Brief Background to the Bridges of Gundagai
There are two significant heritage listed timber bridges in Gundagai – an old road bridge and a rail bridge.
Nowadays there is no railway line and the highway by passes the town. There was a very major flooding of the area in the 1852 which resulted in 89 deaths or about a third of the population of the town. The town was rebuilt on higher ground. The building of the road and rail bridges were forced to be raised above the flood plains of the Murrumbidgee River. The flood plains are wide and this resulted in the bridges being of such a length.
Some Structural Features
Both bridges used steel trusses to span the main channel of the river but the many approach spans across the flood plains used Australian hardwood in tall trestle piers. The road bridge approach spans were shorter so timber beams were used whereas timber trusses were employed for the rail bridge with the tracks on the top deck.
Other timber structures exist in Gundagai such as the supporting tower for the water tanks used to fill the steam engines at the station.
The heritage listed timber truss railway bridge comprises 77 timber trusses based on the American Howe truss concept.
Each truss is approximately 35‐40 feet long and the single rail track is on top of the bridge. These trusses are supported well above the flood plain of the Murrumbidgee River on tall timber trestles.
Locally sourced materials were the preferred choice in the era when the railways expanded rapidly in Australia. Australian eucalypt trees, and in particular the “Ironbark” variety possess great strength and stiffness.
Solid timber members are used in the trusses and trestle supports to carry compression loads and in the deck where bending resistance is required. Iron rods are used to carry tension forces. Careful checking of the timber components means that most of the forces are transferred by direct contact.
All images by Peter Kneen
