Bridges

Following a major slip in April 2017, New Zealand’s (NZ) State Highway 3 through the Manawatū Gorge was impassable, and the road was permanently closed. The Te Ahu a Turanga project re-aligns the state highway, providing a safe and resilient 11.5 km connection between Ashurst and Woodville. The project is being delivered through an Alliance model consisting of the NZ Transport Agency, Iwi Partners, Fulton Hogan, HEB, Aurecon, and WSP.

The project features the Eco-Viaduct, a seven-span, 305m long composite weathering steel bridge, that navigates the new route through an ecologically and culturally sensitive area. The viaduct, comprising of 47m main spans supported on reinforced concrete (RC) columns and bored piles, leaves a light-touch on the landscape and is detailed to reduce ongoing maintenance in the area through fully integral piers.

The project is in an area of high seismicity. Seismic loading developed through a Site-Specific Seismic Hazard Assessment (SSSHA) determined peak ground acceleration 50% higher than acceleration derived from NZS1170.5. To address the high seismic loading, ductile RC substructure elements were detailed to enable formation of plastic hinges at the top and bottom of columns during Damage Control Limit State (DCLS) and Collapse Avoidance Limit State (CALS) loading. Non-hinging elements are protected through capacity design principles.

The project minimum requirements mandated that seismic assessment was required to adopt both the force-based and displacement-based design methods detailed in the NZ Bridge Manual v3.3. The Eco-Viaduct provides a useful case study comparing the two design approaches.

This paper discusses how the substructure design responded to the site constraints including seismic analysis methods and detailing of the substructure elements; the innovative design of integral piers that achieve aesthetics and material efficiency, and how the project mitigated risk due to uncertain and challenging ground conditions.