Central Motorway Junction (CMJ)

Commonly referred to as "Spaghetti Junction", the Central Motorway Junction (CMJ) in Auckland, New Zealand, is where the Southern, Northwestern and Northern Motorways converge, just west of the city centre. The area is a geometrically complex system of motorways and connecting ramps at multiple levels, carrying around 200,000 vehicles per day, all within a very restricted area.

Built in the 1970s, the CMJ is now receiving a much-needed upgrade. Stage 1 of the upgrade was completed in December 2004, and Stage 2 is in progress.

Elevated viaduct structures and retaining walls form a major component of the upgrade, and detailed design of these structures was undertaken by Parsons Brinckerhoff (PB), in their Brisbane office, as subconsultants to Beca Infrastructure (Beca). This followed an earlier "trans-Tasman" alliance between PB and Beca on previous Transit New Zealand projects (Grafton Gully and CMJ Stage 1).

Detailed design of structures

Detailed design undertaken in Australia included: design of three continuous precast girder viaducts up to 260 m long, and design of one smaller single-span, integral precast girder viaduct; load capacity assessment and design modification of an existing two-span box-girder bridge and an existing continuous post-tensioned viaduct; and design of new anchored-pile retaining walls.

Beca and PB also worked together on the complex road geometric design, and PB carried out a ventilation assessment for a partly enclosed area under an existing elevated roadway, including air quality and fire analysis.

Challenges, solutions and innovations

Major challenges for design of the structural works were the physical constraints of the area, construction staging requirements allowing for the need to keep the motorway operating during construction, complexities in upgrade of existing structures, and design for a seismically active area.

Physical constraints and keeping the traffic flowing

Beca's Design Manager, Alan Powell, commented, "There are space constraints in every dimension in the available corridor - with existing structures, with the services and utilities that are already in place, and with the surrounding buildings.

"Combined with the need to keep the motorway operating at all times, this meant, for instance, that the ability to locate bridge foundations and supporting structures was very constrained. The design required a very clear and thorough understanding of the geometric requirements of the three-dimensional design model."

Existing structures

David Lloyd, PB's Principal Structural Engineer, said that existing structures had to be investigated for load capacity, and upgraded as required. "We carried out a detailed investigation and analysis of the existing structures at the tender stage, which involved high-level design methods. We were able to change the geometric design of the upgrade to reduce the amount of modification work required by the initial scope. The changes achieved cost savings and made a major contribution to the success of the Complete Joint Venture tender," David said.

Design solutions and innovations

New elevated viaduct structures required complex geometry to wind their way through the tight alignment. For the first time in New Zealand, viaduct structures were designed using precast prestressed "super-T" girders, made continuous over the piers using in situ concrete diaphragms.

The varying flange widths of the girders were detailed to allow for the horizontal curvature, and the continuity over the piers removed the need for support bearings, provided improved seismic performance, and allowed for increased span lengths of up to 38 metres for the 1500 mm deep precast girders. This resulted in maximum economy of the girders and span arrangements. To minimise costs, previously constructed foundations were utilised where possible.

Piers were typically 1350 mm diameter concrete columns supported on single bored piles socketed into Auckland's Waitemata sandstone. The length of structure between expansion joints was extended up to 250 m for the longest structure by undertaking a detailed assessment of column flexibility and the soil-structure interaction.

Detailed design of structures was in accordance with the New Zealand Transit Manual and NZS 3101 Concrete Structures Standard. Seismic analysis was a part of the design which required particular consideration. The New Zealand Transit Manual specifies earthquake resistance design criteria, the magnitude of the seismic event varying as a function of the site location and foundation material.

Auckland is less seismically active than other parts of New Zealand, and for this reason a site-specific design event was provided. Even so, the structure response to this design seismic event was such that significant ductile behaviour was required, and this was achieved by the detailing of "plastic hinges" in the substructures using capacity design theory.

SAM Integrated Bridge Software was used by the designers for live load analysis, stage analysis, and prestressed girder design. "SAM offered significant advantages for our Australian engineers designing to New Zealand standards," said PB's Design Manager, Rob Alexander. "In particular, the NZ Transit Manual vehicle loadings are fully automated in the SAM Load Optimisation module, which was of significant benefit for the continuous structures which have more complex load patterns."

SAM uses influence surfaces to automatically position the New Zealand "normal" and "overload" vehicle loadings in the correct location. The software is fully integrated so that the analysis results are automatically transferred to the prestressed beam design modules. These features were considered a major benefit to the designers, in particular due to the time savings achieved in establishing critical load patterns according to the NZ Transit Manual.

Design integration between Australia and New Zealand

Remote project teams are becoming increasingly more viable with advancements in communication technology and web-based document control systems. PB completed the detailed design of viaducts in Brisbane, integrating seamlessly with the Auckland-based Beca project team. The road design and construction phase services were carried out by Beca in Auckland. A web-based document management system provided the interface between the PB structural design team and the Beca project team, and all drawings were transferred through this system.

Says PB Design Manager Rob Alexander, "One of the great successes of the project has been our ability to undertake the detailed structural design work remotely. Building on this success, we have been able to use similar systems and models for remotely designing bridges and other structures for the Lane Cove Tunnel in Sydney, and EastLink in Melbourne."

Construction of CMJ Stage 2 structures is in its final stages. The opening of the completed project is expected in late 2006.