As Safe as Houses

As a 2002 Fulbright New Zealand Graduate Student, engineering student Gavin Wight began a project to safeguard New Zealanders and their homes from the devastating effects of earthquakes. Gavin’s doctoral research came into fruition in June, with the completion of the world’s first post-tensioned masonry house to built to current seismic requirements.

Gavin Wight with new home owners the Marsters family

Four years ago I began a doctoral study at the University of Auckland, in the Department of Civil and Environmental Engineering. The objective of this study was to demonstrate the improved performance characteristics of post-tensioned concrete masonry walls when subjected to earthquake forces, with the ultimate goal of building the first house of this type in New Zealand.

This technology falls within the scope of performance-based structural engineering design, a philosophy that became prevalent amongst researchers in the early 1990s. This philosophy focuses on not only designing structures for life safety during earthquakes, but also designing structures to perform in such a way as to minimise damage and reduce the large financial cost of such events.

Post-tensioned concrete masonry walls have the same appearance as regular masonry walls built using concrete blocks, but have a different steel reinforcing design within the wall. Instead of embedding or bonding the reinforcing steel in the wall, the vertical steel is installed within a duct to separate it from the concrete, and is then stressed during a process called post-tensioning.

Essentially the steel bars are stretched, and remain stretched within the wall, compressing the masonry and providing wall strength. This allows the wall to rock when subjected to earthquake forces, minimising the extent of wall damage and providing a restoring force that returns the wall to its original vertical position at the end of loading.

The majority of my laboratory research was conducted during an eighteen month Fulbright exchange to North Carolina State University between November 2002 and April 2004. While in the United States, I constructed a number of full-scale walls and subjected them to real earthquakes using equipment called a shake table.

Rectangular walls, along with walls containing window and door openings, shrinkage control joints and a small structure were built and tested. The shake table had sufficient capacity to subject each wall specimen to various ground excitations recorded during actual earthquakes from around the world. This research validated the performance of the wall system that had been developed specifically for the residential market in New Zealand.

Upon returning to New Zealand, I contacted the Manukau branch of Habitat for Humanity. Founded in 1976, Habitat for Humanity is a global, non-profit, non-denominational, Christian housing organisation, which assists low-income families into their own homes by providing a ‘hand up’ rather than a ‘hand out’. It was proposed that an alliance be formed between Habitat for Humanity, the University of Auckland and numerous consulting and material supply companies, with the goal of building New Zealand’s first post-tensioned concrete masonry house.

The proposal was met with much enthusiasm by all parties involved and the idea became reality. A feature of the house design is that all incorporated products are readily available in the market, with no proprietary products having been specifically developed for post-tensioned masonry. Consequently, it is hoped that this house will be a showcase for the exposure of the technology in New Zealand.

The Marsters family home under construction

The single storey, four bedroom house comprised of a simple rectangular floor plan, and incorporated mortarless concrete masonry blocks in the wall construction. The unique design of this block removes the need for the traditional mortar layer between each block, greatly enhancing construction speed. This was one of the first projects to use this block type, which had only recently appeared in the New Zealand market.

The floor consisted of a concrete raft design, typical of the floors used in residential masonry construction in this area of New Zealand. A simple timber truss roof with iron cladding enclosed the structure.

Construction was undertaken in the first half of 2006, and I found myself onsite from the beginning of site works through to the post-tensioning of the masonry walls, at which point Habitat took over and completed the roof and internal fit-out.

During my time on-site, we welcomed a team from the United States who were in New Zealand to volunteer with Habitat and travel the country. The relationship between New Zealand and the US has therefore played a significant role in the design and construction of this particular house for a South Auckland family.

It was a rewarding experience working with the future home-owners and the diverse group of volunteers that help build these houses. Observing the practical application of four years of research and knowing that it has led to a solution that is benefiting New Zealand families is incredibly satisfying.

- Gavin Wight (2002 Fulbright New Zealand Graduate Student)