Bridges

A feature of the central part of the 1000 km Bibbulmun Walk Track through natural forested areas of south-west Western Australia, had been the re- use of a disused railway bridge over the Murray River at Long Gully until this timber trestle bridge was burnt down in the bushfires of February 2015.

A new site was chosen for the replacement bridge to cross the Murray River some 12 km downstream, relocating part of the track to more appropriate forest park land with river and forest vistas and points of interest. The chosen site allows for 100 year ARI floodwaters to pass under the suspended superstructure with an 82 metre span and minimal impact on the natural surroundings. Life-cycle cost comparisons demonstrated that this would be more economical than a shorter bridge that could be vulnerable to washout in floodwaters due to debris build-up or impact.

Due to the remoteness of the site, the bridge needs to be relatively fire resistant and maintenance free. The design philosophy differs from that of the Australian Bridge Design Code in that a far more relaxed approach to dynamic behaviour is accepted. Instead the structure is compliant with AS 2156.2 “Walking Tracks: Infrastructure Design”.

The design philosophy of foundations, including deadman anchors on slopes of granular soil with boulders, is discussed in terms of preferred modes of failure for safety and sustainability.

The suspension bridge design features the innovative use of weathering steel plate cold-formed into structural sections. This not only aids in visual blending of the structure in its surrounding, but is considered a practical means of controlling corrosion and fire risk without the environmental impact of paint. Other sustainability features discussed in the paper include the use of dual suspension cables, FRP gridmesh decking, proprietary containment mesh and proprietary removable huck-bolts.