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.
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:
NZS 1170.5 assigns each region a hazard factor (Z) representing expected earthquake intensity.
Soil type has a major impact. Sites are classified A–E, from hard rock to soft soil:
Building irregularities also raise design demand.
Under the Building Code, each structure is assigned an Importance Level (IL) that reflects the consequence of failure:
Each IL corresponds to a return-period factor (Ru), effectively scaling the design earthquake forces:
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.
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.
Higher design forces mean larger beams, columns and braces. This increases steel or concrete tonnage, fabrication effort, erection cost, and transport weight.
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.
Heavier loads mean denser reinforcement, stronger connections and tighter tolerances. These add labour hours and fabrication cost, and sometimes require specialist contractors.
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.
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.
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%
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.