San Francisco - Oakland Bay Bridge

In the early 1900's, settlers of the San Francisco Bay Area considered the San Francisco-Oakland Bay Bridge, which spans over eight miles long and 500 feet above the water at its highest point, an impossible feat. However, with only a few complications, the bridge was completed by November, 1936. The San Francisco-Oakland Bay Bridge consists of two bridge types. The east section is a cantilever bridge, and the west, a suspension bridge. The 1.78 mile western span of the bridge between San Francisco and Yerba Buena Island presented the first obstacle. The bay was up to 100 feet deep in some places and required a new foundation-laying technique. Engineers developed a type of foundation called a pneumatic caisson to support the western section. A series of concrete cylinders were grouped together and then capped-off, having the air pressure of each cylinder identical to balance the beginning of the structure. From there, the workers added sets of new cylinders until the caisson reached the bottom of the bay. Then, in order to reach the bedrock, they inserted long drills down the cylinders, digging until they reached bedrock. After the caisson was balanced at the bottom of the bay, workers filled it with 1 million cubic yards of concrete, more concrete than was used for the construction of the Empire State Building! This caisson connected the two suspension bridges that make up the western part of the bridge. The eastern span was an incredible engineering feat as well. The cantilever bridge from Yerba Buena Island to Oakland was the longest bridge of its kind at the time. Yerba Buena Tunnel connected the western half of the bridge to the eastern half and measured 76 feet wide, 56 feet high, and 1,700 feet long. The spoils from constructing this tunnel were used in part to make Treasure Island. The foundations used in the east section differed from those in the west section because the bottom of the bay contained clay and was too deep to reveal bedrock. The steel superstructure of the cantilever bridge was the largest order for steel ever placed when the bridge was constructed. They built the bridge in two pieces, intending to join them in the middle. However, extreme temperatures from sunlight caused the side of the bridge facing the sun to expand while the opposite side contracted. When they were ready to join the two pieces, they did not initially fit.