Menangle Viaduct
The historic Menangle Railway Bridge carries the Main Southern railway line across Menangle Road and the Nepean River between the stations of Menangle Park and Menagle stations in New South Wales. The river is subject to sudden and severe flooding.
It was the first large iron railway bridge on the New South Wales Government Railways network, and survives as the oldest metal railway bridge in NSW. The ironwork was fabricated in England by the Canada Works in Birkenhead and despatched to Australia in two ships in October 1861. Unfortunately, one of the ships, carrying the girder material for the first and third spans, was wrecked shortly after leaving the Mersey. Replacement ironwork arrived in NSW in late 1862. There were three pairs of Fairbairn-type box girders, each of 150 ft span, joined to by plates give a continuous beam 486 ft (148.2m) long.[1] [2]
The fundamental design was by John Whitton, and the detailed design and supervision in England was undertaken by John Fowler acting on behalf of the NSW Government, working in conjunction with Peto, Brassey and Co. The box section girders were 9 ft high and 20" wide, with cellular box section members above and below the side girders, these each having two cells 18" square. At intervals of 3 ft there are internal cross plates at top and bottom joining the pairs of side plates, leaving a space 2 ft high at mid height (presumably allow access for inspection before closing the ends of the girders). The girder plates were mostly 1/2" thick. The iron spans were approached by timber viaducts. This information is condensed from an interesting 1863 article in 'The Engineer', which also describes the broader contest, the method of testing and the measuremnt of deflection of the girders.[3]. The outside of the box girders features decorative arch ribs.
The bridge opened on 1 July 1863 with the line from Campbelltown to Picton. In 1907, the bridge was strengthened with intermediate piers at mid span, to accommodate increased axle loads. The strengthening is interesting, given that shortly after construction, John Whitton was criticised for being extravagant and making the bridge too strong! The design and expenditure were defended in an editorial in 'The Engineer' in 1864, which closed with the view that 'In such matters the colonists will yet find that wisdom in pence may become lamentable folly in the matter of pounds.'[4]
Note: For similar surviving bridges in the UK, see Torksey Viaduct and Findhorn Viaduct (Forres).