Earthquake-prone buildings

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Decisions on the earthquake-prone buildings policy review

The Government has announced changes to the management of the earthquake-prone building system.

If there are concerns about building safety in earthquakes, get professional engineering advice and act on it.

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What are the current provisions in the Building Act 2004?

To make buildings safer for occupants and users, the Building Act 2004 introduced provisions to improve the likelihood of existing buildings withstanding earthquakes.

This long-term strategy focuses on buildings that are most vulnerable in an earthquake. It does not include small residential buildings (ie, houses).

The strategy allows local territorial authorities in different areas to take into account their area's particular seismic, economic and social conditions.

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Why the law was changed in 2004

Changes to the law on earthquake-prone buildings were introduced with the intent of reducing casualties in major earthquakes.

The changes reflected lessons learnt from the effects of earthquakes both internationally and in New Zealand, and growing recognition of the inadequacies of past earthquake design practices when viewed in terms of current knowledge. The changes were developed in close consultation with the engineering profession and were designed to allow consultation between local territorial authorities and building owners and users in developing their policies.

The current legislation is directed at existing buildings (other than small residential buildings) that are less than one-third of the strength required for a new building.

Design standards for buildings in earthquakes were first introduced into 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 that 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 reflect the increased knowledge of the seismicity of New Zealand, material properties and the response of buildings to earthquake shaking.

These developments over the years mean we now realise that many existing buildings fall short of the standards now required for new buildings.

The buildings concerned are not just those of brick masonry, which were already covered by legislation in New Zealand, but include particularly those built before 1976.

The increase in the scope of what could be an earthquake-prone building introduced in the Building Act 2004 recognised the need to keep earthquake risk reduction on the agenda. The provisions are directed only at the worst of existing buildings. Buildings with less than one third of the strength of a new building would have at least 10 times the risk of serious damage or collapse when compared to a new building.

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.

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Local policies

Territorial local authorities were required to develop and adopt a policy regarding local buildings most vulnerable in a moderate earthquake following the commencement of the Building Act 2004. As part of their policy development, territorial authorities were required to consult with their community 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.

A territorial authority's policies must describe:

  • the approach the territorial authority will take
  • the priorities for that approach
  • how the policy will apply to heritage buildings.

Territorial authorities have provided a copy of their policies to MBIE. To keep policies up to date with any advances, the Act requires them to review their policies every five years.

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What is ‘earthquake-prone’?

The risk of earthquake-prone buildings can be measured by comparing the assessed performance of each building to the performance required of a new building.

A key factor for territorial authorities in determining their policy for earthquake-prone buildings is likely to be the level of earthquake risk in their area: what is the probability and severity of earthquakes and what impact could they have on life and property?

The technical definition of an earthquake-prone building is set out in the Building Act 2004 (section 122) and in the related regulations that define a moderate earthquake. This definition is significantly more extensive and requires a higher level of structural performance for buildings than that provided under the previous legislation. Over time this aims to make it safer for people to live and work in buildings that could be vulnerable in the event of a major earthquake.

The definition now encompasses all buildings, not simply those constructed of unreinforced masonry or unreinforced concrete. Small residential buildings are exempt from these provisions.

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How is a moderate earthquake defined?

A moderate earthquake is legally defined as:

in relation to a building, an earthquake that would generate shaking at the site of the building that is of the same duration as, but that is one-third as strong as, the earthquake shaking (determined by normal measures of acceleration, velocity and displacement) that would be used to design a new building at the site.

This definition was consulted on in October and November 2004, and took effect from 31 March 2005.

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