begun.

Financing

Construction of the Bay Bridge was financed through a series of bonds. Net revenues from the Bay Bridge have been combined with funds generated from the San Mateo-Hayward and Dumbarton Bridges.

This fund financed the Bay Bridge reconstruction in 1958, the new San Mateo-Hayward Bridge ($ 70 million), and the new Dumbarton Bridge ($70 million). The bulk of toll revenues currently are turned over to the regional transportation planning agency The Metropolitan Transportation Commission, and redistributed to the Bay Area Rapid Transit District (BART), San Francisco Municipal Railway and Alameda County Transit.