Richard Sharpe is a Wellington-based earthquake engineer, a Distinguished Fellow of Engineering NZ and a life member of the NZ Society for Earthquake Engineering.
OPINION: Those involved in the work to make New Zealand buildings earthquake resistant have surely had a sense of deja vu in our society’s response to the Covid-19 pandemic.
Severe earthquakes and pandemics occur sporadically and mostly with little warning. After major earthquakes, we mandate entry into buildings via a traffic light analogy and enforce no-go red zones. Helmets and masks mandatory. There are aftershocks and some parts of our building population are more vulnerable than others. Some regions are more vulnerable than others.
Common to both is the challenge of communicating what we mean by “safe”. Safe compared to what? It generally depends on our individual perception of risk, our lived experience and our daily tolerance to all surrounding risks.
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The recent spate of building closures related to seismic assessments reflects a natural response to concern about safety. It also highlights the need to better understand what these determinations actually mean and to have a better national conversation about risk and treatment options.
An existing building’s seismic resilience rating is described by a misleading term, “percentage of new building standard” (%NBS). This metric was developed by a committee of experts to compare the expected performance of a building to a new one built according to current standards.
An NBS of 34% equated to about 10 times the risk of new construction. At the same time, upgrading a building to at least two-thirds (67%NBS, about 5 times the risk) was considered a reasonable compromise. This aspect was not included in the legislation and the market set its own de facto standards such as 67% or 80%.
A building designed to current building code is expected to avoid collapse with a loss of life in seismic shaking that is expected to be exceeded at least once every 500 years or so at that location. It can be rendered unoccupiable at much lower thresholds if essential services are lost or damaged to the extent that it is write-off, but it should have a considerable margin against complete collapse and people should be able to get out in safely when the shaking stops.
Our building code and regulations prescribe design methods based on both classical engineering theory/physics and our experiences from previous earthquakes. The main emphasis is on integrating robustness into the structure, which means respecting simple concepts allowing the return of seismic forces to the ground and redundancy if certain elements do not carry the load prematurely.
For buildings that the company wants to be operational after an earthquake normal to the design level (for example, critical parts of hospitals), the building code raises the design level by 80% (and 30% for spaces where a large number of people can gather). It is a matter of relative risk and society’s view (via regulation) of acceptable risk. This may change over time, particularly following major events.
A tremendous effort has been made to apply lessons learned from the past decade of earthquakes to improve the consistency of assessment across many building types, ages, and constituent materials. The guidelines now mandatory for use when councils decide whether a building should be declared earthquake-prone, seek to identify any critical structural weaknesses that would compromise life safety. Its assessed resilience sets the score (%NBS).
Our legislation currently sets the number of years that an earthquake-prone building can be occupied before the reinforcement works are completed. This reflects an implicit societal tolerance for rare events and recognition of the time and resources needed to upgrade or retire our building stock.
However, occupational health and safety law holds those in charge of a workplace personally liable for exposing occupants to unacceptable risks. Thus, it is inevitable that an abundance of caution will influence the decision to evacuate a building deemed seismic.
It comes down to a definition of “safe” and how we quantify it. Our recent experience with earthquakes means we are learning how to patch some vulnerabilities and understand the costs and benefits of such work. These gains must be coupled with conversations about managing seismic “risk”. We clearly have a conflict between the interpretations of the earthquake prone building legislation and the Occupational Health and Safety Act, which now need to be reconciled to avoid undue disruption.
On the bright side, we continue to develop innovative “vaccines” to improve building resilience to earthquakes. There is a rapidly growing use in New Zealand of tuned viscous dampers (coffee pistons on steroids embedded in diagonal struts) which in many cases are a more economical and effective method than basic insulation for improve seismic resilience (including damage reduction) to levels well beyond the somewhat arbitrary seismic design levels.
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