Seismic Strengthening of the Mohaka Township Concrete Bridge with FRP Fabric and FRP Spike Anchors: Case Study in New Zealand

Abstract

The Mohaka Township, New Zealand, reinforced concrete bridge was designed in 1958, measuring 239 m long and divided into 14 spans, each 17.1 m long. The piers are doubly reinforced concrete walls over a pile cap with 12 piles. Four prestressed T-beams and a singly reinforced deck form the superstructure. All individual structural members were deemed to comply with current standards, with the main issue being the connection between substructure and superstructure. The integral connection did not have enough reinforcement bars to resist lateral and rotational movement of the girders, resulting in significant cracking at the beams’ ends. A solution was devised to release those movements by physically separating the beams and the pier cap and installing an elastomeric bearing. However, the new detailing required an enlarged pier cap, which in turn increased the lever arm and the moment demand on the pier cap—from both traffic loads and seismic loads. Vertical layers of fiber-reinforced polymer (FRP) were bonded on the face of the wall piers, anchored at the top and bottom using FRP spike anchors. The capacity of the new pier was calculated using section analysis, but no method was available at the time to design the anchors, which were grossly oversized. More current research enables this design, which would result in significant savings in materials and labor. An innovative method using metallic U-tubes was used to try to minimize drilling through concrete members, but it was not satisfactory, owing to constructability.