Earthquake-prone buildings

The Building Act 2004 introduced the requirement for local authorities to develop policies on earthquake-prone buildings in their area by 30 May 2006.

This is a long-term strategy that focuses on the buildings most vulnerable in an earthquake. It recognises the need to keep earthquake-risk reduction on the agenda and reduce it over time.

The changes reflect lessons learnt from the effects of earthquakes internationally and in New Zealand, and growing recognition of the inadequacies of past earthquake design practices when reviewed in terms of current knowledge.

The new legislation is directedat existing buildings (other than small residential buildings) that are less than one-third of the strength required for a new building. The changes were developed in close consultation with the engineering profession. They were designed to allow territorial authorities to consult with building owners in developing their policies.

History of New Zealand's earthquake design standards

Design standards for buildings in earthquakes were first introduced in New Zealand in 1935, following the Napier earthquake. Significant developments in design approaches came in 1965 when three different earthquake zones were identified, reflecting the growing knowledge of the different seismicity of various parts of New Zealand. Further significant changes were made in 1976 which allowed for more variation in seismicity over New Zealand. More importantly, the 1976 changes introduced requirements to design and detail building structures to protect vertical load-carrying elements (known as the 'capacity design' or 'strong column/weak beam' approach). Further refinements to the 1976 approach were made in 1984, 1992 and 2005. These generally reflected the increased knowledge of the seismicity of New Zealand, material properties and the response of buildings in earthquake shaking.

Older buildings need to meet today's standards

These developments over the years mean that many existing buildings fall short of the standards now required for new buildings. In many cases this shortfall is not significant, but past earthquakes, particularly those in Kobe, Japan (1995) and Northridge, California (1994), have served as reminders of the possible deficiencies in past design methods in some cases. These reminders apply just as much to New Zealand as to the US and Japan, since developments in earthquake standards in these countries have strong parallels.

The changes in the Building Act are in line with how other countries deal with earthquake risk, notably in the US where the Californian Earthquake Hazards Reduction Act of 1986 was enacted following the devastating earthquake in Mexico City in 1985. That Act led to the development of a California Earthquake Loss Reduction Plan which includes requirements to deal with existing buildings.

The new provisions in the BuildingAct 2004 are directed at the worst of existing buildings. Buildings with less than one-third of the strength of a new building have about 10 times the risk of serious damage or collapse when compared to a new building.

The buildings covered by the legislation are not just those with brick masonry, which were already covered by legislation in New Zealand, but include all buildings, irrespective of construction material, particularly (but not necessarily limited to) those built before 1976.

Territoral authorities must have a policy

The Act requires each territorial authority to have a policy on earthquake-prone buildings, but in so doing allows territorial authorities to decide on the approach, priorities and timetable to be followed. The Act requires territorial authorities to consult locally in developing their policies. This is to ensure a balance between the need to address earthquake risk and other priorities, such as the social and economic implications of implementing the policy.

We envisage that implementing the provisions of the Act will take several decades in most cases. Territorial authorities must provide a copy of their policy to the Department of Building and Housing. To keep policies up to date with any advances, the Act requires territorial authorities to review their policies every 5 years.