DESIGN AND CONSTRUCTION
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.
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Design & Construction | Cable Spinning Autos & Trains | Charles H. Purcell | Celebration