Onehunga Foreshore Bridge

Perspective for Resource Consent: The bridge will be a distinctive gateway on State Highway 20, with a different character experienced when travelling east and west.

As a signature element of the Onehunga Foreshore Restoration, the pedestrian and cycle bridge will become a key linking element between the existing Onehunga Bay Reserve and the new coastal parkland and beaches. It will form part of a recreational loop and connect with the Waikaraka Cycleway and the future Taylors Bay coastal walkway.  The project is currently making its way through Auckland Council’s Resource Consent process.

Plan for Resource Consent: The project seeks to re-establish the natural character of Onehunga Bay through the creation of 6.8ha of usable parkland and rocky promontories as well as dynamically stable gravel and sandy beaches.

Bridge design

The bridge design was led by Isthmus who developed the key concept and aesthetic elements of the proposal within the parameters determined by the Principals Requirements. This included the need to balance the gateway directive with the need to ‘fit’ and being keeping with the Onehunga environment and to completely span the motorway.  A key component of the concept was for the bridge to belong to the land and therefore the Onehunga community and this differentiated it from the series of cable stay bridges currently on the motorway network. With the concept embedded Isthmus worked with the URS bridge engineers to develop a steel truss system that could be clad. Similarly Isthmus worked with Tonkin and Taylor Civil and Geotechnical engineers to develop the form of the abutment mound and degree of cladding.

Developed Design: structure and cladding design

The bridge and approaches provide a gateway that is elegant, low and unobtrusive as well as being responsive to coastal and heritage values through material selection and detailing. Constructed over a steel truss and with a concrete deck the bridge has a hardwood timber balustrade to the windward side and a ‘folded flax’ marine grade weathering steel balustrade to the leeward side. The timber balustrade element references the wharf and maritime history of the port of Onehunga, once a thriving west coast port as well an major trade node for Maori. The folded flax speaks of the vegetation that formerly grew on the coast as well as its use as a raw material by Maori.

Concept Design digital model

The bridge is embedded into earth embankments at each abutment  to ensure that the bridge feels part of the land rather than the motorway;  each embankment cut is partially faced in exposed basalt aggregate panels to reference the local volcanic crater. The 5m wide bridge completely spans the motorway and has a 48m central span and two 16m back spans. The spans are supported by two concrete piers. Concrete upstands on the piers are detailed as entranceway features and suggest a traditional palisade post in the approach and departure directions while the developing pier form below the bridge deck is suggestive of an anchor stone when viewed from oblique angles.

Preliminary physical model

Early in the preliminary design phase a physical model was developed to aid conceptual development of the bridge and to allow for visualization of external parties such as the Client team. A key practical consideration was the adherence to NZTAS structural code for the maintenance of the main structural members. A key practical consideration was the adherence to NZTAS structural code for the maintenance of the main structural members. Balustrades were designed to be demountable to allow visual assessment and servicing of the bridge structure and was designed in conjunction with the structural engineers.  Digital modelling and visualization was undertaken to further develop the concept.

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