Pou Kapua
Glued-in rods
Peters and Cheung Ltd decided the best option was to use Macalloy bars (threaded rods used for pre-stressed applications) to fix the main carving to the top plate of the steel shaft. This would involve drilling holes into the narrow base of the main carving, inserting the rods and screwing them through steel anchor pins drilled into the trunk higher up. The rods would then be pre-stressed at the base of the main carving by tensioning them against the top plate of the steel shaft. Grouting the rods with an epoxy would distribute the tension forces along their length.
The first task was to ensure that the steel shaft would be aligned along the vertical axis of the pou and beneath its centre of gravity. With no "as-builts" to work off, a state-of-the-art laser survey technique was used to obtain a "point cloud" representing the surface of the deeply carved main sculpture. Structural analysis showed that the connection required eight stressed Macalloy bars. Survey software was then used to find the best locations for the bars and to determine the size of the top plate on the steel shaft.
A literature search for guidance on the design of the connection revealed recent New Zealand research on glued-in-rods in plywood, similar Scandinavian research, and the draft Eurocode 5: Design of Timber Structures. The design for the connection was based on a combination of first principles and the draft Eurocode 5.
For guidance on the material properties of kauri a detailed report published by the New Zealand Forest Service in 1956 was used. Although the timber of the pou appeared to be well seasoned and dry, the moisture content was found to be around 30%, which was above code recommendations. The anchor mechanism for the bars was therefore conservative: both the glue and the anchor pins were designed to separately provide sufficient anchorage.
To avoid over-stressing the timber at any particular point, the anchor pins within the main carving were staggered between 2.0m and 3.5m up from the base. Eight locations were carefully chosen among the intricate carvings within this zone, and radial holes drilled into the trunk for the anchor pins. Eight longitudinal holes for the Macalloy bars were then drilled into the base of trunk to intersect with the radial holes.
A 120mm-diameter steel anchor pin was inserted into each radial hole, and a 26mm diameter Macalloy bar was inserted into each longitudinal hole and screwed through the anchor pin where they intersected. The Macalloy bars were then stressed and the longitudinal holes grouted with epoxy.
