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Maintaining insurance cover and concerns raised by tenants are the key drivers at present for seismic assessments. Councils are also requesting owners of commercial and multi-level residential buildings to provide seismic assessments. The starting point is an Initial Evaluation Procedure (IEP) inspection and report.
The IEP is the initial assessment of a building’s likely seismic risk and performance based on its age, materials and structural form, size and occupant load according to the building use, and the likely underlying ground conditions. The assessment methodology has been developed by the New Zealand Society for Earthquake Engineering.
The IEP provides an initial assessment of the ability of a building to resist earthquakes as a percentage of today’s building code requirements for a new building, known as the New Building Standard (NBS). We note that seismic loads may be adjusted as new knowledge on seismicity leads to changes in the loadings code AS/NZS1190 and thus the NBS. So a 50% score could reduce to 45% or increase to 55% in the future. It’s all to do with faults – not human, but those fractures in the earth’s crust that may not be apparent.
In regulatory terms, buildings which have their seismic capacity exceeded by an earthquake load 33% or less of NBS, AND are likely to collapse causing death or injury, or damage to other property are currently defined by the New Zealand Building Act 2004 s122 and Regulation 2005 (SR 2005/32) as being earthquake-prone. In practical terms, engineers and councils seem to be using the 33% level as the sole determinant of whether a building is earthquake prone.
We have found that IEP assessments of seismic capacity do vary between engineers so a second opinion may be sensible. There are other measures being considered to reduce such variability, but they are not available as yet.
If an IEP assessment confirms NBS of 33% or less, then a Detailed Engineering Evaluation (DEE) is required. If the evaluation shows the building is still earthquake prone this will eventually lead to substantial structural upgrading, or potentially replacement. So it’s best to consider seismic evaluation, prior to purchase, when other substantial work is being considered, or when a tenancy is approaching the end of a lease.
Tenants and insurers are often asking for higher NBS percentages than 33%. The drivers are occupant fear and concerns; and risk aversion from tenants, owners, insurers and lenders. A minimum seismic capacity of 67% of NBS seems to be in vogue at present.
Just as assessing NBS performance will vary between engineers, what is consid-ered to be an acceptable level of NBS above the regulatory minimum will vary between cities, provincial towns, the quality of accommodation, the nature of the tenant and the availability of other buildings.
Sean Marshall – Director