Seismic Retrofit of the Mohaka River Bridge NZ

Mohaka Bridge is 216.4 meters long and was constructed in 1962. It is a steel truss bridge supporting a concrete deck. The trusses are supported on two abutments and two central piers. Two approach spans are supported on two secondary abutments. The New Zealand Transport Agency (NZTA) initiated a detailed assessment of the bridge, carried out by Opus in 2004, and concluded that it should be strengthened to resist earthquake loads with a 1000 year return period so that it can continue to perform its function for future generations. In the longitudinal direction, one of the main Abutments was strengthened so that it could resist the longitudinal seismic loads. In the transverse direction, the steel bracing system, originally provided for the steel trusses, was upgraded. Strengthening was also provided for the steel bracing in the transverse vertical plane of the main abutments and of the central piers. The horizontal bracing in the plane of the truss bottom chord adjacent to the piers was also strengthened. Seismic upgrading of the connections between the concrete bridge deck and the steel transoms and the wind shoes (shear keys) at main abutments and at piers were included in the strengthening. Geotechnical investigations and analysis identified the vulnerability of Abutments E and F to slope displacement, and the slope was strengthened using post-grouted soil nails. The steep slope below Pier D was also strengthened using rock bolts to reduce the risk of possible instability. The retrofit was completed in 2009 at a total cost of $1.5 Million.