Our Harbour Arch Bridge: The World’s Record.
After the closing of tenders on 16 January 1924, the team of Bradfield, Butler and Stuckey had worked intensively to produce a recommendation to the Minister for Works and the Cabinet by the end of February. Simultaneously Butler seems to have produced this article about the winning tender and thus the bridge that would be built, to allow publication in The Sydney Mail on March 12. The more usual by-line on these articles is 'Butler from the notes of Bradfield', but this article is credited:
By Miss Kathleen Butler, confidential Secretary to Mr. J. J. C. Bradfield, the Bridge Engineer.
The images of the bridge chord cross section or the main bearing pin have not been noted as appearing in any other of Bradfield's papers, so perhaps the piece is more Butler'a alone than other articles. Butler's style of listing off bridge specifications is noted elsewhere.
CABINET (as already intimated in the “Mail”) has approved the Chief Engineers recommendation, and has accepted the tender of Messrs. Dorman, Long, and Co., of Middlesborough, England, for a two-hinged arch bridge in accordance with Mr. Bradfield’s plans and specifications on which world-wide tenders were called, when tenderers had the option of submitting tenders for both arch and cantilever bridges.
Hell Gate arch New York, of 977ft 6in span, at present holds the world’s record in arches. Six years hence “Our Harbour” arch of 1650 feet span will with a majestic sweep of steel gracefully span the harbour and add to its natural beauty.
The silicon steel arches of the main bridge will be 98ft 6in apart, centre to centre, set in vertical planes; the span centre to centre of bearings will be 1650ft and the rise 350ft. At the crown the arch will be 60ft deep, increasing to 190ft over the main bearings. The highest point of the arch will be 445ft above water level – about the same height as Pymble Railway Station.
The four main pins are each 10ft 6in long and 4ft 4in in diameter.
From left to right the figures are: Mr. G. C. Imbault, engineer, Dorman. Long , and Co.; Miss K. M. Butler, secretary to Mr. J. J. C. Bradfield; Mr. J. J. C. Bradfield, chief engineer, Sydney Harbour Bridge; Hon. R. T. Ball M.L.A., Minister for Public Works and Railways; Mr. L Ennis, general manager for Dorman, Long and Co. SARA NRS 17420.
The maximum pressure on each of the four steel pin bearings due to the weight of the bridge itself under the heaviest loading it is designed to carry, and to wind pressure is 43,000,000lb. these huge pin bearings, 10ft 6in long and 52in in diameter, will transmit this 43,000,000lb thrust through steel pedestal bearings weighting 260 tons each, to the granite masonry abutments. Each steel pedestal where it bears on the granite will have a surface area of 17ft 6in x 22ft., i.e. 385 square feet; and the pressure which the granite skewbacks have to withstand is reduced by means of these pedestals to 777lb per square inch. Moruya granite has a crushing strength of 18,000lb per square inch, so there is ample factor of safety against crushing. Through the granite skewbacks the thrust will be transmitted to mother earth and every train and every vehicle passing over the bridge will tend to thrust the shores apart, but once linked with fetters of steel and stone Dawes Point and Milson’s Point will not be separated such puny efforts. Of the 43,000,000lb thrust on each bearing 73 per cent. is due to the weight of the bridge itself; 15 per cent. when the bridge is carrying its full live load; 11 per cent. is due to wind; whilst the effect of the brakes when the trains at full speed have to be stopped on the bridge accounts for less than ½ per cent., and the rise and fall in temperature for the other ½ per cent.
Each arch is 10ft 6in wide; the top chord is 40in deep; the bottom chord varies from 48in to 108in in depth; the thickness of the webs in the bottom chord are from 2¾in to 4½in.
Standing between the webs are Mr. J. J. C. Bradfield, the Harbour Bridge Engineer, Mr. R. T. Ball, and Mr. L. Ennis. The heaviest member of the bridge will have more than three times the sectional area than the heaviest member in the Forth Bridge has, the figures being 2538 square inches and 800 square inches of steel. SARA NRS 17420.
There will be 50,300 tons of steelwork in the completed bridge. At the centre of the arch the bottom chord has a sectional area of 1178 square inches, or over 8 square feet of steel, which increases to 2538 square inches at the main bearings; to cut through this member one would have to sever 17½ square feet of steel. The heaviest member of the Forth Bridge has an area in section of 800 square inches; In the Hell Gate arch bridge, 1392 square inches; in the Quebec Bridge – the longest span cantilever bridge in the world – the heaviest member has an area of 1941 square inches; whilst the heaviest member in Sydney’s arch will have a sectional area of 2538 square inches, or more than three times the area of the heaviest member in the Forth Bridge. The weight of our bridge at the centre, including the decking, will be 24.4 tons per foot run, increasing at the abutments to 66.2 tons per foot run. The Sydney Harbour Bridge will have members heavier than those in any bridge in the world; it will have a span 672 feet greater than the Hell Gate arch.
