| Historical notes: | Timber Truss Bridges in NSW:
The development of bridge technology and design was relatively static for thousands of years, with either simple, short bridges built of timber beams across a stream or substantial stone masonry arch bridges spanning rivers. In Australia, stone (and brick) arch bridges provided the major form of bridge until the middle of the nineteenth century, after which, local engineers turned to timber truss bridges to provide the majority of river crossings, with the (expensive) imported wrought iron bridges reserved for railways and the larger and more heavily trafficked roads. Although timber beam bridges are limited by the dimensions of available materials, timber offered the cheapest and quickest bridge solution. In NSW, the availability of excellent hardwoods provided Public Works engineers McDonald, Allan, Dare and De Burgh with a uniquely strong and durable material for timber truss bridges. Timber beam bridges served NSW well for 150 years as relatively inexpensive structures to aid the movement of goods and people.
The emergence of steel in the latter half of the nineteenth century provided a cheaper, stronger and more adaptable material for bridges than cast or wrought iron. It was rapidly adopted world-wide, its application limited only by its relative cost. In Australia, this meant that its use continued to be constrained until after local manufacture commenced in 1915. In response, the timber truss bridge designs in NSW evolved after 1899 to include steel members in critical locations such as bottom chords, whilst continuing to utilise timber for the majority of the bridge structure.
There were five main types of timber truss bridges erected in NSW, distinguished by the evolving arrangement of the primary truss members. The five types are:
1. Old Public Works Department Truss (PWD) - A basic truss bridge, based upon English models, in use from 1860 to 1886. It took advantage of the local hardwoods for its main members and was a solid and durable design
2. McDonald Truss - Built from 1886 to 1893, the McDonald truss improved upon the Old PWD type by addressing several of its particular shortcomings. These included the placement of cast-iron shoes at the junctions between timber beams, the end members were doubled and splayed for better lateral stability and wrought-iron rods were utilised for vertical tension members.
3. Allan Truss - Built from 1893 to 1929, the Allan type also used cast iron connection pieces and vertical iron rods but was a significant improvement on the McDonald type, with most main members doubled and spaced, a simplified tensioning system and using smaller individual pieces of timber.
4. De Burgh Truss - The De Burgh Truss was built from 1899 to 1905. This truss was a composite truss, utilising timber and steel in combination. It was distinguished by the use of pin-joints in the connections between the steel bottom chords and the steel diagonal rods.
5. Dare Truss - The Dare Truss is very similar to an Allan truss but used steel bottom chords. Designed by Harvey Dare and built from 1905 to 1936, the Dare Truss incorporates the best features of both the Allan Truss and the De Burgh Truss, whilst eliminating the pin-joints of the latter that proved problematic in maintenance. The Dare Truss was the most successful of the timber/steel composite trusses.
Old Cobram-Barooga Bridge, uses De Burgh Trusses. The De Burgh Truss is unique amongst the five timber truss types in NSW, as it was the first to depart from the process of evolution from the previous 'standard type'. The defining features included the 'Pratt' truss arrangement, with timber vertical posts, timber top-chords and steel rods as inclined tension members, bottom chords formed by continuous parallel steel plates, steel plates and diagonal rods connected to the bottom chords by turned pins.
Old Cobram-Barooga Bridge was also a movable lift-span bridge. Opening bridges were required in order to provide clearance for masted vessels. For the inland river system of the Darling, Murray and Murrumbidgee, the majority of craft were paddle-steamers and loaded barges. These craft were typically not tall and could pass under most bridges when water levels were low. However, during high water level periods, particularly floods, additional headroom was required and this was provided by an opening span over the main channel. These telescopic bridges were only suitable for small openings and, where larger ships were operating, swing-span bridges were used. Opening-span bridges were built from 1890 until 1941, after which no new opening bridges were erected on the Murray River.
Agriculture, Navigation and the Commonwealth:
The context for the design and construction of the Old Cobram Bridge was the historical transition of the Murray River from being used primarily for navigation to being used primarily for irrigation water supply. During the 1870s, the closer settlement of the Riverina region for agriculture was struggling with the vagaries of climate and water in western NSW. Selectors obtained land and, in optimal conditions, produced excellent yields, with wheat predominating over other crops. However, rainfall was irregular and tended towards periods of oversupply (flood) followed by long periods of undersupply (drought). Farmers quickly realised that, without some regularity of water supply, their future was doubtful. Consequently, from the mid 1880s, the possibilities of land irrigation were closely investigated.
George Chaffey, an engineer brought out from California, commenced designing a massive irrigation scheme at Mildura in 1887. Whilst this scheme had its own story of success and failure, it represented a concerted state investment to create a permanent agricultural settlement along the Murray River and to institute a means of exploiting the Murray River for agriculture. In 1893, representatives of the three states met in Melbourne and considered the matter of installing locks on the Murray River to preserve supplies for irrigation, whilst permitting ongoing navigation. The prospect of this damming of the river caused considerable concern in South Australia but, when the matter was raised, NSW denied that South Australia had any claim to the waters of the Murray; since no tributaries entered within its territory, South Australia had no rights to water beyond that which flowed across the border. These conflicts of interests tended to stymie any co-ordinated action and the matter of apportioning the Murray's waters between the states remained unresolved. As independent states in economic competition with each other, the idea of co-operative action for mutual benefit was rife with complications and arguments.
Federation in 1901, however, meant that the three states in which the Murray flowed were no longer in competition and, as this coincided with extremely dry conditions in the Murray River regions, there was considerable impetus to address the matter. The government leaders decided that a tri-state Royal Commission should be set up to investigate the 'conservation and distribution of the Murray and it's tributaries for the purposes of irrigation, navigation and water supply'. The Royal Commission's subsequent report of 1902 recommended joint control of the Murray by the three states and a joint funding arrangement for water conservation infrastructure, such as dams and weirs. By the beginning of the twentieth century, this transition was well underway, with river navigation trade almost completely moribund by the 1930s. However, during the early twentieth century, the option for river transport was preserved by, amongst other measures, the continued construction of opening bridges across the Murray River.
Erection and Operation of the Old Cobram-Barooga Bridge:
Public meetings were held in the district almost every year from 1894, urging the local members of Parliament to persuade the NSW and Victorian Government to approve the construction of the bridge. By November 1899, Government Ministers in both states were able to report that decisions had been taken to proceed with the bridge and that specifications were being prepared. In 1900, after question as to the erection of a bridge over the Murray River at Cobram had been under considerable consideration, it was decided that the bridge would be financed and built by the Victorian government. Once completed, it appears that the Victorian Country Roads Board was responsible for maintenance of the Bridge as well.
The construction of the bridge across the Murray River at Cobram-Barooga became evidence for the value of the wool industry in the vicinity and of the economic flows of goods between NSW and Victoria in the late nineteenth century. The story of Cobram-Barooga is representative of the story of the development of the Murray River generally and illustrates the competition between townships for a bridge as a formal border crossing and as a guarantee of ongoing economic development. |