For decades, structural engineers in New Zealand have designed buildings to withstand the violent shaking of earthquakes by focusing on peak ground acceleration, spectral acceleration, and the duration of the seismic event. We have largely modeled the end of an earthquake as a tapering off—a gradual decay of seismic energy as the fault rupture slows. But what happens if the ground doesn't just slowly stop shaking, but instead slams on the brakes?
Recent New Zealand-led research has uncovered exactly this phenomenon, revealing that large earthquakes can stop abruptly with a massive jolt. This sudden termination creates a severe "whiplash" effect, sending an intense, concentrated wave of kinetic energy up through the structure of tall buildings. As detailed in recent expert reactions published by the Science Media Centre, this discovery is poised to fundamentally disrupt how we approach high-rise structural design and dynamic analysis in Aotearoa.
The Mechanics of the Seismic Whiplash
To understand the implications of this research, we must first look at the physics of a tall building during a seismic event. Under current design paradigms, highly ductile structures are engineered to flex. As the ground shakes, the building sways, dissipating energy through controlled damage in designated structural "fuses" or through the inherent flexibility of moment-resisting frames.
However, when an earthquake stops instantly, the building is often at the maximum amplitude of its sway. The sudden halt of the ground beneath it means the kinetic energy stored in the building's upper levels has nowhere to dissipate smoothly into the foundation. The structure violently snaps back.
"When a rupture stops abruptly, it generates a sharp stopping phase. For a tall, flexible building, this is akin to driving a car at 100 km/h and hitting a brick wall. The chassis might stop, but the top of the structure experiences a violent, high-frequency whip effect that our current linear and non-linear models may not fully account for."
Comparing the Paradigms
The traditional approach to seismic loading differs significantly from the realities of the newly identified whiplash effect. The table below outlines the core differences in structural demand:
| Parameter | Traditional Seismic Attenuation Model | Abrupt Termination (Whiplash) Model |
|---|---|---|
| Energy Dissipation | Gradual decay; energy dissipates over multiple tapering cycles. | Instantaneous ground halt; trapped kinetic energy forces rapid structural snap-back. |
| Peak Structural Demand | Occurs during peak ground acceleration (PGA) in the middle of the event. | Can occur at the very end of the event due to high-frequency shear forces at upper levels. |
| Vulnerable Structures | Brittle, unreinforced masonry; low-rise rigid structures. | Mid-to-high-rise flexible structures (e.g., steel moment frames). |
| Connection Strain | Cyclic fatigue over the duration of the earthquake. | Acute, extreme shear and tensile stress in a single, violent motion. |
What This Means for the New Zealand Building Code
For engineering professionals, the immediate question is whether NZS 1170.5 (Earthquake actions) and our associated material standards adequately protect against this specific type of loading. Currently, our design spectra and time-history analysis suites are based on historical earthquake records that may not have captured these high-frequency stopping phases with high enough resolution, or where the stopping phase was masked by local soil effects.
If a building is designed to drift by up to 2.5% of its height, a sudden stop at maximum drift places immense shear forces on the connections, particularly in the upper third of the building. This is highly relevant for centers like Wellington and Auckland, where mid-to-high-rise construction relies heavily on flexible steel frames or reinforced concrete core walls.
Concrete Cores vs. Steel Frames: A Diverging Risk Profile
In New Zealand practice, high-rise lateral load resisting systems typically fall into two broad categories: stiff concrete core walls or flexible moment-resisting steel frames. The whiplash effect impacts these systems differently. While a stiff concrete core may transfer the stopping jolt directly into the foundation (risking base shear failure), a flexible steel frame will amplify the jolt as it travels upward, threatening the integrity of beam-to-column connections and floor diaphragms at higher elevations. Understanding these diverging risk profiles will be critical for engineers selecting lateral systems for future commercial towers.
Practical Implications for Design and Retrofit
As this research moves from the realm of seismology into structural engineering practice, professionals across New Zealand need to prepare for shifts in both design philosophy and regulatory requirements. Here are the primary areas of impact:
- Re-evaluating Time-History Analyses: Engineers utilizing Non-Linear Time History Analysis (NLTHA) for complex or tall structures will need to ensure their suite of ground motion records includes events with abrupt terminations. Relying solely on smoothed or traditionally attenuated records may severely underestimate upper-level shear demands.
- Scrutinizing Connections: The whiplash effect places disproportionate strain on the connections between beams and columns, as well as floor diaphragms, in the upper stories. We may need to move away from assuming that the lowest levels always bear the highest critical loads during an event.
- The Role of Base Isolation and Damping: This research strengthens the case for advanced damping systems. While base isolation decouples the building from the ground, supplemental viscous dampers distributed throughout the height of the building may become essential to absorb the kinetic energy of a whiplash snap-back, preventing the energy from overwhelming structural joints.
- Retrofitting the Existing Skyline: Assessing existing tall buildings, particularly those built before the lessons of the Canterbury and Kaikōura earthquakes were codified, will require a new lens. How will a 1980s moment-resisting steel frame in Wellington handle a sudden stop on the Wellington Fault? Targeted retrofits using friction dampers or tuned mass dampers may be required to protect older, highly flexible structures.
Looking Ahead: Adapting to the Jolts
New Zealand has long prided itself on being at the global forefront of seismic engineering. From the pioneering of base isolation to the rapid evolution of low-damage design following the 2011 Christchurch earthquake, our engineering community has consistently turned geological threats into catalysts for innovation.
The discovery of the seismic whiplash effect is the next great challenge. It forces us to look beyond just how much an earthquake shakes a building, and examine precisely how that shaking stops. For consulting engineers, structural software developers, and code-writers at MBIE, the immediate task is to translate this new seismological data into practical, actionable design parameters.
By integrating abrupt-stop scenarios into our dynamic analyses and reconsidering the distribution of damping technologies in high-rises, we can ensure that our skylines remain resilient. The ground beneath us may stop without warning, but our commitment to structural safety and engineering excellence must keep moving forward.