Seismic Design to NZS 1170.5: Understanding Hazard Factors, Site Class and Ductility
Every building consented in New Zealand since the mid-2000s carries an invisible signature: a Z factor, a site subsoil class, and a ductility value, all buried in the structural calculations behind the architectural drawings. In late 2024, thousands of those signatures effectively changed overnight. The National Seismic Hazard Model (NSHM) update revised the seismic hazard picture for much of the country, and for many engineers, clients and territorial authorities, the practical implications of nzs-1170-5-seismic-design methodology suddenly became front-page news rather than back-office technical detail.
This article unpacks the three pillars of seismic demand under NZS 1170.5 — the hazard factor (Z), the site subsoil class, and the structural ductility factor (μ) — and explains why the 2024 NSHM update matters for anyone commissioning, designing or reviewing a building in New Zealand today.
Why NZS 1170.5 Still Matters in 2025
NZS 1170.5:2004, Structural Design Actions – Earthquake Actions – New Zealand, remains the governing standard for determining seismic design loads under the New Zealand Building Code (NZBC) Clause B1 Structure. It works alongside AS/NZS 1170.0 for general design actions and sits within a verification framework administered by MBIE. Despite being two decades old, the standard’s underlying methodology — combining a probabilistic hazard model with site and structural response factors — remains sound engineering practice worldwide.
What has changed is the data feeding into it. The 2024 NSHM update, led by GNS Science, incorporated nearly a decade of new fault mapping, updated ground motion prediction equations, and lessons from the Kaikōura and Canterbury earthquake sequences. The result is a revised national hazard map that increases the hazard factor Z in a number of centres — including parts of Wellington, Napier, Hastings, and several South Island locations — while leaving others largely unchanged.
For structural engineers, this is not a cosmetic update. Z feeds directly into the elastic site hazard spectrum, which in turn drives design base shear. A meaningful increase in Z can translate into materially larger seismic loads, stiffer lateral systems, and in some cases, foundation redesigns for projects already at concept stage.
Key takeaway: Any project with a resource consent or building consent lodged before the updated hazard factors are formally adopted into NZS 1170.5 and MBIE guidance should be reviewed against the new NSHM outputs before construction documentation proceeds.
The Hazard Factor (Z): The Starting Point for Every Design
The hazard factor Z represents the underlying seismicity of a location — essentially, how strongly the ground is expected to shake at that site over a defined return period. Under the current standard, Z values range from as low as 0.13 in parts of the far north to 0.6 in high-hazard zones near major fault systems. Wellington’s CBD, for example, has historically sat around Z = 0.4, reflecting its proximity to the Wellington and Wairarapa faults.
The 2024 NSHM findings suggest some locations warrant upward revision, driven by better characterisation of subduction zone interface behaviour and previously underweighted crustal faults. Other areas — particularly some Auckland and Northland sites — may see limited change, given their comparatively lower proximity to active fault systems.
It is worth being precise about process here: Z factors are formally scheduled in NZS 1170.5, and updating them requires a standards amendment process coordinated through Standards New Zealand and MBIE. As of this update cycle, engineers should treat the NSHM outputs as the best available science while watching closely for the formal incorporation timeline, interim guidance, or transitional provisions that MBIE may issue.
Practical insight: For any project currently at feasibility or concept design stage in a jurisdiction flagged for Z factor increases, run a sensitivity check now. A 10–15% increase in Z can increase design base shear by a similar or greater margin, depending on the shape of the design spectrum in the relevant period range.
Site Subsoil Class: The Ground Beneath the Numbers
Z describes regional seismicity, but the ground directly beneath a building can amplify or dampen that shaking substantially. NZS 1170.5 classifies sites into five categories — A (strong rock) through E (very soft soil) — based on shear wave velocity, standard penetration resistance, or undrained shear strength within the upper 30 metres of soil profile.
