Parallel chord through trusses
The Hampden Bridge was constructed in 1895. It was the first of engineer Percy Allan's 33.5 m span timber truss bridges.
Percy Allan designed the third of the five standard timber truss bridge designs developed in NSW. While there was a clear line of development between the first two standard truss designs, Allan completely reconsidered and redesigned the form of the timber truss bridge in NSW. He based his design on the American Howe bridge but exploited the superior strength and performance of Australian ironbark to reduce it to its barest essentials. In doing so he devised an efficient and graceful truss bridge design, similar to designs recorded by Palladio.
Allan truss bridges were substantially simpler than their predecessors. They used less material, were easier to repair and maintain and enjoyed long operational lives. All the timber elements were pairs of members and the joints allowed for loads to be shed onto just one member if need be. As a result, any individual piece could be replaced easily when it became unserviceable.
The original Allan truss design had a 27.4 m (90') span and a 4.6 m (15') carriageway. It was highly successful and bridges of this type were constructed as late as 1929. In 1895, Allan upgraded the design to span 33.5 m (110') with a 7.4 m (24') carriageway. He used it initially for the Wagga Wagga bridge, then for other bridges throughout NSW. The longest timber truss vehicular bridge built in Australia was an Allan designed truss bridge constructed at Kempsey in NSW in 1900. It had four spans of 46.6 m (153').
The Hampden Bridge was originally designed as a steel structure. However, the tenders were unacceptably high and the timber alternative was developed. With an intended economic life of about thirty five years, it served as the principal river crossing for the City of Wagga Wagga until 1995 when it was decommissioned.
Allan truss bridges remain in service in various parts of NSW. The most accessible trusses form the approach spans for the old Darling Harbour bridge in Sydney.
City of Wagga Wagga
Across the Murrumbidgee River at Wagga Wagga, NSW
The bridge is a Howe through truss with timber top and bottom chords, timber compression diagonals and vertical steel tension rods. It has three 33.5 m (110') truss spans and nine trestle and beam approach spans. All the timber elements in the main spans are spaced pairs of members while timber to timber joints are cast iron compression seats.
The design is efficient in materials. The sizes of members reduce towards the centre of the truss spans. The tie rods reduce from three rods in the panels closest to the supports, to a single rod each side of the centre panel. The top and bottom chords are spliced with steel connection plates. The bottom chord splice plates incorporate vertical square ribs to transfer tension more efficiently while the top compression plates are simple spacing and locating plates.
Allan, Percy 1897, 'The Wagga Wagga Timber Bridge, NSW' Minutes of Proceedings, Institution of Civil Engineers, vol. 128, pp. 222-225
Department of Main Roads NSW 1987, Timber Truss Bridge Maintenance Manual, Department of Main Roads NSW, Sydney.
Fraser, D. J. 1985, 'Timber Bridges of New South Wales', Paper G1169, Multi-disciplinary Engineering Transactions, The Institution of Engineers, Australia.
O'Connor, Colin, 1985, Spanning Two Centuries: Historic bridges of Australia, University of Queensland Press, St Lucia.
Palladio, A. 1965, 'The Third Book of Andrea Palladios Architecture', The Four Books of Andrea Palladios Architecture, reprint of 1738 editions published by Issac Ware, London, Dover Publications, New York, Chapter VIII, Plate 4, p. 66.
1. Top chord, 2/450 x 150 mm
2. Bottom chord, 2/330 x 175 mm
3. Web, 2/435 x 170 mm
4. Web, 2/330 x 170 mm
5. Web, 2/285 x 150 mm
6. Cross beam, 400 x 350 mm
7. Strut, 2/300 x 125 mm
8. Tension rod, 65 mm Ø
9. Cast compression seat
10. Suspension plate and bolts
11. Graded packer
a. The Hampden Bridge over the Murrumbidgee River