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Concrete In Australia : September 2013
Concrete in Australia Vol 39 No 3 59 Design and construction of the South Road Superway Lucas Wise and Peter Selby Smith Bridge Engineer, GHD Pty Ltd Technical Director -- Bridges, GHD Pty Ltd e South Road Superway is a new elevated highway under construction in Adelaide. When complete, the 2.8 km-long structure will provide three or four lanes of traffic in each direction, as well as local ramps. e bridge will ultimately connect to the Northern Expressway and facilitate heavy road freight movements on the main north-south route through Adelaide. South Road Superway is a project for the state of South Australia and is administered by the Department of Planning, Transport and Infrastructure, commonly known as DPTI. e project is being delivered under a design-construct contract, with the construction joint venture being a consortium of John Holland, Macmahon and Leed, known as Urban Superway Joint Venture (USJV). Major subcontractors to the project include Rizzani de Eccher and DEAL, which are assisting with planning, equipment for segment casting and deck erection with Fressyinet, and Structural Systems responsible the post tensioning systems and installation. e design team is led by GHD Pty Ltd, supported by International Bridge Technologies and SMEC. e elevated roadway deck is constructed with a precast segmental superstructure. Typical spans are 60-66 m long, with an 85 m main span over the Grand Junction Road intersection and are built in balanced cantilever, in which segments are joined to the existing structure in balanced pairs about the centerline of the pier. e newly erected segments are held permanently in place with cantilever post- tensioning tendons in the top slab. Once opposing cantilevers meet at mid-span, they are joined by a small closure pour and secured with additional continuity post-tensioning in the bottom flange. e asymmetric, constant depth section is adapted with multiple variations to accommodate the frequently changing geometry along the alignment. Typical segments towards the southern end of the alignment have a constant 3.5 m depth and range from 13.5 m to 15.5 m wide. e central area of the viaduct allows for the main span over the Grand Junction Road intersection by adopting varying depth segments ranging from 5mdeepatthepiersto3.5mdeepatmidspan.Atthe northern end of the alignment, in order to accommodate an additional lane from the on and off ramps, the single cell boxes utilise internal steel struts and are up to 19.5 m wide. All segments have transverse post tensioning in the top slab, which results in substantial savings in segment mass and allows for increased segment wing lengths. e thickness of the webs and depth of the bottom flange of the segments reduce along the length of each cantilever to meet the specific demand requirements and optimise the mass of each segment. e optimisation of the segment mass leads to a reduction in substructure loads and savings in superstructure post tensioning quantities. e piers are both architecturally interesting and structurally challenging. Due to the narrow available corridor, both during construction and for the final alignment, each pier in the southern areas begins with a single, central stem, located within the median of the existing South Road. After a section with a constant width, the legs of the piers split apart and dramatically flare outward on a constant radius, allowing for the vertical clearance requirements over South Road. e outside edges of the superstructure box girders align with the tips of the flare, and the two legs are tied together at the top with a curved, post- tensioned beam. is basic structural scheme is carried through all of the varying roadway conditions encountered along the northern parts of the alignment, the ramps and bifurcation areas and into the wider four-lane box girders. Because of the length of the bridge, expansion joints were necessary every four to five spans to accommodate movements due to temperature, concrete creep and shrinkage, and to relieve loads in the super- and sub-structure. e preferred method for constructing expansion joints is with a concrete hinge, located at the span quarter point. However, for the South Road Superway, a mid-span joint was selected, as it is more easily integrated with balanced cantilever construction. ese mid-span expansion joints employ steel needle beams across the joints, secured in segments with internal diaphragms, to provide continuity for moments and shear, while allowing longitudinal expansion. is is the first application of this type of joint in Australia. Major construction has been under way for nearly two years, and substantial progress has been made. All of the foundations, abutments and piers have been constructed, with the pier head diaphragms nearing completion. Over 2000 of a total of 2203 segments have now been cast in the purpose-built casting yard. Erection is currently progressing on several fronts, using a variety of equipment to erect the segments, including an erection gantry, smaller segment lifters and ground- based mobile cranes, with full or partial cantilevers erected on 40 piers to date. e structure is expected to be complete in December.