How TS1170.5 Impacts Seismic Demand (and Construction Costs)

 New Zealand’s seismic standards are evolving. TS 1170.5:2025, a new Technical Specification for earthquake actions, has now been published and will progressively replace NZS 1170.5:2024 once it is formally adopted. Because earthquake design requirements are a major driver of construction cost, this update matters to anyone involved in project development, budgeting or delivery. 

This article explains what TS 1170.5 is, why it was introduced, and how it may affect both seismic demand and construction cost across the country. 

Note: This article builds on our first piece in the series that defined seismic demand and how it impacts construction costs in New Zealand.  

1. What Is TS 1170.5?

TS 1170.5 is a Technical Specification that updates how New Zealand defines earthquake loading for new buildings. It is intended to: 

• Improve alignment with current seismic hazard science 

• Provide clearer design guidance 

• Support better and improved building performance 

• Eventually replace NZS 1170.5 once adoption and citation occur 

While engineers will handle the technical detail, TS 1170.5 has practical implications for project cost and risk, because it adjusts how seismic forces are calculated — and those forces drive the size of structural elements and foundations. 

2. What Is Changing Under TS 1170.5?

TS 1170.5 does not completely rewrite New Zealand’s seismic design system, but it refines several key areas that directly influence seismic demand. 

a. Updated Seismic Hazard Values

TS 1170.5 incorporates the latest geological and seismic hazard modelling. 

• Some regions will see their expected shaking levels increase, others decrease. 

• Areas with complex or emerging fault data may experience more significant shifts. 

For project teams, this means that the same building on the same site may attract different design forces than before - either higher or lower. Having said that, most regions in NZ will experience an increase in seismic demand. 

b. Clearer Soil Amplification Guidance

For developers, this could mean: 

• More conservative design actions on soft sites 

• Reduced uncertainty in geotechnical-structural coordination 

• Clearer expectations around foundation complexity 
  

c. Enhanced Treatment of Building Period and Dynamic Behaviour

Tall, flexible buildings behave differently than low-rise structures. TS 1170.5 provides clearer guidance on how to calculate these effects, improving consistency in design. This tends to benefit: 

• Simple, regular buildings, which often attract more predictable design forces 

• Irregular or complex buildings, which may see more explicit penalties for torsion or discontinuities 
  
  

d. Continued Use of Importance Levels (IL2, IL3, IL4)

Importance Levels are unchanged, but updated hazard factors mean IL3 and IL4 buildings - such as schools and hospitals - may see increased or decreased design forces depending on location. 

For example: 

• An IL4 hospital in a region where seismic hazard values have increased may now face higher structural and foundation demands. 

• An IL3 school in a region with reduced hazard may see slightly lower seismic forces and therefore lower cost. 

3. What This Means for Construction Cost in NZ

a. Some projects will cost more, some less

Because TS 1170.5 updates the underlying hazard data, design actions will shift up or down depending on region and soil class.  This means project budgets may move in either direction and early feasibility assessments may need to be revisited. 

b. Soft-soil sites may see more cost pressure

If soil amplification factors become more conservative, projects on soft ground could face: 

• Larger or deeper foundations 

• More reinforcement 

• More complex structural systems 

These are already high-cost sites under NZS 1170.5; TS 1170.5 may sharpen that effect. 

c. Complex or irregular buildings may attract higher design forces

Clearer rules around torsion, setbacks and other irregularities mean some building forms may now require: 

• Additional bracing or walls 

• Increased stiffness 

• More robust detailing 

Early design decisions will matter more than ever. 

d. Better fit with modern seismic science to reduce long-term risk 

Although some projects may face higher upfront structural cost, the intention is to: 

• reduce under-design, 

• improve building resilience, 

• reduce repair and downtime costs after earthquakes. 

In other words, the whole-of-life cost profile may improve even if some capital costs increase. 

4. The Opportunity: Designing Smarter, Not Just Stronger

On projects where cost is likely to increase, it creates an opportunity for smart design and value engineering.  TS 1170.5 tilts the balance away from simply making everything stronger to considering ‘alternative’ solutions. 

That means greater opportunity for: 

• Damping systems that absorb earthquake energy 

• Bracing systems with low-overstrength 

• Rocking wall systems that limit damage 

• Structural simplification early in design 

These strategies can offset increases in seismic demand and help keep cost under control. 

This sets the stage for our next article in the series:  How DMAX changes the equation by reducing seismic demand - and the cost associated with it. In the meantime, you can check out how our DMAX seismic braces work.

In Summary 

TS 1170.5 represents a significant update to New Zealand’s seismic design framework. 
By refining hazard values, soil amplification, and dynamic behaviour, it changes how engineers calculate earthquake forces — and therefore affects construction cost. 

Some buildings will see higher design forces, some will see lower forces, and all will benefit from alignment with more current seismic science. 

For the construction industry, the key message is simple: 
Understanding these changes early will lead to better budgeting, smarter design decisions, and more resilient buildings 

To learn more about how Tectonus can assist you with TS 1170.5 compliance, please set up a call with one of our experts. And, if you haven’t already, take a few minutes to check out our first article in this series.