Bridges

The bridge on Burns Bay Road over Tarban Creek in the inner western Sydney suburb of Hunters Hill is an iconic structure in Sydney Harbour. The main span of this bridge is 90 m long and comprises five prestressed concrete ribs. Each rib consists of two inclined portal legs supporting a cantilever beam. The beam and the portal legs form an elegant arch-shaped structure. Cantilever beam extensions support the adjacent approach spans at half joints. Severe corrosion was identified in the tendons of each portal leg and a retrofit was carried out in 1970. This retrofit was investigated in 2004 because of maintenance concerns and increasing traffic loading on the bridge.Due to the high risks it was decided that further strengthening of the portal leg structure was justified. Bridge inspections have also revealed that the bearings and cross girders at the half joints located at the end of the cantilever beams are severely damaged and require replacement. RMS Bridge and Structural Engineering initiated and developed an innovative support scheme design. In this design, the cantilevered beams of the main span will be supported and jacked from new permanent pier frames to precompress the portal legs. This will balance unacceptable tensile stresses under serviceability loadings and thereby strengthen the arch shaped structure. This design is currently under construction and has substantially lower cost and provides easier constructability than other proposals considered in the past. The new pier frames provide a permanent working platform for rehabilitating the half joints and for their inspection in the future.This paper outlines the lessons learnt from the various retrofit design options investigated and describes the adopted innovative support scheme design to strengthen the structure.