The San Francisco-Oakland
Bay Bridge involved four distinct projects: three bridges and one tunnel.
The western span between San Francisco and Yerba Buena Island is actually
two suspension bridges joined by a common anchorage since, at the time,
building a 2-mile suspension bridge was impossible. Chief Engineer Charles
H. Purcell knew that the anchorage would have to be as “solid as an island”
and rise more than 200-feet above the water. He consulted with Daniel E.
Moran, the top deep-water foundations expert and builder of the piers for
the George Washington Bridge in New York. Following Moran’s plan, workmen
at the Moore dry-dock in Oakland constructed the “Moran-Purcell caisson”
half the size of a city block and consisting of fifty-five vertical steel
cylinders—each fifteen feet in diameter. The caisson was then towed to
its site in the Bay and concrete was poured into the spaces between the
cylinders, sinking it into the water as its sides were built higher. When
the caisson reached the bottom, the cylinders were uncapped and clamshell
buckets removed 6,800 pounds of mud at each bite. The 508-foot caisson
was thus built from the top down and rammed through 100 feet of mud into
bedrock to tower 288 feet above the water. This anchorage contains more
concrete than the Empire State Building.
The Yerba Buena Island tunnel
crossing consists of three 12 x 12-foot tunnels bored through the hill
which were then broken out to become one horseshoe-shaped excavation.
The island section of the bridge is 2,950 feet long and roughly 80 feet
wide by 60 feet high.
The East Bay Crossing between
Yerba Buena Island and Oakland is also almost two miles long. Because the
bedrock drops off steeply and anchoring a suspension bridge proved to be
too costly, Purcell decided to build a cantilever span consisting of one
1,400-foot cantilever section, with two 510-foot anchor arms. These anchor
sections were built using huge derricks to lift large sections of steel
that were then riveted into place. One of the most anxious moments during
construction came when giant cranes lifted the final 21,000-ton section
185-feet above the water. At that moment, a cold wind cooled the northern
side of the span, while the sun warmed the southern side causing a failure
to connect the two sections by 4 inches. Hours later when the temperature
at the two ends of the span became balanced, the riveting was completed.