The difference between site classes is not trivial. A structure founded on Class D (deep or soft soil) can experience spectral accelerations 30–50% higher than an identical structure on Class B (rock) at the same location, particularly in the short-to-medium period range relevant to low and mid-rise buildings. Christchurch’s post-earthquake experience is the clearest domestic case study: extensive liquefaction-prone Class D and E soils across the eastern suburbs materially influenced both damage patterns and subsequent rebuild design requirements.
Geotechnical investigation is therefore not a compliance formality — it is a primary determinant of seismic demand. Under-classifying a site (assuming better ground conditions than actually exist) is one of the more common and consequential errors seen in preliminary design, particularly on constrained urban sites where a full geotechnical investigation is deferred until later design stages.
Real-world example: A four-storey commercial building on reclaimed land in a harbourside location may sit on Class D or E soils despite being only metres from a neighbouring site on shallow rock classified as B. The seismic design spectra for these two adjacent sites can differ dramatically, underscoring why site-specific geotechnical data should never be assumed from regional mapping alone.
Ductility: Designing for Controlled Damage, Not Just Strength
The third pillar — structural ductility factor μ — is where engineering judgement plays its largest role. Ductility describes a structure’s capacity to deform beyond its elastic limit without collapse, dissipating seismic energy through controlled, repairable damage rather than brittle failure.
NZS 1170.5 allows designers to reduce elastic design forces through the ductility factor, recognising that a well-detailed ductile structure can survive the same earthquake with lower design forces than a nominally elastic one. A reinforced concrete moment frame detailed for full ductility might adopt μ = 4, substantially reducing design base shear compared to a nominally ductile system at μ = 1.25. However, this reduction comes with an obligation: capacity design principles, adequate detailing, and confinement reinforcement must ensure the intended ductile mechanism actually forms during a major event.
As one senior structural engineer involved in post-Kaikōura reviews put it: “Ductility on paper means nothing if the detailing doesn’t deliver it in the field. The 2016 and 2011 events taught our profession that assumed ductile behaviour has to be verified through rigorous detailing, not just assumed through a code factor.”
Key takeaway: Selecting a higher ductility factor is not simply a load-reduction shortcut. It requires commensurate investment in detailing, quality assurance during construction, and, for importance level 3 and 4 buildings, often independent peer review.
Bringing It Together: What Clients and Owners Should Ask
For property owners, developers and asset managers, understanding these three variables — even at a high level — supports better-informed conversations with design teams. Useful questions include:
- Has the hazard factor Z used in this design been checked against the 2024 NSHM outputs, and is there a risk of revision before consent is finalised?
- What site subsoil class has been adopted, and is it based on site-specific geotechnical investigation or a regional default assumption?
- What ductility factor has been assumed, and what does that mean for detailing requirements, construction tolerances, and future retrofit flexibility?
- For existing buildings, has a seismic assessment been updated to reflect revised hazard data, particularly for buildings near the %NBS earthquake-prone threshold?
Key takeaway: Existing buildings assessed under the previous hazard model may see their %NBS rating shift once updated Z factors are formally adopted. Portfolio owners should proactively commission reassessments for buildings sitting near critical thresholds (34% and 67% NBS) rather than waiting for a statutory trigger.
Looking Ahead
The 2024 NSHM update is a reminder that seismic design standards are living frameworks, built on evolving science rather than fixed certainty. NZS 1170.5 will continue to be refined as MBIE and Standards New Zealand work through formal incorporation of the updated hazard data, likely alongside broader review processes already underway for the standard more generally.
For engineers, the discipline required is technical rigour combined with proactive client communication. For clients, the discipline required is asking the right questions early, before design assumptions are locked into consented drawings and constructed form.
Chambers Consultants works directly with developers, asset owners and design teams across New Zealand to interpret evolving seismic hazard information, review site-specific ductility and detailing strategies, and ensure structural designs remain robust as standards evolve. If your project or portfolio may be affected by the 2024 NSHM update, contact our structural team to arrange a seismic design review before your next design milestone.