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Making Suspension Bridge Cables

Although suspension bridge cables are said to be spun, the wire in them is not twisted as it is in the production of textiles. In reality, bridge cables are made up of a large number of continuous parallel wires. In order to make the cable manageable during production, the wires are first laid up in a series of bundles or strands. In the George Washington Bridge there are 61 strands in each of the four main cables.

The galvanized wire is delivered to the worksite on large spools. The process of spinning the cables begins when one end of a wire is affixed to a strand shoe. It is then pulled from the spool and carried up and over the towers and around a strand shoe in the opposite abutment. The process is repeated back and fourth until the spool is empty. When one wire ends, another wire is spliced to it, and the process continues until a predetermined number is reached and they are bundled into a strand. As the wires are put in place, each is drawn up or let out to insure the proper sag. The main cables of the George Washington bridge contain a total of 105,896 individual wires.

To create a round shape, the bundles were compacted by a movable hydraulic press. Arc-shaped shoes squeezed the wires with more than 720 tons of pressure per square inch. At the same time the device forced the wires into an almost solid form, the exterior was wrapped with a continuous wire. The outer wrapping both insured that the cable retained its shape and protected interior wires from the weather. The deck or roadway was suspended from the main cables by smaller suspender cables that looped over them.

 


George Washington Bridge

By early 1924, bridge engineer Othmar H. Ammann had completed a proposal for a suspension bridge that would span the Hudson River between Ft. Lee, NJ, and Ft. Washington, NY. Ammann earnestly promoted his project--designed on speculation--to professional, legislative, and civic groups throughout the region. Against formidable odds, his proposal prevailed. The newly established Port of New York Authority was given approval from the state legislatures to construct the facility. Ammann was hired as the Authority's first Chief Engineer and set to work on the project that would place him on the world stage.

After the bridge was designed, contractors and suppliers were asked to submit bids for the construction. Although the designers had carefully researched estimates for the project, the true cost could not be determined until the construction contracts were awarded. With fiscal constraint in mind, the engineers took bids on two very different suspension networks. One featured a suspension cable, the other a suspension chain. Hundreds of spools of steel wire are spun in place to make a suspension cable. By contrast, a suspension chain is made by bolting together thousands of thin metal bars. On aesthetic grounds, suspension cables were considered superior because they render a more fluid line in space. The contract for the George Washington Bridge suspension network went to the low bidder: a cable spinner. John Roebling & Sons was awarded the contract. The firm, a New Jersey company founded by the designer of the Brooklyn Bridge, manufactured and spun the 107,000 miles of wire used to make the George Washington Bridge's main suspension cables.

Bridge construction commenced October 21, 1927. Hundreds of construction workers hazarded their lives to erect and suspend one of civilization's most beautiful rebuttals to gravity. Twelve lives were lost in the course of constructing the bridge. The bridge opened to traffic October 25, 1931, eight months ahead of schedule. Due to the speed of construction and precision of engineering, it cost one million dollars less to build than originally estimated.

The George Washington Bridge was designed with a capacity for expansion. It opened in 1931 with a single road deck handling six lanes of traffic. As the automobile culture grew, traffic increased. By 1949, two additional lanes had to be opened. Roughly 12 years later, construction on the lower level began. In 1962, its six lanes opened, making the George Washington the world's only 14-lane span, and distinguishing it as the bridge that carries the greatest volume of traffic among all vehicular bridges in the world.

 


Othmar H. Ammann

Engineering genius Othmar H. Ammann (1879-1965) was born and educated in Switzerland and immigrated to the United States at age 24. He gained valuable experience designing railroad bridges while working in Pittsburgh and Philadelphia, but hit his professional stride in the 1920s while practicing in New York. Bridges and tunnels spanned the East and Harlem Rivers before 1920, but no vehicular connection had been established across the much broader Hudson River. Numerous plans for bridging the Hudson had been put forth since the early 1800s. Ammann's 1923 proposal for a vehicular suspension bridge between Ft. Washington and Ft. Lee ultimately prevailed. Named in honor of the first president, the George Washington Bridge opened to traffic in 1931. This engineering triumph brought Ammann world renown and a succession of coveted bridge commissions.

In all, Ammann designed and built six bridges that collectively lace together New York's metropolitan highway system: the George Washington Bridge, Bayonne Bridge, Triborough Bridge Complex, Bronx-Whitestone Bridge, Throgs Neck Bridge, and Verrazano-Narrows Bridge. Several of the six bridges boast record-breaking spans. Indeed, Ammann's career charts an unflagging series of technical innovations. However, Ammann's stature as a leading 20th-century designer derives as much from his architectural sensibilities as from his technical brilliance. Never satisfied with simply the most economic engineering solution, Ammann designed each structure to convey an air of dignity, power, and grace appropriate to its site and function. As he stated near the end of his life, "In bridge design, the aesthetics are quite as important as engineering details. It is a crime to build an ugly bridge."

 



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