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This paper describes the evolution of large-truss road bridges in New South Wales (NSW), Australia, citing specific examples of various genres. In particular, the high proportion of iron bridges constructed in northern NSW over approximately a 25-year period from around 1870 is highlighted. The proportion here was greater than the rest of NSW for the same period. Various postulates are canvassed as to why that might have been so. With the onset of periods of financial astringency, the engineering profession had to adapt and make changes to take account of worsening economic conditions and political imperatives of the times. Typical of such major changes at the time was a dramatic swing from substantive iron road bridges to much more slender, but astutely designed, timber truss bridges. These colonially designed, but US-influenced, ‘lean and mean’ timber bridges were a far cry from the earlier, stockier, high-maintenance versions that were inherited from British/European designs. In some respects, such innovative local design was a symbolic way of releasing the restraining shackles of the colonial past and the growth of a nation. For over 40 years, these new style timber bridges, of various forms of successive improvement, dominated bridge construction in NSW to the extent that NSW was euphemistically known as the ‘timber bridge state’. It was not until innovations and improvements were made in steel production, steel fixing and concrete technology in the early 1930s that the newer materials started to replace timber.

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During the period 1855-1930, in NSW, Australia many larger timber bridges with spans up to 100 m and overall lengths of 500 m were designed and constructed with local timbers. To justify the safety factors of these structures, the early characterisation of Australian timbers undertaken in the late 1800s is compared with contemporary characterisations. In this paper, a historical background to the early characterisation of Australian hardwoods is contextualised and compared with a more contemporary approach to demonstrate how to assess extant timber bridges. The strength of round structural bridge girders can be inferred from their in-service elasticity, and such knowledge can be incorporated both in new designs and in maintaining those extant. As a girder deteriorates, its modulus of rupture against modulus of elasticity vector will follow a predictable path. By interpreting this path the current and predicted carrying capacity of extant heritage-listed timber bridges can be assessed by loading with a low-mass vehicle precluding the need for proof testing.