Bridges

Te Ahu a Turanga project provides a new 11.5 km long resilient connection between Ashhurst and Woodville on State Highway 3 in New Zealand. The project is being delivered using an Alliance model consisting of NZ Transport Agency, constructors (Fulton Hogan & HEB), consultants (Aurecon, Gaia, Roadlab, and WSP), and five Iwi partners.

The new route features the Parahaki Bridge, a 295 m long balanced cantilever concrete box girder bridge spanning over the Manawatū River. The bridge is a four-span structure with a main span of 88m. The superstructure is supported by seismic isolation lead rubber bearings on shallow foundations at the abutments and deep piled foundation at the piers.

Parahaki Bridge is situated in area of high seismicity and near to the major fault system running through New Zealand. Seismic loading for the project was derived from a Site-Specific Seismic Hazard Assessment (SSSHA) which demands are between 20% to 50% higher than NZS1170.5 loading. Conventional elastic or ductile design of the Reinforced Concrete (RC) substructure was determined to be not economically feasible, and a base isolated solution was developed. The final design required near to the largest commercially available structural bearings using 1410 mm and 1360 mm diameter elastomeric rubber bearings with a 390 mm diameter lead core at abutments and piers respectively.

Design requirements for base isolation are outside the scope of the New Zealand Bridge Manual and the project specific design criteria was developed using guidance from AASHTO. The RC substructure was designed under bi-axial bending to be elastic at the Damage Control Limit State (DCLS) and form a plastic hinge at the base of the pier in the Collapse Avoidance Limit State (CALS).

This paper will outline seismic design philosophy and design method developed including analysis of lead rubber bearings using non-linear time history analysis, seismic detailing of substructure.