Brooklyn_Bridge

The Brooklyn Bridge was the longest suspension bridge at its time of completion. Built over the East river in New York, it connects the cities of Manhattan and Brooklyn. Before its completion the only way of crossing the river was by the ferry which was often slow, crowded and inefficient. The bridge was designed by a German engineer by the name of John Roebling. He came to the conclusion that a suspension bridge needed to be constructed. The East River was one of the busiest waterways on earth, with hundreds of crafts of all sizes sailing through it at any time. Any bridge spanning the water would have to allow for ships to pass beneath it, meaning a suspension bridge was the only practical solution. Just as work was beginning on the bridge in the summer of 1869, tragedy struck. While examining locations for a tower site, Roebling's foot was crushed on a pier by an incoming ferry. Roebling later died of tetanus as a result of the injuries. Immediately following Roebling's death, his son, Washington, took over as chief engineer of the Brooklyn Bridge. Materials used in the construction of the Brooklyn Bridge were mostly steel and granite. The Brooklyn Bridge is the first steel-wire suspension bridge in the world. Earlier suspension bridges of the time had been built of iron, but steel would make the Brooklyn Bridge much stronger. Because of its compressive properties, granite blocks were used in the two support towers. Work started 3rd January, 1870 on the towers that supported the cables. The north tower was built on land but the south tower had to be constructed in the actual river itself. For this to happen caissons were constructed on land then towed to the site where the foundations are to be made. The caisson was then sunk to the bottom of the river by loading blocks of granite on top. Once it had sunk to the bottom of the river, air was pumped into the caisson forcing the water out the bottom. The caisson had to be pressurised to keep the water from flowing in. An airlock was used to maintain the air-pressure so that the men could get in and out of the caisson without it flooding. The men had to dig down to the bedrock using only a pick and shovels. Washington Roebling used to help with the digging until he got ‘caisson disease’ caused by changes in air pressure that affect nitrogen levels in the bloodstream. He was bed ridden and observed progress from his bedroom window. Once they had reached the bedrock the caisson was filled with cement making the foundation. The tower was then built using granite and mortar. In the meantime two anchorage points were made at either end of the bridge. When the towers reached their ultimate height, 85 meters above the water, work began on the four enormous cables that would support the roadway. In each cable there are 5,500 wires. Suspension wires were then lowered down to where the roadway was to be. Sceptics believed that the bridge was not strong enough to hold the weight of its traffic. Diagonal stays were put in as a precaution. The bridge later worked out to have a safety factor of six. The bridge opened to traffic on the May 24, 1883. A total of 24 people died including John Roebling during the construction of the Brooklyn Bridge. It cost a total of $15 million to build, which was double the amount that was estimated. In 1884, P.T. Barnum demonstrated the safety of the bridge by parading across it with a herd of 21 elephants. An average of 150,000 vehicles cross the bridge daily.