Building a Bridge Pier
James B. Eads faced many obstacles when he built the first steel bridge in the world over the Mississippi River, none of which was more daunting than the sinking of the bridges' two piers and abutments to bedrock. Eads plan was based on a similar technology used in Europe: that of using caissons. This demonstration shows the step-by-step process of constructing one of the piers and explains this amazing feat of engineering.
1 - Builders construct a water-tight caisson, over fifty feet long and nine feet high. It is made of oak timbers and covered with plate iron. Contained within the caisson are three open-bottomed air chambers that allow the structure to act as a diving bell - a technology Eads used while salvaging wrecks in the Mississippi.
2 - To sink the caisson, seven-ton blocks are placed atop its roof. Wooden pilings guide its descent, and a passageway is left in the middle for stairs. Two 215-foot workboats are used to support the machinery and maneuver the blocks into place.
3 - Once the caisson reaches sand, men enter the air chambers through the air locks and begin to excavate the riverbed, shoveling sand into a sand pump that suctions it up through a shaft and out through an opening at the top of the structure. Compressed air is pumped into the chamber to keep water out ‚ the deeper the caisson sinks, the greater the air pressure needed.
4 - The increased air pressure in the air chambers makes it difficult for the men to work. Many develop what is known as caisson disease, or the bends, which is an accumulation of nitrogen gas in the body. Sufferers experience dizziness, coughing, and in extreme cases, death. To prevent the disease, a worker should decompress two hours for every two he works at a depth of 100 feet. At first, workers spend only a few minutes in the air lock. Of the 119 who suffer from the disease, 14 die.
5 - Signs of bedrock reveal that the excavation is nearly complete. The caisson rests on the bedrock and serves as the foundation of the mighty pier. To fortify this foundation, the air chamber is filled with concrete and, as the men ascend to the surface, the core is filled behind them.
6 - The construction of the east and west piers, which descend over 100-feet below the surface of the Mississippi, takes seven months, 600 workers, and 40,000 tons of concrete. The St. Louis Bridge establishes James Eads as one of the most successful engineers of his time.