Cat 10 01 Mary Potter Apartments 028

Te Ara o Puanga - Mary Potter Apartments


Daniel Moroder / PTL Structural And Fire

10. Innovation timber engineering award

Photographed by: 
Andy Spain / Mary Potter Apartments / Archaus / Daniel Moroder

The Te Ara o Puanga - Mary Potter Apartments is a 5-storey, 41-unit apartment building in Newton, Wellington. The rental income from the apartments will provide a stable and reliable revenue stream to fund Mary Potter Hospice services into the future.

Innovation and challenges:

This was the first multi-storey timber building in New Zealand using PLT floors on prefabricated light timber frame walls with continuous steel tie-downs.

The proprietary continuous steel tie-down system was adapted for New Zealand use, especially for seismic design, timber species, durability, and fire requirements.

The building showed that multi-storey timber buildings can be adapted to extremely difficult building sites, with hybrid materials and careful design.

The innovative design features of this building allowed PTL engineers to assist with the BRANZ Multi-Storey Light Timber-Framed Buildings Guide.

The structure had to be constructed on a very small site, and both construction traffic and noise levels had to be kept to a minimum.

Timber solution for form and function:

Timber was the obvious material of choice for an extremely difficult steep site, and a building with many structural irregularities, including an L-shape floor plate, re-entrant corners, split levels, offset bracing walls etc. This, combined with strict resource consent requirement, resulted in a structurally complicated building designed for high seismic loads.

Wall design:

For the walls, light timber framing (LTF) handles the high gravity and seismic loads. This wall system of LVL framing, plywood bracing, and a continuous steel rod system for the wall hold-downs is highly adaptable and efficient, only using structure where it is required. The LTF walls met thermal and acoustic requirements, thinner than possible with CLT on a very tight site. The steel rods resist lateral loads, account for any shrinkage and for temporary bracing during construction. Wall panels were prefabricated off-site with linings, membranes and windows installed.

Floor design:

The floors of the building were parallel laminated timber (PTL) panels with a robust diaphragm load path under seismic loads. A floating timber floor met the required vibration and acoustic performance. The floor panels were installed quickly without introducing any wet trades, reducing cost and time on site.

What sets this building apart from other materials?

The main benefit of timber is buildability and speed of construction. Prefabrication increased the speed of construction, and reduced exposure to the weather. Only 2 weeks to install one storey of walls, then one week to install the floor, with a total time savings of 4 months compared to traditional heavy construction.

In a steel-concrete design, structural steel would have been used for lateral loads and gravity loads, with light timber framing only used as infill walls. Floors would be in situ concrete, adding seismic weight and introducing a wet trade.

The sustainability analysis indicated sequestration of 100 kg CO2 per m2, compared to a steel and concrete building with emissions of 250 kg CO2 per m2. This is a significant saving to meet the client’s sustainability requirements.