During the colonial period, and particularly following the discovery of large quantities of gold in many places during the 1850s, Australia’s population increased rapidly and became widely dispersed. Road bridges were needed and the lowest cost and most readily available structural material for them in New South Wales was timber. At one time there were over 4,000 timber bridges in the state. New South Wales was sometimes known as the timber bridge state.
To meet the need, engineers in the Public Works Department (PWD) developed designs of timber truss bridges of the kinds illustrated in this article. 407 timber truss road bridges were built in NSW between 1861 and 1936 by the PWD. 63 remain, 48 of them have been maintained by the former NSW Roads and Traffic Authority (RTA) now the NSW Roads and Maritime Service (RMS), and 15 by local councils. RMS has plans to conserve 25 of these for the long term and to retain them in use. The ones retained would be a representative sample of the main types and would be widely distribute around the state.
A succession of engineers in PWD introduced improvements in the design of these bridges, and their designs were original and unique to NSW. They are an important example of Australian technology development and engineering history. In a recent ASHET project two retired Chief Bridge Engineers with the NSW Roads and Traffic Authority, Brian Pearson and Ray Wedgwood, give an oral history interview which summarises the history and gives an up-to-date outline of the efforts and plans to conserve a representative sample of the bridges and to maintain a permanent record of their design and history. A transcript of the interview is now available. The interview was conducted and the transcript prepared by Frank Heimans.
The project has been assisted by funds allocated by the Royal Australian Historical Society through the Heritage Branch of the NSW Office of Environment and Heritage.
A small number of timber truss bridges were built by the NSW Railways, but John Whitton, Engineer-in Chief from 1856 to 1890, had a strong preference for following British practice and building railway bridges in iron or masonry wherever funds permitted.
History of timber truss bridges in NSW
PWD on the other hand had insufficient funding to build a large number of road bridges in iron or masonry. However NSW had ample and widely distributed sources of strong and durable hardwood. The first timber bridges in NSW were simple beam bridges with timber decks, but were suited only to short spans. William Bennett, PWD Engineer for Roads from 1862 to 1869, introduced his own design of timber truss for road bridges, based on a Palladian design of truss widely used in building. They became known as the Old PWD truss. 147 were built but only two survive. They were single lane bridges, designed for a load of 16 tons and had spans up to 90 feet.
In North America, where timber was plentiful, timber trusses were quite common and several types of timber truss were developed. American bridge technology did not reach NSW until the early 1870s when two bridges of an American design known as the McCallum truss were built that had 150 foot spans.
J.A. McDonald succeeded Bennett at the PWD in 1886 and a number of improvements to the old PWD truss resulted. The McDonald trusses, of which around 90 were built and 5 remain, allowed for higher loads, and had features to ease replacement of individual bridge members and combat timber shrinkage. McDonald also introduced composite construction in timber and steel for the longer span bridges. The bridge at Cowra replacing an earlier McCallum truss bridge had three 160 ft spans with steel lower chords.
NSW bridge design gained further pace under Percy Allen, who followed McDonald in 1893 as the principal bridge designer. He was able to make good use of American technology to achieve efficient truss design, and make effective use of the results of Professor Warren’s timber testing program at Sydney University. He introduced many design features that reduced costs and improved durability and maintainability.
Following American practice, Allen employed an American Howe truss design that had vertical steel tension members and diagonal timber compression members. The timber members employed two parallel timbers spaced apart, adding to strength and stability, while allowing the use of smaller and cheaper timber sections. This also facilitated maintenance as individual timbers could be readily replaced. Cast iron clamps and shoes were employed at joints. Stability was assisted by employing deep trusses with the top chords joined by cross members. 105 Allen truss bridges were built between 1893 and 1927; 19 survive.
Bridge engineers in the PWD who succeeded Allen made further innovations in timber truss bridge design, and the names are associated with the bridges they designed. From 1899 E.M.DeBurgh designed Pratt trusses with steel bottom chords and with construction details that added to stability and maintainability. His designs were best suited to long spans, and Allen trusses continued to be used for the shorter span bridges. There were 20 DeBurgh bridges built and 9 survive.
