It starts with the shaking

Since the February 2011 Christchurch earthquake, there’s been ever-growing recognition of earthquake risk in New Zealand, says Dr Ken Elwood CMEngNZ, who experienced the quake firsthand while visiting from Canada. The now New Zealand-based Chief Engineer (Building Resilience) for the Toka Tū Ake EQC and the Ministry of Business, Innovation and Employment (MBIE) says there’s wide-ranging recognition of the need to address earthquake risks across many different sectors now as a result of recent seismic activity.

He says while shaking hazard is just one part of the story when it comes to building performance and earthquakes, it’s the fundamental basis on which everything else is determined. October saw the release of the updated National Seismic Hazard Model (NSHM), which calculates the likelihood and strength of earthquake shaking that may occur in different parts of the country over specified periods. It was released by GNS Science, MBIE and Toka Tū Ake EQC.

“Without any shaking there’s no damage, so it really starts with the shaking. When a new hazard model like this comes out, it gives us an opportunity to think about how we approach the rest of the building design and assessment process.”

Ken says the NSHM is used very broadly by many different organisations and decision makers who need to estimate the likely impact of earthquakes on Aotearoa’s land, buildings and infrastructure, from civil defence planning through to helping insurance companies assess risk.

“It helps in the management of government-owned assets and in determining how buildings and structures need to be designed and built. Furthermore, the NSHM informs the risk settings of our building regulations and requirements in relation to earthquake loadings.”

So, what impact does the updated model have on the way engineers work?

“The NSHM on its own doesn't change anything, however, it will lead to eventual changes in standards and guidelines that will have an impact on how engineers approach design and assessment in the future.”

But he says it will take some time to understand the results of the updated NSHM and what it really means for the design of new buildings and the assessment of existing buildings.

“The NSHM provides an assessment of the likelihood of earthquake shaking. On its own it doesn't predict the likely performance of a building. It can't fully capture the risk to a building and its occupants during an earthquake.”

It’s critical to have an up-to-date understanding of the seismic hazards facing New Zealand, and to ensure the risks from earthquakes to the built environment can be appropriately managed. Developed in the 1980s and last updated in 2010, Ken says the NSHM was out of date.

“The primary science behind the 2010 model came from 2002. This 2022 revision reflects international best practice and, significantly, the learnings from recent seismic activity in New Zealand.”

He says the Canterbury earthquake sequence and the Kaikōura earthquake provided “a tremendous amount of data, both from an earth science perspective and from a building performance perspective”.

“Considering how these regions were impacted by the earthquakes helps us better prepare New Zealand so we can reduce risks we face from earthquake events in the future.”

But the updated model doesn't automatically change the way buildings should be designed. Ken says building professionals should continue to use the existing building standards to demonstrate that their work complies with the Building Code.

MBIE is now considering what this update to the NSHM means for new building design standards and working with Te Ao Rangahau to assess what changes are required, and how to include the results of the NSHM into regulatory settings for new buildings.

MBIE aims to consult on proposed changes to building standards in late 2023.

Aotearoa is not alone in the time it takes it to convert from the outputs of the NSHM to design provisions – Ken says in the USA it typically takes two to four years for NSHM updates to be incorporated into seismic design provisions.

Designing for uncertainty

MBIE encourages the use of the recent advisory Earthquake Design for Uncertainty, jointly prepared by New Zealand Society for Earthquake Engineers (NZSEE), Structural Engineering Society New Zealand (SESOC) and New Zealand Geotechnical Society (NZGS). It recommends approaches engineers can take to help ensure reliable performance of a building, regardless of the uncertainty in the seismic hazard. It recognises that shaking hazard is just one component of what controls the performance of a building in earthquakes.

“This guidance helps an engineer recognise all of those other aspects to building design, how one lays out the structural system and the detailing one puts into the design of the building. It will help ensure reliable seismic performance regardless of the fact that there’s uncertainty in what level of shaking the building is going to face in the future.”

Building closures prompt action

When Ken started in his role at MBIE in December 2021, he also took on the responsibility as convenor of the Joint Committee for Seismic Assessment of Existing Buildings which includes members from SESOC, NZSEE, NZGS, Toka Tū Ake EQC and MBIE.

With the decision to close and vacate some high-profile buildings earlier this year, including Hutt Hospital, it seemed suddenly everyone was talking about percent New Building Standard, or %NBS, an assessment of how well an existing building fares against today’s building standards. Ken says this brought forward MBIE’s plans to provide guidance to building owners, users and tenants about how to interpret their seismic assessments in order to make more informed, risk-based decisions about their buildings.

The Joint Committee found people were thinking too much about the %NBS and needed better ways to make risk-based decisions on their buildings based on more than just the %NBS rating. Ken believes the use of plain English was a key factor behind the success of MBIE’s ensuing Seismic Risk Guidance for Buildings, released in July. It explains what %NBS is, and what it isn’t, and the information a seismic assessment actually provides, and offers language tools for better communication between engineers and tenants and building owners. While a low %NBS rating does indicate a high life safety risk in the event of an earthquake, it doesn't mean that a building is imminently dangerous. And it's not a predictor of building failure. Ken says understanding the vulnerability of different building elements, and the consequences of their failure, will always be more important than a building's %NBS rating.

The Joint Committee is currently providing input on a strategy to determine how, when and why the Seismic Assessment of Existing Buildings Guidelines should be updated.

“This strategy needs to carefully balance the need to regularly update the guidelines based on new research and engineering practices, while at the same time provide certainty for the regulatory systems that use the guidelines, in particular the earthquake-prone building system,” Ken says. He adds that the strategy also needs to consider that the guidelines are used outside of the earthquake-prone building system for all forms of buildings. He expects the Joint Committee will start a broader discussion on that strategy by mid-2023. Other priority topics for future work programmes include case studies of seismic assessments, the development of retrofit guidance, and the development of resilience assessment guidance.

New challenges ahead

Ken says there’s a lot of talk right now in engineering circles about the need to better consider post-earthquake functionality and the repairability of buildings. This would require engineers to “think beyond the life-safety performance objectives that currently dominate the way we design, integrate better non-structural and structural design, and consider the constraints in a post-earthquake environment”.

“These are things that engineers haven't been forced to deal with in the past. This isn't just a challenge for the engineer, but rather the whole design team – the architect, the engineer, through to the contractor building the structure. I see this as coming in the future, maybe not next year, maybe not the year after, but coming in the future.”

This article was first published in the December 2022 issue of EG. The full version of EG is available in your member area.