MBIE says there are common misconceptions on how a NBS (New Building Standard) percentage rating should be interpreted when assessing a building’s seismic risk. To address those, new Seismic Risk Guidance has been published to assist with a better understanding of seismic assessments so that risk-informed occupancy decisions can be made.

MBIE’s key messages

• Compared to most business-as-usual risks, earthquakes are a low probability risk
• An NBS rating is a relative assessment of seismic risk, not a predictor of building failure in an earthquake
• Understanding the relative vulnerability of different building elements (and potential consequences of their failure) is more important than the overall NBS rating
• A low NBS rating does not mean that the building presents an imminent danger as the life-safety risk is still low
• A low NBS rating is a trigger for planning, funding and implementing a seismic upgrade, addressing any vulnerabilities and mitigating risk
• Most seismically vulnerable buildings can be occupied while owners plan, fund and undertake seismic remediation works
• Owners can mitigate seismic risk through emergency planning, training and restraining heavy objects etc
• Occupancy decisions should always still involve an engineering assessment, preferably which has been independently reviewed.

The new Seismic Risk Guidance

The Guidance:

• provides information and tools for engineers and building owners; and

• aims to help building owners, tenants and users understand seismic assessments and make risk-based decisions about continued occupancy if a building has a low NBS rating.

Part A: Obtaining and understanding Detailed Seismic Assessments (DSAs)

A building may be classified as earthquake-prone or seismically vulnerable if it is calculated as less than 34% NBS by an engineer working to a specified DSA methodology. Although earthquake-prone buildings may carry a higher risk than new buildings in an earthquake, it does not mean that they must cease to be occupied immediately. A low NBS rating also does not mean the building is a ‘dangerous’ (as defined by s121 of the Building Act 2004). To be ‘dangerous’, other risks must be identified with the building.

Once deemed earthquake prone, the Building Act specifies timelines for owners to carry out seismic works (ranging from 7.5 to 35 years depending on the seismic risk in that particular area) and does not preclude continuing use and occupancy of the building in the meantime.

Part B: Process for making occupancy decisions

The Guidance includes a step-by-step chart that helps building users, owners, and tenants make balanced and risk-informed occupancy decisions. It focusses on risk assessment and provides questions to consider or ask an engineer about.

Before any occupancy decision is made, the Guidance says DSAs should be independently reviewed and the questions set out in the Guidance for owners about occupancy should be fully assessed.

The Guidance says that in most cases an earthquake-prone building can be justifiably occupied while seismic work is being carried out.

Part C: Managing ongoing earthquake risk and communicating with staff

If a decision is made to close a building, its occupants should be given reasonable time to vacate the building unless there is immediate danger. If the building owner decides to keep the building open, the Guidance lists a number of ways to reduce seismic risk, including limiting access to particularly vulnerable parts of the building. MBIE also suggests that the Guidance can be used as a tool to help communicate the decision so tenants and other building users understand the reasons for the decision and the decision-making process.

What this means for you

As the Guidance says, deciding whether buildings can still be occupied while seismic works are done is an emotive decision. The key takeaway is that earthquake-prone buildings do not necessarily present any immediate danger and in some circumstances there may be no need to immediately close or restrict access to a building while seismic works are planned or carried out.

We welcome the Guidance given MBIE’s report that there are 4200 buildings in New Zealand that have been identified as earthquake-prone, with more likely awaiting identification/assessment. Often building owners and users have to make difficult decisions about continued occupancy, balancing all of their legal obligations. We have seen regular reports of decisions being made which are later challenged as incorrect because they do not appropriate balance any risk, or because they were based on incorrect or limited information.

The Guidelines make it clear that an NBS rating is not the be all and end all for occupancy decisions. We think they will help move building owners and users away from what some have described as a ‘fixation’ with NBS ratings to a better understanding of what they mean and what other information should be considered when making risk based decisions about continued occupancy if a building has a low NBS rating. If faced with that decision, we recommend building users, owners and tenants follow the Guidance and obtain professional advice from a Chartered Professional Engineer and, where necessary, other advisors.

Get in touch

Our team of experts are available to speak to you about how to use the Guidance and the earthquake-prone provisions in the Building Act.

You can find the Guidance here.

Special thanks to Jennifer Liu for her assistance in writing this article.