The Sydney Harbour Bridge: Tenders Close in Three Weeks.
MR. BALL, Minister for Works and Railways, has met the following representatives of firms abroad, who have been or are in Sydney in connection with their tenders :- Sir John Hunter and Mr. Hutchinson, representing the firm of Sir William Arrol and Co., of Forth Bridge fame: Mr. Tarver, of the Cleveland Bridge Co. of Darlington, England: Mr. Jonathan Jones of the McClintic Marshall Products Co. of Pittsburgh and Pottstown, U.S.A.: Mr. Cruttwell, of Sir John Wolfe Barry and Sons, of Tower Bridge fame, and associated with Sir Wm. Arrol and Co. in Sydney’s bridge: Mr Kerihuel of Baume et Marpent, Belgium and associated with Goninan and Co., Newcastle, and Strauss, of Chicago: Messrs. Paddock and Porter, of the Canadian Bridge Co., of Quebec Bridge fame: and Messrs. Ennis and Imbault of Dorman Long and Co., Middlesborough, England. Other representatives are hastening Sydneywards.
The cantilever bridge across the St. Lawrence River at Quebec, 1800ft span, and the cantilever bridge across the First of Forth, Scotland, of 1700ft span, are world-famous bridges, as is the also the Manhattan Suspension Bridge, of 1470ft span, across the East River, New York, the most graceful long-span bridge in the world. The two former carry heavy railway traffic, the latter street-car and highway traffic; hence the difference in type. Another suspension bridge, the Williamsburg, of 1600ft span, also across the East River, New York, is the reverse of a graceful suspension bridge.
Research carried on during the war and subsequently has increased our knowledge of the properties of metals, and alloy steels and heat-treated carbon steels of a higher grade than formerly are now available for constructional purposes, and it was this knowledge which enabled Mr. Bradfield to design an arch bridge 650ft greater in span than the largest arch bridge yet erected, and be satisfied in his own mind that its erection could be successfully consummated.
Heat-treated carbon steel eyebars with an ultimate tensile strength of 132,600lb per square inch, an elastic limit of 89,800lb per square inch, an elongation of 6.3 per cent. in 18 feet, and a reduction in area of 22 per cent. have been fabricated, whilst molybdenum and tungsten alloyed with steel have produced a much higher grade alloy steel than was formerly available. These are facts of considerable importance in connection with the Sydney Harbour Bridge.
MR BRADFIELD has designed a suspension bridge, a cantilever bridge, and an arch bridge for Sydney. For a long span bridge of this character the bridge should have the maximum amount of rigidity vertically under the rolling load and laterally under wind pressure, so that by its freedom from vibration – when carrying the maximum traffic, or when resisting the pressure of a raging gale – it may gain the confidence of the public and have the reputation of being the most rigid and the strongest bridge in the world. The greatest economy consistent with the ultimate usefulness of the bridge must obtain, but utility of purpose should be leavened – not dominated – by aesthetic considerations. Naturally its structural relationship to the city as a whole and its place in the surrounding landscape must be taken into account.
Each of Mr. Bradfield’s bridges is designed for exactly the same loading, and with the latest steel available best suited to each type. Under full live load the arch bridge will have a deflection of seven inches at the centre, the cantilever bridge 13¼ inches, and the suspension bridge 11ft 3in. The arch or the cantilever bridge will stand four-square to every breeze that blows, but the stiffening truss of the suspension bridge, hung from the straight wire cables by wire ropes and cradled within the towers will rock to every wind as it sings its lullaby.
A suspension bridge with stiffened eyebar cables would not have as much movement as a suspension bridge with straight wire cables, but its appearance would not be as handsome as either the arch or the cantilever bridge, whilst its deflection under live load and temperature would be at least six times as great as either of these bridges, which were selected on account of their greater rigidity as bridges best fitted to withstand the wrack of heat and cold and storm and the stress of passing loads.
The arch bridge, two or three hinged, may prove to be the cheapest bridge and , taking local conditions into account, perhaps cheaper to erect, whilst the curves in the approach to the arch bridge will be much easier than can be with either a cantilever or a suspension bridge.
Romance is about to become a reality. Tenders close on January 16th next and the one outstanding thought in the minds of our visitors is where will the contract for the bridge be placed? Part of it in New South Wales, for a certainty; but where the rest? In the lowlands of Scotland, the North Country of England, Flanders Fields, the Land of the Maple Leaf, or that of the Stars and Stripes. We shall know before very long now, but not immediately after the date named, for it will take some time to go through the various tenders and decide definitely as to the form of the structure – arch, cantilever, or suspension. But there can be little doubt that the matter will be settled at the earliest possible moment. – KATHLEEN BUTLER
Each bridge is designed to carry four lines of railway traffic, six lines vehicular traffic and two footways. Each track is to accommodate a train 1000ft long headed by two electric locomotives weighing 160 tons each. Bradfield thesis.
