Earthquake design is a defining feature of building in New Zealand. Every new structure must be designed to cope with a certain amount of seismic demand i.e. the earthquake forces that a structure must be designed to resist. These forces determine the size, strength, and ductility of structural elements and have a direct influence on construction cost.
Understanding what drives seismic demand and its impact on construction cost is the first step to understanding what can be done to mitigate those costs. This article explores the current set of earthquake design requirements and how they influence seismic demand and construction cost.
Please note that this article is based on the current seismic code NZS 1170.5:2004 which will gradually be replaced by TS 1170.5:2025 once formally adopted. Our next article will talk about the impact of TS 1170.5 on earthquake design requirements and construction cost.
What is Seismic Demand?
Earthquake design requirements (as defined in the New Zealand Building Code) define how strong and flexible a building must be to withstand ground shaking without collapsing.
Engineers refer to this as 'seismic demand' i.e. the earthquake forces that a structure must be designed to resist. These forces determine the size, strength, and ductility of structural elements and have a direct influence on construction cost. All other things being equal, higher seismic demand means higher construction cost.
In New Zealand, seismic demand is governed by NZS 1170.5: 2004 – Earthquake Actions, which considers:
- Earthquake hazard at the site,
- Soil conditions,
- Building’s importance level, and
- Structural form and dynamic behaviour of the building.
What Influences Seismic Demand?
A. Seismic Zone (Hazard Factor Z)
NZS 1170.5 assigns each region a hazard factor (Z) representing expected earthquake intensity.
- Wellington, Marlborough and Canterbury have high Z values and therefore higher design forces.
- Auckland and Northland have lower Z values and correspondingly lighter design actions.
B. Site and Building Characteristics
Soil type has a major impact. Sites are classified A–E, from hard rock to soft soil:
- Class A (rock) sites amplify shaking the least.
- Class D and E (soft soils) can significantly increase accelerations, particularly for taller, flexible buildings.
Building irregularities also raise design demand.
- Vertical irregularities (setbacks, soft storeys, podiums) can concentrate forces in certain levels.
- Plan irregularities (L-shaped or asymmetrical layouts) create torsion and uneven load paths.
Irregular buildings generally require stronger or more complex structural systems.
C. Building Importance Level (IL)
Under the Building Code, each structure is assigned an Importance Level (IL) that reflects the consequence of failure:
- IL 2: Standard commercial or residential buildings.
- IL 3: Buildings with higher occupancy or community importance, such as schools or airports.
- IL 4: Essential post-disaster facilities, including hospitals and emergency services.
Each IL corresponds to a return-period factor (Ru), effectively scaling the design earthquake forces:
- IL 2: 1-in-500-year event
- IL 3: 1-in-1,000-year event
- IL 4: 1-in-2,500-year event
That means IL 3 buildings are designed for roughly 30 percent higher seismic forces, and IL 4 for up to 80 percent higher, than IL 2 structures.
D. Structural System and Ductility
A building’s ductility (its ability to deform and dissipate energy through targeted damage and designated failure) can reduce the forces it must resist. Systems such as moment frames, braced frames, or shear walls offer different balances of strength and flexibility. More ductile or well-damped systems allow engineers to design for lower effective seismic demand, reducing material quantities and cost. More on this later.
How do Earthquake Design Requirements Affect Construction Cost?
Larger Structural Members
Higher design forces mean larger beams, columns and braces. This increases steel or concrete tonnage, fabrication effort, erection cost, and transport weight.
More Demanding Foundations
Those forces must be safely transferred to the ground. On soft soils, foundations may require piles, thicker pads or ground improvement, often adding substantial cost and programme time. Switching from a concrete pad to a pile foundation, for example, can add significant project costs.
Increased Detailing and Build Complexity
Heavier loads mean denser reinforcement, stronger connections and tighter tolerances. These add labour hours and fabrication cost, and sometimes require specialist contractors.
Controlling Building Movement
Higher-importance buildings may need to limit inter-storey drift to protect façades and internal systems. Meeting these performance targets typically requires additional bracing to increase strength and stiffness. These structures usually have specific performance criteria more demanding than the minimum code requirements.
Compounding system effects
Heavier loads leading to thicker columns increase structural mass. Increased structural mass leads to greater forces that need to be transferred to the foundations. Thicker columns are also likely to require bulkier beam-to-column connections. Increased seismic demand sets off a cascade of additional costs through the entire structural system.
Turning Requirements into Opportunity
Earthquake design requirements, or seismic demand, are among the strongest cost drivers in New Zealand construction. They reflect the intersection of location, soil, building form and importance level, all influencing how much force a building must resist. Those forces have a direct impact on material and labour cost.
While higher seismic demand naturally drives up cost, it also creates a value engineering opportunity. Engineers can achieve compliance in different ways - not just by adding material strength but by reducing seismic demand at its source.
Technologies such as buckling restrained braces, viscous dampers, friction dampers, rocking systems, and proprietary moment-frame connections enable smaller structural members, leaner foundations and faster construction, while still delivering code-compliant safety and resilience.
The cost-saving impact of such systems and technologies will be explored in depth in a subsequent article.
Lincoln University's award-winning Waimarie Science Building reduced steel and concrete works by adopting Tectonus braces and hold downs, creating cost savings and reducing embodied carbon by 40%
Sound familiar?
If you're working on a project where columns, walls and/or foundations are getting out of control, it might be worth considering a different structural system. On many projects, smart structural engineers have used Tectonus technology to drive savings across structure and foundations - sometimes totalling millions of dollars.
For a no obligation discussion, reach to one of our engineering experts. Let’s talk.