Risk Guidance for Buildings with Low Seismic Ratings
5 July 2022
Engineering design standards and our understanding of earthquakes have advanced over time, in particular as a result of learnings from the 2011 Canterbury and 2016 Kaikōura earthquakes. Consequently, many older buildings do not meet the standards required of new buildings.
In light of this, the Building Performance team from the Ministry of Business, Innovation, and Employment (MBIE) has today released a new Seismic Risk guidance document:
This guidance helps building users, tenants and owners to understand seismic assessments of their buildings and make risk-informed decisions about continued occupancy of these buildings when they have a low seismic rating.
The guidance also provides the tools and language for engineers and their clients to discuss seismic assessments and what these mean for building performance in an earthquake.
The document provides guidance on three key areas:
- Part A provides background material on when to obtain and how to interpret a seismic assessment, including the limitations of the New Building Standard (%NBS) metric.
- Part B describes a process for building owners and tenants to go through when making decisions on occupancy of seismically vulnerable buildings.
- Part C provides guidance on how to manage ongoing earthquake risk and communicate this information with staff and other stakeholders.
Understanding Seismic Risk
The purpose of seismic assessments is to inform building owners and users about their building vulnerabilities, encourage strengthening of vulnerable buildings and lead to the improvement of our building stock over a reasonable time period. When the outcome of a seismic assessment is a low percentage of New Building Standard (%NBS) rating, this should be a trigger for planning, funding and implementing a seismic upgrade, addressing the identified vulnerabilities and mitigating risk.
The %NBS is an index used to characterise the expected seismic response of a building to earthquake shaking. It identifies buildings that represent a higher seismic risk than a similar new building, built to the minimum life safety requirements of the Building Code (or New Building Standard).
The aim of the %NBS metric is to provide a relative assessment of seismic risk. It is not a predictor of building failure in any particular earthquake. In most cases, seismically vulnerable buildings can be occupied while you plan, fund and then undertake seismic remediation work.
It is recommended that occupancy decisions should not be made until you have received a detailed seismic assessment (DSA) that, if necessary, has been independently reviewed, and have had time to discuss and work through Part B of the guidance document with your engineer and other key stakeholders.
It is very difficult to reverse a building closure decision, so make sure you are confident in the information you have received and decision process you have followed.
It is important to remember that you can never eliminate seismic risk. Even if a building is vacated, staff and building users will be exposed to seismic risk in their homes and other buildings. However, seismic risk can be mitigated through emergency planning and training, as well as restraining plant, services, and contents within the building.
If you have questions, please email the team at building@mbie.govt.nz
