Bridges

Inspections and load ratings commissioned by WaterNSW discovered that the Bridge over the Nepean River was in poor condition, concluding that the bridge was unusable and needed replacing. With the Avon Dam Main Access Road being a critical route for the maintenance and serviceability of the dam as well as a fire access route, the construction duration of the new bridge needed to be short, whilst complying with the numerous site constraints. The Bridge design solution had to enable the bridge it to be constructed; within 19 weeks commencing September 2020; entirely with no plant or equipment in the waterway and no environmental impacts; form only one point of access, using a maximum 350t crane with a maximum reach of approximately 51m at an 8t capacity; to conform strictly with design standards, TMR and RMS B specifications. The 2.85m clear width existing bridge, comprised RMS planks, in-situ deck with 4 spans approximately 10m each. The replacement bridge was rebuilt on the existing mass concrete substructures, strengthened with rock anchors.

New precast headstocks were anchored into the substructures and enabled QLD TMR planks to be tied down. The bridge soffit level could not be lowered, and changes to the vertical alignment and road approaches were to be avoided. The bridge was designed to AS5100, for SM1600 vehicular loading, with dispensation sought to reduce the braking loads to enable the re-use of the existing substructures. The articulation was developed to enable a free-floating deck without imposing large longitudinal forces on the substructures. The paper will discuss the findings of condition inspections and reasons to replace the bridge, the bridge design approach and solution to achieve a short construction duration, whilst complying with environmental and construction constraints. It will present reasons for sustainably reusing and strengthening the existing substructure using Rock anchors and the challenges encountered. Additionally, it will discuss, maximising the use of precast components with design considerations for cranage limitations and construction tolerances for a modular style construction.