Changes to the New Zealand Building Act in 2004 have raised the requirements of the structural performance of older buildings to withstand earthquakes.
The changes reflect increased efforts to mitigate loss of life and damage caused by earthquakes to buildings and nearby properties.
Building owners and developers need to be aware of the earthquake prone status of their buildings and their legal obligations to assess and strengthen them.
They also need to be aware of the timeframes and issues associated with modifying and upgrading affected buildings.
Background
New Zealand experiences about 14,000 earthquakes annually, and while most are small, about 100 to 150 are strong enough to be felt.
The biggest known earthquake in New Zealand was in Wairarapa in 1855, with a magnitude of 8.2 on the Richter Scale – a large earthquake by international standards (the largest occurred in Chile in 1960 with a magnitude of 9.5). The biggest New Zealand earthquake since instrumental recording began was in Hawke’s Bay in 1931, with a magnitude of 7.8, and which claimed at least 256 lives.
Records dating from the 1840s show, that on average, New Zealand can expect several magnitude 6 earthquakes a year, one magnitude 7 a decade, and a magnitude 8 a century. The recent Gisborne earthquake on 20 December 2007 was magnitude 6.8 on the Richter Scale and as at 14 January 2008, the Earthquake Commission had received over 3,100 insurance claims amounting to $16 million.
The understanding of earthquakes and building performance in New Zealand has been steadily developing since the devastating Napier earthquake of 1931. With this greater understanding, codes and standards for the seismic design of buildings have been refined since they first appeared in the 1935 “Loadings” Standard.
With the introduction of the original Building Act in 1991, unreinforced concrete and masonry buildings were required to be strengthened, however these initial requirements are inadequate in terms of preventing loss of life and the extensive property damage that brick and unreinforced concrete buildings could sustain.
The Requirements of the Building Act 2004
The recently enacted Building Act 2004 covers all building types and requires older buildings to have the performance capacity of at least one third of that of a new building – a significant step in terms of seismic structural performance.
Given this change, buildings previously strengthened under the requirements of the original 1991 Act may now be defined as earthquake prone and require further strengthening. However, buildings designed in line with the 1976 Loadings Standard are generally less likely to be earthquake prone.
The new earthquake-prone provisions in the Building Act 2004 apply to all buildings other than those “wholly or mainly for residential purposes other than those of two or more storeys and containing three or more household units”. This means the Act applies to all commercial, industrial, institutional, recreational buildings and larger residential (apartment or multiple unit style) buildings.
Under the Building Act 2004, territorial authorities (TA) have a Statutory Obligation to adopt an Earthquake Prone Buildings policy to ensure all earthquake-prone buildings are identified and measures are taken to reduce the potential loss of life or injury, and damage to other properties even in a moderate (code level) earthquake.
Some TA’s are proactively dealing with earthquake prone buildings by seeking and assessing buildings likely to be at high risk of damage or danger to occupants and distributing letters to notify building owners of potential earthquake-prone nature of their buildings. These letters require the building owner to engage a structural engineer to assess their building to confirm the earthquake-prone status and formally respond to the territorial authority. Failing to do so within the legislative timeframe could mean the building is listed as “earthquake prone” (when it may not be), limiting its development and the ability to make alterations.
Assessment of Building Structures – The Process
A structural engineer must assess the earthquake prone nature of the building. The New Zealand Society for Earthquake Engineering (NZSEE) has developed recognised methods for assessment in its “Assessment and Improvement of the Structural Performance of Buildings in Earthquake” publication (2006).
The first step in the assessment process – an Initial Evaluation Procedure (IEP) – characterises buildings based on a percentage of the “New Building Standard” (NBS) – i.e. the standard that would be used to design a new building.
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Buildings that have an assessed performance greater than 67% NBS are categorised as “not an earthquake risk”.
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Those with an assessed performance of between 33% and 67% NBS are categorised as “being an earthquake risk, but not required to be strengthened” under the Building Act 2004.
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Those with an assessed performance less than 33% NBS are categorised as “earthquake prone buildings” and are required to be strengthened.
The NZSEE IEP also proposes a “Grading for Seismic Risk” system based on the assessed performance of the building, ranging from A+ for greater than 100%, to A for 100% to 80%, down to E for less than 20%. This system is considered easy to understand by the general public.
© Sinclair Knight Merz
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Who does this affect?
Building owners and developers who need to be aware of the earthquake prone status of their buildings and their legal obligations to assess and strengthen them.
What do I need to do?
Gain an understanding of the processes and assessment measures and legal requirements for upgrading or strengthening earthquake-prone buildings.
Author: Louise Tucker
Based in Wellington, Louise Tucker is a Senior Structural Engineer with SKM. Louise and the team of structural engineers in the Auckland and Wellington offices have extensive experience in delivering structural engineering solutions to meet the needs of the New Zealand environment and specialise in seismic engineering and strengthening.
© Sinclair Knight Merz
Requests to re-publish achieve articles should be made here