More than two years ago on from the Kaikoura earthquake, our Capital’s property market is still dealing with the impact.
The high demand for ‘safe’ workspaces is ongoing. Most clients want to simply be reassured their workspace is fit for purpose and meets an acceptable seismic standard (as measured against the New Building Standard or NBS). What they don’t understand is what all the jargon and percentage ratings really mean.
So, here’s a simple breakdown…
Earthquake ratings – what do they really tell us?
The %NBS rating evaluates the performance of a building compared with a similar new building in terms of protecting life. It’s calculated as part of a seismic assessment of a building.
However, the %NBS doesn’t measure compliance with the current Building Code. If a building scores 100%, it doesn’t mean it fulfils all the requirements of the Code. Rather it 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 doesn’t measure a building’s ability to function after an earthquake. It says nothing about the damage that building could be expected to sustain or whether it will be able to be used again.
The basics
If a building has a seismic rating of 50% NBS, then there is an expectation it will perform to a similar standard as a building rated 100%NBS would in twice the level of shaking, all else being equal. In Wellington, office buildings are designed and built to withstand, with a high level of reliability, earthquake shaking with a return period of one in 500 years. A building with a 50%NBS seismic rating would be expected to withstand, to a similar level of reliability, a one-in-100-year quake (the scale is exponential).
Above 67%NBS, a building is considered to be an acceptable seismic risk by the New Zealand Society for Earthquake Engineering (NZSEE).
The risks to your organisation
The following table is a useful way to understand the level of risk associated with the various NBS ratings:
|
% NBS rating of a building |
Approx. risk to life in a building compared to a similar new building |
Indicative probability an earthquake will occur over a 12-year lease period that will theoretically cause the building to exceed its defined capacity* |
Relative risk description |
|
100% |
<1 |
<2.5% |
Low risk |
|
75% |
2 times |
5% |
Low risk |
|
67% |
5 times |
6% |
Medium risk |
|
50% |
7 times |
12% |
Medium risk |
|
34% |
10 times |
22% |
High risk |
|
<20% |
More than 25 times |
>45% |
Very high risk |
The table shows that, being in a building rated at 50%NBS (which is considered a medium risk), means there is around a 12% chance during a 12-year period of experiencing earthquake shaking that could result in the defined capacity being exceeded.
While occupying such a building is likely to be approximately seven times riskier than a new one, it’s not considered high risk, relatively speaking.
It should be pointed out that the assessed capacity of building is not set at a level where collapse is a certainty and, therefore, there is an even lower probability again that fatality could result if collapse occurs.
What about the buildings rated below 34%NBS?
A building with an earthquake rating less than 34%NBS fulfils one of the requirements for the Territorial Authority to consider it to be an Earthquake-Prone Building (EPB) in terms of the Building Act 2004. A building rating less than 67%NBS is considered as an Earthquake Risk Building (ERB).
New legislation now applies to earthquake-prone buildings, primarily around identifying the buildings and timeframes for strengthening.
There is a perception that earthquake-prone buildings can’t be occupied, but that’s not correct. They can be occupied, it’s just a question of safety and managing risks. There are consequences under the Health and Safety in Employment Act if owners or occupiers don’t take practical steps to minimise ‘hazards’ in the workplace. However, according to Worksafe, provided steps are actively being taken to address earthquake-prone buildings in accordance with the requirements of the Building Act, then the Health & Safety at Work Act will not impose a higher standard of life safety than the Building Act requires. If these risks aren’t being addressed, then owners and occupiers run the risk of being in breach of the HSAW Act. There’s an exception that serious hazards that are easily addressed will be attended to in a reasonable timeframe.
What recent earthquakes have shown us
While these statistics make sobering reading, things do not always play out like this way in real life!
Some buildings in Christchurch experienced a one-in-2000-year earthquake in 2011 and some buildings in Wellington experienced a one-in-500-year event in 2016 with different levels of performance. A number of buildings were demolished because of immediate safety concerns or because they weren’t considered economic to repair. Many were able to be occupied without repair.
There are no guarantees how a building will eventually perform in a major shake, but the NBS rating system seems to be the best tool we have for now to help us make decisions on managing safety obligations in the workplace.
