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Concrete In Australia : September 2014
Concrete in Australia Vol 40 No 3 31 shrinkage, particularly the difference in creep and shrinkage effects between the eastern and western carriageways. One significant effect was the offset horizontal centroid of the sections, which varied from 400 mm for the typical segments up to 800 mm for some segments in the bifurcation area. This resulted in rotated principal axes ranging from 2° to 8° to the horizontal and stress variations of up to 8 MPa across the width of the top slab. In extreme cases, non-symmetrical post tensioning was utilised and not capturing these effects would have led to an inadequate design. Movement joints To accommodate movements due to temperature, concrete creep and shrinkage and to relieve loads in the superstructure and substructure, expansion joints were required every four to five spans. To enable balanced cantilever construction to proceed without disruption, mid-span joints with needle beams were adopted, the first time that this approach has been used in Australia. With expansion joints at mid-span, the steel needle beams span across the joints and maintain the relative attitude of the ends of the two cantilevers, while allowing longitudinal expansion as shown in Figure 11. The needle beams are supported on bearings at internal diaphragms in the girders. The beams are placed inside the box girder after the completion of the first cantilever, then pulled back into the girder. When the second cantilever and the continuity stressing has been completed, they are pulled back across the joint and secured in place. Moments in the needle beams cause equal loads in each of the bearings but shear causes unequal loadings. Because the shear at mid span is usually low this is not a significant issue, but large torsions can be generated by live loads, in particular if live loads are placed on the north eastern and south western sides of a joint. This can create significant shears in the beams and differences in the bearing loads of up to 12 MN. To transfer the vertical loads from the bearings into the webs of the girder very substantial diaphragms are required. The bearings are supported on steel boxes cast into the diaphragms, and these boxes have large reinforcement bars welded to them, to carry the load up or down into the diaphragm itself. To transfer this load into the webs without creating unacceptable tensions in the diaphragm, additional reinforcement and draped prestressing was installed to supplement the reinforcement welded to the imbedded steel box. This post-tensioning runs through the webs and bottom slab of the segment and is anchored at the top, as shown in Figure 12. Conclusions The design and construction of the South Road Superway project provided a number of interesting challenges to the design and construction teams. The construction method adopted demonstrated an innovative solution, which overcame technical and geometric challenges and resulted in an aesthetically pleasing viaduct with limited maintenance required for its 150 year design life. The authors would like to acknowledge the contributions made by the entire design and construction team and the assistance provided by DPTI. The innovations outlined in this paper were a direct result of a successful design and construction team relationship. Figure 12: Needle beam diaphragm segment showing steel casing and draped prestress. CIA 40-3 FINAL.indb 31 CIA 40-3 FINAL.indb 31 26/08/14 9:19 AM 26/08/14 9:19 AM