Harvey Dare redesigned Allen’s truss to incorporate a steel bottom chord and simplify the bottom chord joints. After 1905 there were no further DeBurgh trusses built and nearly all the truss bridges from then on were of composite timber and steel known as Dare trusses. In all 44 were built and 13 survive. The last timber truss bridge in NSW was built in 1936.
From the end of World War I steel trusses were generally used for spans over 30 ft. and steel beams for those over 20 ft. So there were very few new timber truss bridges. The first reinforced concrete bridges appeared at the end of War I, and eventually became the most usual material for new and replacement road bridges in NSW.
The demise of timber truss bridges was in part a result of the need for bridges to carry heavier loads, for which steel and concrete proved more economical. Other factors were the shortage of suitable timber and of the skilled carpenters needed for construction and maintenance of timber bridges. Once the superiority of steel and concrete bridges was established, timber truss bridges have been progressively replaced. The ones that survive are mostly in locations where traffic conditions can still be met with single lane bridges with low load capacity. Some of the timber truss bridges have been upgraded by adding steel members or replacing timber with steel. Durability has been increased by installing laminated timber decks in some bridges.
Conservation of timber truss bridges
It is remarkable that in a period of development of new engineering materials and technology, timber truss bridges were built in NSW for nearly 100 years. This happened only because the original design as incorporated in the Old PWD bridges dating from 1861 was progressively improved by the efforts of a succession of highly competent and innovative engineers in the NSW PWD.
These bridges represent an important part of NSW engineering history and heritage. This has been widely recognised and RTA (now RMS), which owned most of the remaining bridges, has proposed a strategy for conservation which involves retaining 25 of the 48 that it currently owns, and divesting the remaining 18. 12 of the 18 proposed for divestment are on the State Heritage Register, so RTA does not necessarily have a final decision on their demolition or transfer to a local council. RTA published a report in July 2011 that includes details of the studies that led to its proposed strategy, and submitted the report to the State Heritage Council as well as inviting public comment.
The Heritage Council formed a committee to review the proposed strategy and having received a draft of the report several months in advance of publication, was able to release its own report also in July 2011. The Heritage Council stopped well short of endorsing the RTA report but recommended that subject to a number of conditions the Heritage Council should advise RTA that its proposed strategy is an appropriate basis for community discussion and for the various notifications required under the Heritage Act. The Heritage Council noted that while divestment by RMS of redundant bridges offered some possibilities for their retention such as adaptive reuse or transfer to a local council, they would rarely be feasible and demolition was the most likely outcome.
It seems clear that a representative sample of the main types of timber truss bridges will be retained, and that these will be under the ownership and management of RMS. But in the absence of an agreed strategy and any guarantee of government funding the future of individual bridges is uncertain.
The oral history interview with Brian Pearson and Ray Wedgwood provides an excellent brief summary of the history of timber truss bridges and their heritage significance. These two former Chief Bridge Engineers have a detailed knowledge of the engineering, and were members of the National Trust review team so their knowledge of the conservation aspects is right up to date.
For those seeking further information, Don Fraser’s 1985 article in the Transactions of the Institution of Engineers Australia provides a detailed account of the engineering of these bridges, with an extensive set of references. It is not available on line. The RMS report and the Heritage Council review are both available on line. Also on line is a video made by RMS showing the recent restoration of the timber truss bridge at Dunmore.
Sources and further reading
Fraser D.J., Timber bridges of New South Wales, I.E.Aust Multi- disciplinary Transactions, 1985.
Oral history interview with Brian Pearson and Ray Wedgwood, October 2012, audio record held in State Library of NSW oral history collection. See link to transcript below.
Timber truss road bridges: A strategic approach to conservation, NSW Government Transport, Roads and Traffic Authority, July 2011, www.rta.nsw.gov.au/newsevents/2011/2011_07_timber_bridges. html
Timber Truss Road Bridges of NSW: review of Roads and Traffic Authority’s proposed approach to conservation, Heritage Council of NSW, July 2011. www.environment.nsw.gov.au/…/ TimberTrussBridgeStrategy.pdf
The project was been assisted by funds allocated by the Royal Australian Historical Society through the Heritage Branch of the NSW Office of Environment and Heritage.
A small number of timber truss bridges were built by the NSW Railways, but John Whitton, Engineer-in-Chief from 1856 to 1890, had a strong preference for following British practice and building railway bridges in iron or masonry wherever funds permitted.