Concrete Arch Bridges
It is an observation of no great insight that John Bradfield had a preference for arch bridges. Apart from the main span of the Sydney Harbour Bridge, he built a steel arch bridge at North Sydney to carry the eastern railway route onto the harbour bridge over the Bradfield Highway, an open spandrel arch over Lavender Street, concrete arches over Fitzroy and Burton Streets at Milsons Point, three concrete arches on the southern approach viaduct adjacent to Cumberland Street and an arch to carry the southern road and rail approaches over the Argyle Cut. Near Central the City Railway crossed three roads. Two of these - Campbell and Hay Streets are spanned by concrete arches and although Eddy Avenue bridge might at first be identified as an arch it is in fact a reinforced concrete beam. There is also a concrete arch over Euroka Street between Waverton and North Sydney. The underground spaces, for trains, platforms and pedestrian subways at St James and Museum stations are also concrete arches.
The use of reinforced concrete for larger bridges was not a long-practiced art in railway engineering in the early nineteen-twenties. A reinforced concrete arch had been built in 1899 at Hilltop, but it was an overbridge carrying road traffic, not trains. A couple of very small span (4.5m) bridges had been built at Glebe (1919) and Banksia (1925) to carry trains, but really they were encased steel girders more than ‘reinforced’ concrete. A simple reinforced concrete span for double track over O’Riordan Street, Botany, opened in October 1925 was technically the first reinforced concrete underbridge by a few months, but in such a public location and carrying six tracks the Hay Street and Campbell Street were a bold design, though perhaps not as bold as Eddy Avenue.
The skewbacks are mass sandstone concrete, carried to rock at about 27 feet (8m) down, all excavated by hand. Shear connection between arch rib and skewback was gained by stepping the joint, hand placement of irregular sandstone rocks in the surface of the wet concrete and steel starter bars in the planes of the arch reinforcement.
The arch rings were cast in ‘bluestone’ concrete in a single pour, though not for the whole width of the bridge. There was a limit to the amount of concrete which could be produced on site and placed by wheeled barrows. In particular, at Hay Street three mixers were used – two on the finished formation level either end of the bridge and a third at road level with the concrete lifted by an electric crane, one barrow at a time.
At Hay Street the arch rib is 24-inches (600mm) thick at the crown, reinforced on both faces by ⅞-inch (22mm) bars at 4-inch (100mm) centres. The shorter Campbell Street bridge is only 15-inches (380mm) thick with ¾-inch (19mm) bars at 8-inch (200mm) centres.
The bridge over Euroka Street at Waverton is built to the same plans as Campbell Street, without the sandstone façade, though the foundations are site specific.
