Pictured: Milford Alliance Avalanche Response Team Building

NZ National Seismic Hazard Model Update: A ‘One Page Explainer’ for Building Owners, Managers, Developers

RBG has been operating in New Zealand for over 6 years. Our leadership team is made up primarily of NZ-trained engineers who have returned to NZ after working abroad in one of our many RBG offices across the globe. We are engineers who ‘grew up’ on seismic engineering, refining and expanding our technical skills while working on some of the world’s most complex engineering projects.

New Zealand can be a tough place for buildings, and it doesn’t get much tougher than the Milford Alliance Avalanche Response Team Building, one of RBG’s first completed projects in NZ. Located on State Highway 94 near the Homer Tunnel, the site is close to NZ’s Alpine fault, near the expected epicentre of a south-to-north rupture scenario. To achieve an efficient design at such a demanding site lead us down an ‘all timber’ route to keep the building light on its foundations while achieving the seismic resilience so often demonstrated by light timber-framed structures. Since then, RBG has completed many projects, peer reviews, and assessments requiring specialist seismic knowledge.

Drawing on our specialist experience in seismic design, we’ve developed a ‘One Page Explainer’ on the 2022 NZ National Seismic Hazard Model update for building owners, managers and developers, detailing what the NHSM means for the industry – here’s what you need to know…

What’s this about?
New Zealand’s updated National Seismic Hazard Model (NSHM) was released on the 4th of October 2022. The new model represents the latest scientific knowledge on earthquake hazards, and it is an important input into managing earthquake risk for buildings and structures. In their press release, GNS Science stated that ‘Forecast ground shaking hazard has increased across New Zealand with an average increase of about 50% or more’.

Why do we care?
The NSHM is not a design standard however, it will inform the development of future design standards for buildings. This means that, in the near future, the goalposts may shift in terms of minimum seismic design requirements for new buildings, and for seismic assessments of existing buildings.

Could my building now be earthquake prone?
Not without a change in legislation. The current definition of an Earthquake Prone Building (EPB) is benchmarked to the earthquake loading standard current at the EPB Act’s commencement date (1 July 2017).  That standard is NZS1170.5:2004 and, regardless of any standard update that results from the 2022 NSHM, the 2004 document will remain the benchmark for EPB classification until there is an Act of Parliament to enact that change.

Could my building’s %NBS score be affected?
Not yet. The New Building Standard (NBS) for earthquake loading remains NZS1170.5:2004. In order for the New Building Standard to change requires: 1. the earthquake loading standard NZS1170.5 to be updated, and 2. the new version of this standard to be cited in the building code (specifically, within Verification Method B1/VM1 issued by MBIE). RBG understands that this is unlikely to happen until 2025.

How does the updated NSHM change the earthquake risk for my building project?
The risk hasn’t changed, our understanding of the risk has changed. While the earthquake shaking imposed upon buildings has always been (and always will be) uncertain, good engineering design addresses this uncertainty directly. Good design anticipates the possible effects of larger loads and displacements, ensuring that the structure will perform well despite the uncertainties of earthquakes.

Where can I find more information?
The following references provide more detailed background and guidance relating to the NSHM and what it means for the building industry.

GNS Science – NSHM Resources
SESOC – Interim Advice on the 2022 National Seismic Hazard Model
NZSEE – Earthquake Design for Uncertainty

Tom Watson, Principal, & Sam McHattie, Senior Engineer, RBG Wellington, NZ

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