The demand for ‘safe’ workplaces remains a high priority, as clients seek reassurance that their space is fit for purpose and meets acceptable seismic standards, as measured against the New Building Standard (NBS). 

However, for many people, understanding the jargon and percentage ratings can be challenging. So here’s a simplified breakdown for clarity:

Earthquake ratings – what do they really indicate?

The % NBS rating is a measure of a building’s seismic performance when compared to the seismic performance of a similar new building constructed under current standards and codes. It is calculated as part of a seismic assessment.

However, the %NBS rating does not assess compliance with the current Building Code. A 100% rating does not guarantee that all Code requirements are fulfilled. It merely indicates how the building should perform to meet the minimum seismic performance objectives of the Code in terms of protecting people. 

The %NBS rating also does not assess a building’s ability to function after an earthquake. It provides no information on the expected damage or the building’s usability post-earthquake.

The basics

A seismic rating of 50% NBS means the building should perform similarly to a 100% NBS-rated building but only under half the level of shaking, assuming all other factors are equal. In Wellington, office buildings are designed and constructed to reliably withstand earthquake shaking with a return period of one in 500 years. A building with a 50% NBS rating should withstand a one-in-100-year earthquake with a similar level of reliability (note: the scale is exponential).

The New Zealand Society for Earthquake Engineering (NZSEE) considers buildings with a rating above 67% NBS to be acceptable in terms of seismic risk.

The risks to your organisation

The following table helps understand the level of risk associated with different NBS ratings:

The table demonstrates that occupying a building rated at 50% NBS (considered medium risk) carries approximately a 12% chance over a 12-year period of experiencing earthquake shaking that could exceed the defined capacity of the building. In other words, there is statistically a 12% chance over 12 years that the building will experience severe shaking.

Although occupying such a building is around seven times riskier than a new one, it is not considered high risk relative to other options.

It is important to note that the assessed capacity of a building is not set at a level where collapse is certain. Thus, the probability of fatality resulting from collapse is even lower.

What about buildings with ratings below 34%NBS?

Buildings with earthquake ratings below 34% NBS meet one of the requirements for being considered Earthquake-Prone Buildings (EPBs) under the Building Act 2004. Ratings below 67% NBS are considered Earthquake Risk Buildings (ERBs).

New legislation now addresses earthquake-prone buildings, primarily focusing on identifying these buildings and establishing timeframes for strengthening.

Contrary to popular belief, earthquake-prone buildings can still be occupied, but safety and risk management are crucial factors. The Health and Safety in Employment Act imposes consequences if owners or occupiers fail to take practical steps to minimise hazards in the workplace. However, WorkSafe clarifies that as long as steps are actively being taken to address earthquake-prone buildings in accordance with the Building Act’s requirements, the Health and Safety at Work Act will not impose a higher standard of life safety. If these risks are neglected, owners and occupiers may be in breach of the HSAW Act. Serious hazards that are easily addressable should be attended to within a reasonable timeframe.

Insights from recent earthquakes

While the statistics are informative, real-life scenarios may not always align with them.

Major earthquakes in Christchurch and Wellington in the past decade were exceptional events with different levels of building performance. Some buildings were demolished due to immediate safety concerns or because repairs were deemed uneconomical. However, many buildings were still usable without the need for repair.

Although there are no guarantees of how a building will perform in a major earthquake, the NBS rating system remains the best available tool to assist in making decisions regarding safety obligations in the workplace.

Update on what’s changing

GNS have recently released an update of the National Seismic Hazard Model (NSHM) which forecasts across NZ the expected strength of ground shaking that will occur during an earthquake. The outputs from the model feed into the design standards for new buildings. Engineering NZ has developed a draft of potential changes to the NZ Standard (NZS1170.5:2004) used to determine the seismic loads for the design of new buildings. 

While these are yet to adopted into a new (updated) Standard, we can expect an increase in design standards at some point in the future. The implications of this is that the %NBS of existing buildings will be lowered against their current rating. MBIE will need to carefully consider what a further increase in design standards will have on the market’s view of existing building stock from a safety perspective and demand/rent implications.