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Concrete In Australia : September 2014
Concrete in Australia Vol 40 No 3 23 consists of twin carriageways along its entire length. South of Grand Junction Road each carriageway is capable of carrying three lanes of traffic, but north of the on- and off-ramps to Grand Junction Road each carriageway can carry four lanes of traffic. The project also includes substantial upgrades to the surface drainage of this very flat area, including water treatment works to improve the quality of discharges to the environmentally sensitive Barker Inlet wetlands. The project has been delivered under a design-construct contract, by the Urban Superway Joint Venture (USJV), a consortium of John Holland, Macmahon Constructions (now part of John Holland) and Leed Contractors. Major subcontractors to the project included Rizzani de Eccher and Deal who assisted with planning, equipment for segment casting and deck erection, with Fressyinet and Structural Systems responsible the post tensioning systems and installation. The design team was led by GHD, supported by International Bridge Technologies of the USA, and SMEC. Structural form The South Road Superway will act as a gateway to the northern end of the city of Adelaide and as such is architecturally important. During the tender DPTI clearly desired a structure that would have a positive aesthetic impact on the site, with primary ways of expressing this being the shape and design of the piers and an open appearance of the area under the bridge. DPTI specified that the minimum span length typically be 60 m, but 72 m across Grand Junction Road, and that the soffit of the girder be a minimum of 9 m above existing ground level. A span of 60 m is too long for precast beams or span by span construction using precast segments, meaning that the most economic solution was to construct the elevated roadway as a precast, prestressed concrete segmental structure, built in balanced cantilever and a segment depth of 3.5 m was selected. To achieve the longer span at Grand Junction Road, the typical section was modified by adopting deeper, haunched segments to span the extra distance, and provide an improved layout for the major intersection at Grand Junction Road. This span was made 83 m long, and was 5 m deep at the piers, using variable depth segments cast in special casting cells. To meet site constraints and provide the space required for the ground level intersections with the turning circles and sight distances required by the large A-double and B-triple vehicles, the typical spans varied between 60 and 69 m. The structure was built in balanced cantilever, with pairs of segments joined to the existing structure in balanced pairs about the centreline of the pier. The newly erected segments are held permanently in place with post-tensioning tendons located in ducts within the top slab. Once opposing cantilevers meet at mid-span, they are joined by a 400 mm long closure pour and secured with additional continuity post-tensioning located in the bottom flange and in the webs. To assist with carrying some of the shear loads without reducing the cross section of the webs, external draped tendons were also used. South of Grand Junction Road Superway was built within the existing corridor of South Road and south of Grand Junction Road. Two lanes of traffic, a footpath and cycleway were required at grade, in each direction. This severely limited the possible width of the pier at ground level and meant that the two box girders had to be located above the ground level carriageways. GHD, USJV and architects Wood Marsh developed a design in which the piers flare outward on a constant radius, to provide the required vertical clearance over South Road. The curves of the outside edges of the piers continue to the tips of superstructure box girders with composite drain covers and the two legs of the piers are tied together at the top with curved, post-tensioned beams, as shown in Figure 1. In order to provide DPTI with a pleasing but constructible pier shape, some unusual features were introduced into the design. To reduce the width of the top of the piers, the outer cantilevers of the box girders were made 2 m longer than those on the inside; and the bridge was raised by 3 m so that the soffit was 12 m above ground level, in order to reduce the curvature of the piers. The piers were constructed with 40 MPa concrete and reinforcement rates of up to 4.5%. North of Grand Junction Road North of Grand Junction Road there are on- and off-ramps, each two lanes wide, which rise to the level of the main carriageway, run parallel to the existing three lanes and then merge to provide four lanes of traffic in each direction. Further north again there are two lane on-and off-ramps, while the main carriageway crosses over Salisbury Highway and divides to connect to the Salisbury Highway in both directions. In the future there will be a grade separated interchange in this area, with most traffic continuing north along the Northern Connector. At grade, South Road becomes a two lane arterial road, running along the centre of the corridor and designed for Figure 2: Structural arrangement – ramp sections of bridge. CIA 40-3 FINAL.indb 23 CIA 40-3 FINAL.indb 23 26/08/14 9:18 AM 26/08/14 9:18 AM