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Concrete In Australia : September 2014
Concrete in Australia Vol 40 No 3 27 Pier design The piers were structurally challenging. In the southern area, both carriageways rest on a single curved, Y-shaped pier. The superstructure is monolithic with the substructure, and the top of the Y is tied together with a post-tensioned cross beam, shown in Figures 1 and 4. This basic structural scheme is carried through all of the varying roadway conditions encountered along the alignment, the standard three-lane portions, the ramps and bifurcation areas and into the wider four-lane box girders. The global demand for the design was calculated using Bentley’s RM Bridge, Larsa 4D and Midas. These programs are multipurpose structural analysis software specifically written for bridge analysis, and allow for time-dependent staged- construction analysis, so that the erection scheme and schedule agreed with USJV could be incorporated into the analysis. The sequence and program for joining the cantilevers was particularly significant, and the closures for a given “frame” (i.e . a section of structure between expansion joints) had to be cast and stressed in parallel, so that continuity prestress from one girder did not “leak” into the other, and cause unequal prestress in the two girders. Nominal continuity prestress was applied to the east girder first, followed by nominal prestressing of the west box. Final continuity prestress was applied to each span after all the closures in the frame were cast. The age of the segments at erection was assumed to be 40 days and the foundation springs assumed “drained” soil properties. A comparison with results using “undrained” parameters was undertaken, which showed that this assumption had minimal effect on the design of the superstructure. In the analysis of the superstructure, the models captured the vertical and plan deviation of the roadway as well as the variations in pier geometry. The box girders, needle beams, and individual members of the piers were modelled as beam elements with the appropriate section properties, with rigid elements used to locate the members at their geometric centroid, where appropriate. While striking in appearance, the geometry presented several design and construction challenges. The curvature of the piers had significant effects on the load in the column, in particular the fact that longitudinal bending from the superstructure caused longitudinal bending and torsion in the top of the curved pier. As the load travels toward the base and the differential between the tangent of the curve and the vertical stem reduces, the torsional component becomes longitudinal bending, until the column becomes vertical and the moment matches the load applied by the superstructure, as is shown in Figure 5. Similar to flexure, the load path from torsion in the deck is complicated by the shape of the pier. Generally speaking, if the deck had torsion applied outward, which is normally the case for the southern section of the bridge, where the long wing is located on the outside face of the pier, there would be a high compression on the outside edge of the column. In this case, however, the outside edge of the column is the narrow tip at the end of the curve and does not offer the stiffness of a vertical column. Finite Element Models were developed to assess the exact distribution of force within the region where the transverse tie beam meets the curved pier leg. It was found that ‘flexure’ does not develop in the leg of the pier until well below the tie beam, which was particularly important to recognise when assessing the boundary conditions for design of the diaphragms. Traditional design tools would have underestimated the flow of forces through the connection and could have led to an inadequate design. Torsion in the piers is high during erection and cannot be reduced by altering the torsional stiffness as it is required for equilibrium. However, once the superstructure closures at mid- span are complete, the transverse stiffness of the continuous superstructure can be relied upon and the torsional constant may be reduced for long term effects, traffic loads, thermal loads and seismic loads. Pier head design To make the piers integral with the superstructure, a cast- in-situ diaphragm (with an access doorway) was used. Two mirrored precast shell segments were used to contain the diaphragm, as shown in Figure 6, with the gap between them matching the dimensions of the top of the pier limiting the amount of formwork required. Until the diaphragm was poured, the pier head segments were supported either on brackets stressed to the top of the piers or on temporary props, but no temporary propping was required over live traffic. The vertical reinforcement from the piers continues into the diaphragm and is anchored at the top with terminators, which can be used because of the three dimensional constraint which exists at the top of the diaphragm. There is reinforcement connecting all the vertical faces of the pier head segments to the insitu diaphragm, with couplers cast into the segments coupled to reinforcement which extended into the diaphragm. The two segments are post tensioned together with longitudinal prestress in the top and bottom flanges of the bridge and also with transverse prestress running through the diaphragm between the cantilevers of the segments. In the transverse direction, the critical design action was Figure 6: Pierhead segment. CIA 40-3 FINAL.indb 27 CIA 40-3 FINAL.indb 27 26/08/14 9:19 AM 26/08/14 9:19 AM