Bridges

The Captain Cook Bridge (CCB) is an iconic bridge crossing the Brisbane River and is a vital piece of infrastructure connecting the Brisbane CBD to the City's south. The CCB is a post-tensioned precast concrete box girder bridge 550 m in length. The bridge is an anchor cantilever type structure with ìdrop-inî suspended spans supported by halving joints providing the articulation. Since opening, loading on the structure has increased from three lanes of 33 tonne vehicles to four lanes of traffic, catering to 50 tonne vehicles. Inspections identified live concrete cracks at the halving joints and a set of expansion joint bearings had failed with the sliding material extruding from the bearings. Assessment of the halving joints confirmed insufficient structural capacity to carry current loads.

The deteriorating original sliding bearings were replaced with new spherical bearings designed to accommodate the in-service loads and movements of the bridge. These bearings were carefully designed to accommodate the bridgeís geometric constraints and limited height available to lift the structure. Load-measuring sensors in the bearings were also used to lower and set the bridge into its final position without overstressing the structure. Strengthening the halving joints involved thickening the box-girder webs and post-tensioning the existing bridge in three dimensions to form a composite section and provide an adequate load path from the newly placed concrete to the original concrete. Solutions were developed to ensure that construction could occur under live traffic as well as minimise the risks associated with coring into, and post-tensioning an existing prestressed concrete bridge. An ultra-high tensile strength posttensioning bar system was used. Development of early age set, and strength and very low shrinkage super workable concrete mixes were used. BIM and parametric models were also used to address the many construction challenges, characteristic of undertaking complex strengthening works on existing and significant structures.