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Earthquake requirements for Class 1 buildings

Builders and designers often query whether Class 1 buildings are required to be designed to comply with AS 1170.4 Earthquake actions. The information below provides an overview of the compliance requirements in NCC 2019 Volume Two for earthquake design and detailing for Class 1 buildings.

The NCC requires that earthquake actions be considered under Performance Requirement P2.1.1 Structural stability and resistance to actions. Part 3.0.1 outlines that the design of a building in accordance with AS 1170.4 will meet the Performance Requirements. 

AS 1170.4 provides for simplified design for domestic structures (Class 1a and 1b) such that most houses are excluded from requiring specific detailing.  However apartments, taller or larger homes, or those in high seismic activity areas will require detailed earthquake design and construction to suit.

Here’s more detail on what that all means:

Earthquakes and AS 1170.4

Earthquakes are high-risk, low-return events and it is therefore unreasonable to design every building to withstand all potential earthquakes. As a result, AS 1170.4 works to ensure the protection of people during an earthquake event by allowing the building to withstand the earthquake long enough for people to exit the building, balancing the need for additional construction requirements with the probability of an event and the harm caused to life and the economy. 

This is achieved by scaling the loads for structures against their expected impact on this balance.  
Earthquake loading is based on the location, height and weight of the structure and the soil conditions of the site. The loading induced in a structure during an earthquake is dictated by its own mass and height. 

The loads on taller and heavier structures will be higher than for shorter, lighter structures.  Those founded on softer soils will also be subjected to higher loading. Any combination of these factors may result in the anticipated loading being high enough to warrant earthquake detailing.

AS 1170.4 maps out the seismic areas in Australia of greatest earthquake hazard. The hazard factor associated with the location of a structure is one of the key considerations in the overall design requirements for a building. The hazard factor can be identified only from the maps included in AS 1170.4. 

How does AS 1170.4 affect design of domestic structures?

Appendix A of AS 1170.4 allows for simplified assessment of domestic structures less than 8.5 metres high and with a least plan-dimension of 16 metres, where traditional construction techniques are employed. 

Importantly, houses within these criteria and located in areas where the hazard factor is 0.11 or less do not require any additional earthquake detailing, as domestic structures are considered to perform sufficiently in an earthquake if they are designed for the local wind conditions.

Houses with a third level, highly irregular plan or alternative construction methods are required to undergo a full assessment and include earthquake construction details to suit.  

Houses proposed in locations where the hazard factor is 0.12 or greater that use traditional construction methods can have a simplified assessment completed, where it is likely that some construction detailing will still be required. 

Detailing for earthquakes

Houses required to be designed to resist earthquakes require stiff plates at floor and roof levels and sufficient walls to transfer earthquake loading to the ground. Plates and walls need to be connected together to prevent them sliding away from each other as an earthquake hits. 

Taller structures will generally require increased construction requirements towards the lower levels. A building of heavy construction will also require more inclusions.

Some common detailing requirements for houses that require earthquake detailing include additional bracing at ceiling level, increased bracing capacity for walls and tie connections from footings to slabs.

When does a building require seismic design?

A building may require seismic design if:

  • it is not Class 1 or 10 in accordance with the NCC, or
  • the site is located in an area where the hazard factor is greater than 0.11, or
  • it has more than two levels, unless:
    • one level is entirely underground (note: basement with a garage door does not classify as fully underground)
    • the top level is a light loft with all walls within the roof, or
  • the structure is highly irregular in shape, or
  • it has a least-plan dimension of more than 16m, or
  • the building is to be constructed from non-standard materials (such as rammed earth, hay bale construction and so on).

If a building falls into any of these categories, speak with your engineer and building approval body about the potential for earthquake design to be required. Even if a building does fall into these categories, it may not need any additional detailing.

To find out more, contact HIA’s Building Services team.

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