Electrical Aptitude Practice Test 12

Hanging a Road over the River: The Brooklyn Bridge

¹A suspension bridge hangs its roadway from cables. The main cables pass over tall towers and are anchored firmly at each end, so the weight of the deck and of the traffic on it travels up into the cables, across to the towers, and down through the towers into their foundations. Engineer John A. Roebling designed such a bridge to link Brooklyn and Manhattan across the East River. In June 1869, while he was surveying the site, a ferry pinned his foot against a piling. His crushed toes were amputated, but he developed tetanus and died the following month. His 32-year-old son, Washington Roebling, became chief engineer.

²Construction began on January 2, 1870. Each tower stands on a caisson, a huge wooden box with an open bottom that was sunk into the riverbed. Inside it, workers dug away mud and stone while compressed air kept the water out, and as they dug, the caisson slowly sank. The Brooklyn caisson came to rest about 44.5 feet below high water. The Manhattan caisson went to about 78.5 feet. Probes there showed that bedrock lay deeper than expected, and the risk of caisson disease, now called decompression sickness, was rising with every foot. Roebling judged the sandy soil firm enough, and the caisson was filled with concrete in July 1872.

³Washington Roebling was himself struck by caisson disease soon after work began on the Brooklyn tower foundation, and he was left paralyzed. He directed the work from his bedroom. His wife, Emily Warren Roebling, carried messages between him and the engineers at the site. She learned the engineering mathematics and spent 11 years helping to supervise construction.

⁴The bridge has four main cables, each made of 5,282 galvanized steel wires bundled into 19 strands of 278 wires. One contractor supplied inferior wire, which lowered the bridge's strength from six times what was needed to about four times. The main span measures 1,595.5 feet.

⁵The bridge opened on May 24, 1883, and Emily Roebling was officially the first person to cross it. Six days later, a woman fell on a stairway at the Brooklyn approach, setting off a stampede in which at least twelve people were crushed to death. On May 17, 1884, the showman P. T. Barnum sent his elephant Jumbo leading a parade of 21 elephants across, which helped ease doubts about the bridge's stability.