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Support cables

During the bridge construction, each half arch was held back by a series of cables anchored in tunnels carved in the rock. There were 128 cables on the southern shore supporting the southern arch and 128 on the northern shore.Each cable was 2¾ inches [69·8 mm] in diameter, about 1200 feet [366 m] long, and consisted of 217 single wires twisted together. They were tested to failure at 460 tons [4674 kN] but ‘… in practice were called upon to sustain a load of 115 tons [1169 kN] …’ (Ennis, 1932, p 41) (Note: 1 ton = 2240 pounds, abbreviated to lb.)

When the two half arches were joined and the central arch panels completed, the support cables were progressively slackened off and removed from the link plates and taken out of the tunnels.

What Happened to the Support Cables?

Some of the surplus support cables used to hold up the incomplete halves of the Sydney Harbour Bridge during its construction were used as suspension cables on the Walter Taylor suspension bridge over the Brisbane River at Indooroopilly, Queensland http://www.flickr.com/photos/80651083@N00/95576042. Other surplus cables were transported to Chipinge, Zimbabe and used to support the roadway on an arch bridge designed by Ralph Freeman over the Save River http://www.freebase.com/view/en/birchenough_bridge.

The Construction of the Sydney Harbour Bridge – support cables (excerpt) [The Institution of Engineers, Australia] 11.7 MB

JJC Bradfield standing on one of the bearing pins
Cable anchorage scheme from Sydney Harbour Bridge: illustrated by photographs and line drawings, Dorman, Long, 1932

 

Shown below is a sketch of one-half of the Bridge arch cantilevered out from the shore with cables holding it in place. The force Fc represents the force in the cables. The mass of the half arch is approximately 10,000 tonnes.

sketch of one-half of the Bridge arch

Problem 1

If the reaction (R) at the thrust bearing on shore is 45° to the horizontal acting up and to the right, determine (a) the position of the weight force, (b) the magnitude of Fc if the weight force is 62 mN and (c) the magnitude of R.

(Hint: Print out the image above and use the Three Force Rule to solve the problem.)

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Problem 2

Earlier in the construction, Fc was calculated to be 98 MN. There are 128 cables holding each half of the arch in place. Assuming all cables are equally loaded, determine the tensile stress in each cable. Each cable has a diameter of 69·8 mm.

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Problem 3

How much would each cable stretch (elongate) when loaded to 1146 kN if Young’s modulus (E) for the steel is 210 GPa? Assume the cables to be solid in cross-section.

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Problem 4

What maximum load could the 128 cables support?

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