Techlink :: Tech Practice :: Hard Materials

The Pou Kapua is the result of three years of intricate carving by a 30-strong team from New Zealand, Australia, Alaska, Canada and across the Pacific. The carvings depict Māori and Polynesian beliefs, nautical myths and legends, and the histories of migration to New Zealand.

Although visitors to the Event Centre are likely to remember the 20-metre-high pou as the largest totem of its type in the world, engineers may be interested to know it is also the largest known application of state-of-the-art technology in glued-in-rod connections in timber.

The Pou Kapua stands 20 metres tall and was carved in three sections. The metre-high base is carved from a piece of swamp kauri estimated to be 50,000 years old. Above the base sits Tangaroa, the guardian of the oceans, carved from a 3-metre length of kauri trunk with a diameter of 1.6 metres. On top of Tangaroa stands the main carving of the pou, hewn from a 16-metre kauri trunk donated by the Te Rarawa iwi of Hokianga. Crowning the main carving is a magnificent depiction of the legendary explorer Kupe, who discovered Aotearoa. Adding to the structural complexity, Kupe is flanked either side by totara shields spanning a total of seven metres.

The carvers envisaged that the lower two sections, the base and Tangaroa, would support the weight of the main carving. A steel rib down the back of the pou to the foundations would resist wind and earthquakes forces. Based on this concept, the shape of the pou was developed and carving began.

Pou Kapua

The main pou is connected by glued-in rods to the upper section of the structural shaft.

Hollowing out Tangaroa.

In June 2004, when the carving was well advanced, structural engineers Peters and Cheung Ltd were commissioned to carry out the foundation and structural design. They soon realised that the proposed concept presented two major problems. First, a single rib down the outside of the structure would not resist wind and seismic loads from every direction. And changes to the moisture content over time would cause the timber of the pou to swell and contract; if the main carving was supported by the lower two sections, unpredictable stress concentrations at the connections between the sections could create structural difficulties.

The challenge was to come up with a solution that was not only structurally sound but as unobtrusive as possible. Aesthetics precluded the use of external supports, so Peters and Cheung Ltd proposed that a steel shaft be fitted through the lower two sections to carry the weight of the main carving to the foundation, while also providing wind and seismic stability.

The design concept meant that the core of Tangaroa and the swamp kauri base would have to be hollowed out so the steel shaft could pass through them to support the main carving. But the structural difficulties didn't end there. The carvers had chosen to invert the trunk of the main carving so that the thick end of the trunk was at the top. The narrower end, which had to provide the structural connection to the shaft, was also deeply carved greatly reducing its effective structural diameter. The key to the structural design for the Pou Kapua therefore lay in finding a strong and rigid connection between the narrow base of the main carving and the steel shaft inside Tangaroa.

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.

The size of the shaft was limited by the diameter of Tangaroa, but a flange with a larger diameter was required to fix the pou to the foundation pad. The solution was to fabricate the shaft in two parts. The upper section consists of a length of steel tube which runs through Tangaroa and is connected to the main carving by the Macalloy bars.

The lower section is a shorter, wider length of steel tube welded to a thick baseplate. To help lower the pou into position, the lower section has an internal sleeve, which acted as a guide for the upper shaft. The foundation is an octagonal reinforced concrete pad, founded 1.2 metres below ground-level. The baseplate on the lower shaft is secured by anchor bolts embedded in the foundation slab.

At the top of the main carving, a short length of steel was required to support the 7-metre span of Kupe's carved shields. At the request of the sculptors the column was extended down the entire length of the pou into the foundation so that these shields, along with smaller shields and a decorative rib, could be attached on site. The column was also designed to help erect the Pou Kapua.

The column was fastened to the outside of the main carving in the workshop with a row of large-diameter coach bolts along each flange. The critical loads on the column would occur during the erection of the main carving. A projecting steel T-shaped beam with lifting holes was bolted to the top of the column to help raise it.

The foundation was fabricated on site and the lower shaft bolted on. Tangaroa, the hollowed-out middle section, was transported from the workshop and gently positioned onto a galvanized steel frame above the shaft.

The 14-tonne main carving, already connected to the upper steel shaft, was then lifted by crane and lowered into position, spearing through Tangaroa and fitting onto the sleeve of the wider shaft below. Once this was placed, the upper shaft was welded to the lower shaft. Kupe's shields were then lifted and bolted into position and finally, the pieces of the swamp kauri base were slid around the lower flange and connected to complete the Pou Kapua.

The Pou Kapua is a remarkable tribute to its carvers. It is also a tribute to the problem solving skills of engineers and especially Peters and Cheung Ltd. Faced with a difficult and uncompromising medium, they adopted an innovative approach – encompassing the structural support within the sculpture and making a world first in what is now the largest application of glued-in-rod connections in timber. Together with the teams who undertook the carving, steel fabrication, directional drilling, pre-stressing and grouting, they showed extraordinary ingenuity in adapting their work for this unusual and magnificent kiwi taonga.

Duncan Peters MIPENZ is a director of Peters and Cheung Ltd, civil engineering consultants, Auckland. www.peters-cheung.co.nz