Our Harbour Bridge Its Fabrication and Erection.
This article, written by Kathleen Butler from the notes of John Bradfield, appeared in The Sydney Mail on 26 March 1924. The decision that the bridge would be an arch had only been announced a month before, the contract had been signed by the NSW Minister for Works only two days earlier, and Dorman Long's representative, Lawrence Ennis, had not yet signed the document. ,br>
Several of the details which are set out in this article did not occur as planned at this early stage. The bearings were erected using a small overhead traveling crane arrangement, rather than the main bridge erecting crane high above. The supporting cables during the cantilever phase of the work were not carried over a tall strut to the fourth panel; Smaller creeper cranes were not used to build the central panels; the closing of the arch was not achieved by jacks at ground level acting on the struts holding the tie-back cables. If nothing else this early report illustrates the evolution of the design of both the structure and the erection process.
THE bridge will be fabricated on the area of land now occupied by the railway between Milson's Point and Lavender Bay station. The new station will be opened for traffic in August, and the area of land referred to will become available for constructional purposes, as was specified to all tenderers.
Dorman, Long, and Company intend to erect their workshops on this site for the purpose of fabricating the bridge. These workshops will rank among the heaviest shops of the character in existence, and will be provided with the finest equipment of heavy structural fabricating machinery that it is possible to procure. The shop will he constructed in two portions, one being 600ft long and 149ft. wide to centres of crane rails, consisting of two 73ft. bays, and the other being 600ft. long and 130ft wide, consisting of one bay only. The two-bay shop will be utilised for the preparation of the individual pieces of the bridge members in detail, the processes carried out comprising templating, shearing, planing, drilling, and riveting, etc., and all the necessary machinery will be specially designed for the work. The two 73ft bays in this shop will be equipped with four modern electrically-driven overhead cranes.
The second shop, 130ft wide in one span, will be devoted to the assembling of the various pieces, dealt with in the first shop, to form the finished bridge members. After the bridge members are assembled, they will be riveted up, and the ends will be machined to the finished lengths ready for erection. In this shop will be located the heavy hydraulic and pneumatic riveting machines and the specially-designed end-planing and pin-boring machines. After completion the various bridge members will be assembled in panels in the position ultimately taken up in the finished structure, and all splice and connections at the junctions of the members will be drilled in position to ensure absolute accuracy of workmanship. In this shop will be mounted two 150-ton electric overhead travelling cranes, and after the bridge members have been finally completed the cranes will lift the members in their complete finished conditions and lengths direct to barges designed for the purpose. These barges will be run into a dock, which will be constructed within the width of the shop, so that the overhead electric cranes have complete command of the dock area. After the bridge members are on board, the barges will be towed into position in the harbour, right under the erection cranes working at the bridge.
THE method of erection of the world's largest arch span of 1650ft will be watched with intense interest by the engineering profession the world over. The pictures illustrating this article will make the method clear. In the first stage, after the necessary excavation has been performed, the approach piers will be constructed, and sufficient of the abutment towers to support the decking, then the approach spans will be commenced, and will be erected as shown on timber trestles, with cranes travelling on special tracks laid on the trestles.
THE second step in the erection will be the construction of trestling on the partially completed abutment tower, whose top surface corresponds with the level and plane of the arch top chord. On this trestling the main erection cranes will be built. The cranes weigh 536 tons each, and have been specially designed for this work, to lift members up to 160 tons weight, in this position the cranes will erect the arch hearings and first panel of the arch trusses on each side of the harbour.
One of the most important operations in the erection of the bridge will be the trueing up of the faces of the four granite skewbacks on which the main bearings rest. The faces of the two skewbacks on each side of the harbour must be set in the same plane and at the same levels, as far as human skill can do so, whilst each pair of skewbacks on opposite sides of the harbour must be parallel to each other and exactly the same distance apart, so that at all stages of erection the main arches being built out from either shore will be in line with each other. The main pins are 52 inches in diameter, and have been described in a previous article.
For anchoring the structure to the solid rock steel wire cables will be used, passing through tunnels in a position approximately below the second pier of the approach span from the main bearing. The tunnels will consist of two inclined shafts about 140ft apart, and will be approximately 10ft. by 6ft., connected by two semi-circular cross tunnels. The cross tunnel will be sunk to about 120ft below ground level, depending on the character of the rock.
ON completion of the first panel the erection crane will be moved forward on the top chords of the bridge, and will erect panels in succession until the crane reaches the fifth panel. Immediately behind the fifth panel a further series of cables will be secured so as to transfer the reaction from the end post to this point. The second series of cables will be carried over struts, one standing on the end post of the arch, and the other supported on hydraulic jacks at ground level, serving for final adjustment of the completed half arches. This completes the third stage.
IN the fourth stage, as soon as sufficient cable is provided to sustain the reaction, the cables secured to the end post will be slacked, and the crane will continue to erect successive panels until the tenth panel is reached, leaving four panels on each side of the centre-line to be erected.
THE remaining panels will be erected by means of a lighter crane, whose total weight is only about, 180 tons. The half-arches will finally be adjusted in level and allowed to come together by means of hydraulic rams on the struts supporting the cables just above the ground level. In this condition the arch will be complete as a three-hinged arch, under the dead load of the main trusses and bracing.
IN the sixth stage hydraulic rams will then be inserted in the top chord members, and an initial stress will be put into the members to correspond with the stress which would arise in these members if the arch were built as a complete elastic two-hinged structure of the correct calculated lengths for all members. The total stress required in the top chord is 4,900,000lb.
AFTER the main trusses have been completely erected the erection of the floor will proceed simultaneously and symmetrically at both ends, and abutment towers will then be completed, after which, when necessary finishing and clearing-up work have been performed, the great undertaking will have been successfully brought, to its conclusion, and Sydney will have its long-awaited Harbour Bridge.
