Pheasants Nest bridges: 12 years after prestress retrofitting

The Twin Bridges over the Nepean River at Pheasants Nest were constructed in 1980 by the balanced cantilever method. The bridges are fixed at both abutments and have expansion joints in the middle of the central main span. The expansion joints have been sagging since the completion of bridge construction. Additional prestress was applied both in 1983 and in 1996 which partially corrected the sag. In February 2008, it was noticed that the high tensile anchor bars that fixed the bridge deck to the southern abutment of the southbound bridge had failed. The failure of the anchor bars presented an opportunity to analyse the bridges for the long-term effects of creep and shrinkage together with the effects of prestress applied at various stages. The actual deflections of the expansion joints, which have been monitored on a yearly basis since the bridges were constructed, were compared with those predicted from the bridge analyses. In 2008, concrete temperature readings of the southbound bridge were recorded to assess thermal effects. This paper presents the findings of the above analyses, compares the results of the analyses with field observations and discusses the likely causes of the failure of the abutment anchor bars.