Erection Cable Anchorages.

From Engineering Heritage New South Wales

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    The system actually used for the anchorages of the half-arches embodied the same essential features as that used by Mr. G. C. Imbault when erecting the Victoria Falls bridge, where it had proved entirely satisfactory; although in that case the tension to be sustained was only about one-fiftieth of that at Sydney. The cost of the tunnel and anchorage-cables for the Victoria Falls bridge was correspondingly small, and liberal margins of safety could be allowed without material addition to the total cost of the bridge.

    The maximum tension in the anchorage-cables attached to each half-arch of Sydney Harbour bridge amounted to a total of 28,000 tons, or 14,000 tons for each truss. In the scheme of erection adopted this was sustained by a hundred and twenty-eight cables connected to the end of each upper chord. Each cable formed a U-shaped loop with the two free ends attached to the bridge structure, the base passing through a tunnel excavated in the sandstone rock shores. To ensure that the anchorages would not disturb the rock to which they were secured, it was assumed that movement could only take place if the vertical component of the tension in the anchorage exceeded the weight of rock contained in a column, having as its base the horizontal projection of the lower part, or bearing area, of the tunnel face, with vertical end planes and side planes. This assumption was confirmed by some small-scale experiments on an anchorage of similar form buried at various depths in dry sand. The results of these were consistent, and indicated that the volume of sand resisting disturbance had as its base the projected area of the bearing, vertical ends and sides sloping at 28° to the vertical. The general disposition of the anchorage cables is shown in Figs. 15, Plate 6.

Saddles or Supports for Cables.

    From the attachment above each end post the cables passed straight to supports or "saddles" on the outer walls of the abutment-towers, where there was a change in direction of the cables in the vertical plane. From these saddles the cables followed a straight course to saddles at the entrances to the tunnels, where there was a considerable change of direction leading the cables down to the base of the U-shaped tunnel. Between these two saddles the weight of the cables was carried by three light intermediate supports carrying the weight of the cables only, and not intended to produce any change of direction. At all changes of direction the cables were supported on bearings formed with a radius of 50 feet to avoid any material increase of stress due to the bending round the curve. It was necessary to define the exact course of every cable, and to provide supports and bearing faces at the angles which could not only be designed exactly, but which could also be set up as intended without risk of error; the contriving of these supports or saddles was one of the most troublesome problems of the erection scheme. At the pylon-saddle on each abutment tower the change of direction was about 4 degrees. The saddle consisted of a hundred and twenty-eight tubes, 4½ inches internal diameter and 2 feet 6 inches long, carried on a steel frame; this frame was supported on the concrete platform of the tower just above the back wall, and was anchored back into the concrete platform. At the mouth of the tunnel the change in direction was about 22 degrees, and the pressure on the rock at each saddle was about 5,500 tons, equal to about 12 tons per square foot. In this case similar tubes were used as supports for the cables, but the length required was about 21 feet at the south end and 24 feet at the north end, where a greater length was necessary owing to the difference in the spread of the two sides of the anchorage. These tubes were specially made in England, and sent out ready for use. To ensure that they should be correctly spaced and aligned the tubes for each saddle were set up in three steel plates, perforated for receiving the tubes, and interconnected with one another by steel framing so that when this framework and the plates were fixed in a position which could be accurately defined, the tubes supported by the plate would be correctly placed. After the tubes were fixed, the intervening spaces were filled in with high-quality reinforced concrete to support the tubes and to transmit the pressure from them into the rock below.

    Each saddle was 28 feet high and 9 feet wide.

    To correspond with the grouping of the cables on the tunnel-face, and to lead up to their connections with the bridge structure, the supporting tubes of the tunnel-saddles were arranged in groups of four, the inclination of the groups corresponding with the gradual spreading of the cables from the bridge connection to the tunnel-faces.

Tunnel-Lining.

    The tunnel was excavated so that a space of about 1 foot was left between the intended alignment of the cables and the face of the rock. The actual bearing of the cables on the rock was confined to two quadrants of 50 feet radius at the base of the U, the cables passing down into the tunnel without making any contact either between the tunnel-saddles and the quadrants, or at the base between the termination of the two quadrants. It was important to ensure that the lining of the tunnel throughout the quadrants forming the bearing for the cables was accurately formed, so that each cable had a true seating throughout the length of the quadrant.

    This was achieved by building up a tunnel-lining of corrugated steel sheets, the corrugations being of 1 ½-inch radius, and spaced 3 inches apart, the sheets being accurately bent to a radius of 50 feet.

    The sheets were 2 feet 1 inch wide, in lengths of about 10 feet; both ends of each length were bent over on a radius of 3 feet, leaving a central portion about 9 feet long on the radius of 50 feet. To support these sheets double timbers were set up in the tunnel at intervals of 9 feet, fixed to the correct profile of the bearing face of the cables. The sheets were then secured to these timbers tier by tier, and concrete was packed in behind to fill in the space between them and the rock. One cable out of every four was supported on two cables of the lower tier resting on the corrugated steel strips thus providing a succession of groups of four cables. This arrangement produced an intensity of pressure on the rock of about 9 tons per square foot.


File:SHB First S Photo 4.jpg
This photo does not appear in the original article. 28 July 1923 SARA NRS12685.


File:SHB First S Photo 5.jpg
THE SCENE AT THE TURNING THE SOD CEREMONY.
The mayor of North Sydney is here addressing the crowd. Speeches were also delivered by the Premier and the Minister for Works. 28 July 1923 SARA NRS12685.
File:SHB First d Photo 6.jpg
While the photo adjacent is the one used by The Sydney Mail, another near identical image shows the crowd with umbrellas raised. It was a wet day. 28 July 1923. SARA NRS 12685.


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