THE architectural features of the bridge are the granite faced towers and pylons, and the arch, graceful at the crown, shows depth where it is wanted. The floor line is well marked from end to end of the structure; the arch rib, beautiful in its strength and its simplicity, demonstrates clearly its purpose, taking the eye down to the abutments on either side without camouflage, without interruption. The tower and pylons will be constructed of reinforced concrete and granite rock-faced ashlar masonry with plinth and surbase. Above the deck will be the reinforced concrete pylons, each pierced with an archway through which the outer railway track and footway will pass. The pylons measure 93ft a 50ft at their bases, tapering to 75ftx 34ft at their summits 310 feet above high water. The top of the tower on the Church of England Grammar School, North Sydney, the tops of the bridge pylons, and the top of the tower of the General Post Office, Sydney, will be approximately at the same level.
Dorman, Long, and Co’s. tender for the arch bridge without the abutment towers is £3,499,815; and with the abutment towers, faced with granite masonry, the accepted tender is £4,217,721 11s 10d. The arch bridge without towers is simple and elegant, but would be too severe for its setting.
The bridge provides for a main roadway 57ft wide between kerbs, with two lines of railway on either side of the roadway, also two footways. When the bridge is opened for traffic, the inner pair of railway tracks may be used temporarily for tramway purposes, until electric railways are built to Mosman, Manly, and Narrabeen, the existing tramways being availed of to convey passengers from these localities into the city until the railways are constructed.
Mr. Bradfield’s estimate for the arch bridge, providing for Australian and British steel, and fabrication complete at Milson’s Point, is £4,339,530. His estimate was submitted to the Minister sealed, and was opened when the tenders were opened. Dorman, Long, and Co’s. accepted tender for the arch bridge – £4,217,721 11s 10d – also provides for Australian and British steel and complete fabrication at Milson’s Point. The next lowest tender for the arch was that of Sir William Arrol and Co., providing for 33 per cent. fabrication in Sydney, the remainder in Glasgow, the tendered price being £4,645,351 7s 8d. the Chief Engineers estimate thus comes between the two lowest tenders.
Dorman, Long, and Co’s. tender provides for fabricating the bridge complete at Milson’s Point, and for using as much Australian steel as the Australian rolling mills can produce to comply with the specification. The bridge should contain 50 per cent. Australian steel, and 50 per cent. British steel and will be fabricated wholly in Australia. The value of the plates to be imported from England will be but 8.4 per cent. of the amount of the contract, and these plates cannot be manufactured in Australia at the present time. To manufacture these plates would necessitate the erection of rolling mills and steel production plant costing £3,000,000 sterling; the tonnage of plates used in Australia yearly would keep this plant in operation for six weeks out of the year.
THE piers and abutments will be built of Moruya granite, Nepean River sand, and New South Wales cement, and in this connection the following figures may be interesting: – Australian steel totalling 25,000 tons may be used in the bridge. To produce this steel, 38,000 tons of iron ore, 7000 tons of limestone, 1,800 tons of dolomite, and about 90,000 tons of coal will be required. For lining the furnaces about 10,000,00 fire and silica bricks will be required. Cement approximating 540,000 bags, or 30,000 tons, will be required, the manufacture of which will necessitate the mining of 60,000 tons of limestone, 15,000 tons of coal, and 12,000 tons of shale, Concrete, in addition to the cement above, will require 50,000 cubic yards of Nepean River sand and 100,000 cubic yards of granite metal broken from 1½in to 2½in gauge.
The masonry facing will require upwards of 30,000 cubic yards of Moruya granite, employing 150 men for a period of five years. One million feet super grey gum and ironbark timber will be required for the transoms and planking of the railway tracks.
At Milson’s Point 400 men will be employed in the fabrication of the bridge, and 500 men will be employed on the foundations, concrete and masonry works of the piers and abutments; whilst 200 men will be required to erect the steelwork. When the bridge is in full swing, including the men in the granite quarry at Moruya, between 1500 and 1800 will be employed. The stone, sand, cement, and timber required would weigh upwards of 260,000 tons, and if carried in 25-ton lorries, each requiring a running length of 26 feet, a length of road 260 miles would be required to accommodate the vehicles.
AS the main northern pier of the bridge will come where the present Milson’s Point station is, arrangements have been made to divert the railway traffic from this station to the new station already constructed on the foreshores of Lavender Bay. The station will be reconstructed so that the railway passengers will walk off the platforms to the ferry, as at the present Milson’s Point station. The tramway traffic will be diverted by a short length of tramway from Alfred-street along Dind and Glen streets, from which passengers will be conveyed to the ferry by means of an overbridge, 30 feet wide, and three reversible escalators each four feet wide.
These ae being constructed under the supervision of Mr. J. J. C. Bradfield, Chief Engineer, Sydney Harbour Bridge. Three complete truss frames are now in position, and work is proceeding on the station and yard arrangements. In the background can be seen a dump for spoil from the excavations in progress on the northern approach to the bridge and the tunnel from North Sydney Station.
