Earthquake-prone buildings, explained

What is an earthquake-prone building?

Hayley: A building, or part of a building, is earthquake prone if it is likely to collapse and cause injury or death, or damage to another property, in a moderate earthquake. An earthquake-prone building can only withstand one third of the earthquake shaking that a new building at the same site could take.

Earthquake-prone buildings are given an earthquake rating, commonly referred to as a percentage of the new building standard (NBS). Earthquake ratings mean the degree to which the building, or part, meets the seismic performance requirements of the Building Code. Buildings that achieve less than a third of the new building standard (below 34% NBS) are considered earthquake-prone.

For context, if a brand-new building were built today it should be designed to achieve at least 100%NBS. A building which is 100%NBS should be designed not to collapse in a larger earthquake so that people can get out of the building safely. However it is important to note that the building may not be usable after the earthquake.

A structural engineer undertakes an engineering assessment to establish the %NBS of a building and if the building achieves less than 34% NBS it is considered to be earthquake-prone. Following a review of the engineering assessment by one of the team, the building is issued with an earthquake-prone building notice, which allows the owner of the building either 7.5 years (if it is a Priority Building) or 15 years to either strengthen or demolish the building.

The notice requires that the building owner must either strengthen or demolish the building or part of the building that causes it to be earthquake-prone. The building can still be used and occupied for its current lawful use prior to the notice expiring. The notice must also be displayed on the building in a prominent place, on or adjacent to the building.

There are four types of notices, with the different borders indicating the type of earthquake-prone building. Notices with an orange and black striped border are for buildings with an earthquake rating of 0% to less than 20% NBS, or where no engineering assessment has been provided and the earthquake rating has not been determined.

What do the engineers look at?

Hayley: In the first instance an engineer will be interested in taking a close look at the original plans of the building as well as any drawings that document any previous structural upgrades. They will consider the building age, soil conditions, construction type, number of storeys and if a building is structurally connected to the neighbouring building. They will also undertake an external and internal inspection of the building to ensure the building as built, matches the existing drawings.

An engineer could then undertake a very broad basic assessment, which is called an Initial Evaluation Assessment. This preliminary assessment is carried out as a screening tool to determine a rough indication of the seismic performance of the building.

A Detailed Seismic Assessment (DSA) could be a next stage assessment and this is a more involved evaluation which includes some calculation and/or computer analysis providing a more accurate indication of the seismic performance of a building.

Do you need advice or support?

Whatever your earthquake-prone building issue, we are here to support you and help you to understand the steps you need to take to improve the earthquake safety of your building. Please get in touch. Email buildingresilience@wcc.govt.nz or call 04 499 4444 and ask to speak to one of the Resilient Buildings team members.

If you own a residential earthquake-prone building and find yourself in hardship over earthquake strengthening, then Kāinga Ora is offering a loan scheme. You can find out more and apply on Kainga Ora website.