Seismic Design in Australia Are You Complying With the National Construction Code and Earthquake Standard?

Seismic design is a regulatory requirement in Australia. In Australia an earthquakes exceeding a magnitude of 7 only occurs every hundred years or so, which is low when compared to countries with higher seismic activity. You may be surprised to know that we have minor earthquakes in Australia almost daily, but the majority of them are minor and go unnoticed.  It is impossible to predict when the next big earthquake will hit, or where, and buildings therefore need to be built according to seismic requirements.

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According to Geoscience Australia, there are on average 80 earthquakes of magnitude 3.0 or more in Australia each year.

"Adelaide has the highest earthquake hazard of any Australian capital. It has experienced more medium-sized earthquakes in the past 50 years than any capital because South Australia is being slowly squeezed in an east-west direction by about 0.1mm year,” the Australian Government organisation said.

Australia's largest recorded earthquake was in 1988 at Tennant Creek in the Northern Territory with an estimated magnitude of 6.6, but it was not in a populated area.

A magnitude 6.5 earthquake at Meckering in 1968 caused extensive damage to buildings and was felt over most of southern Western Australia. Earthquakes of magnitude 4.0 or more are relatively common in Western Australia with one occurring approximately every five years in the Meckering region.

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WHO IS RESPONSIBLE FOR SEISMIC COMPLIANCE?

It is the duty of all those engaged in the planning, designing, and construction of buildings to ensure compliance with the seismic requirements in the NCC and relevant Australian standards. For example, architects and designers need to consider appropriate detailing and specification in accordance with AS 1170.4.During construction, it is the responsibility of builders to make sure that all non-structural elements are properly considered for earthquake loads in their design and that the buildings they build comply with all seismic requirements. They also have to oversee the installation of compliant structural and non-structural components on site. Multi-storey buildings must have structural engineering input to allow for inter-story drift. Engineering must also engage with suppliers and manufacturers regarding mechanical, electrical, and architectural systems and components to make sure they have the necessary performance properties to withstand seismic activity. Finally, building surveyors are responsible for ensuring that documentation sufficiently covers AS1170.4 detailing and that the design appropriately considers seismic actions.

OVERVIEW OF AUSTRALIAN BUILDING CODES AND STANDARDS

According to the NCC Performance Requirements for structures, buildings must perform adequately under all reasonable, expected, and extreme design actions, including earthquake actions. The Deemed-to-Satisfy Provisions of the NCC require buildings to be designed and constructed in compliance with AS 1170.4 to meet such requirements. The relevant provisions are set out in Part B1 Structural provisions. In general, all buildings must adhere to AS 1170.4. The only exceptions are Importance Level 1 buildings (small, isolated structures that are rarely inhabited) and certain domestic Class 1a or 1b buildings that are less than 8.5 metres tall and comply with Appendix A and the applicable design standards. Different requirements apply to structural and non-structural components, respectively.

The design of structural components must incorporate a seismic force-resisting system with a well-defined load path, or paths, to transfer both gravity loads and the earthquake actions produced in the earthquake to the supporting foundation soil. Meanwhile, non-structural components and their fastenings must be designed for horizontal and vertical earthquake forces in accordance with Section 8 of AS 1170.4.In June 2020, a new standard for suspended ceilings - AS/NZS 2785:2020 “Suspended Ceilings – Design and Installation” was released, coinciding with the increased focus on seismic compliance. This standard provides valuable guidance on seismic restraints in relation to ceilings and partitions.

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WHAT DESIGN LOADING APPLIES?

Earthquake actions used in design must be appropriate to the type of building, its intended use, exposure to earthquake shaking, and design working life. When determining the applicable design loading, several factors need to be considered.

The factors include:

• location of the site to determine the Hazard Design Factor (see Section 3 of AS 1170.4);
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• Importance Level of the building (Levels 1 to 4),depending on the type and purpose of the building being constructed (see Table B1D3a of NCC Vol.1) ;
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• Site Sub-Soil classification (see Section 4 of AS 1170.4);
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• structure height. Based on these factors, the Earthquake Design Category (EDC) can be determined using Table 2.1 in AS 1170.4.

Note that a seismic engineer must be engaged to determine which EDC applies to the building.

DESIGN OF PARTS AND COMPONENTS

Section 8 of AS 1170.4 details the non-structural building elements and components that must be engineered to withstand both vertical and horizontal seismic forces for Class 2 to 9 buildings. The following is a list of non-structural components that need to have earthquake loads taken into account:

• walls that are not part of the seismic-force-resisting system;
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• appendages, including parapets, gables, verandas, awnings, canopies, chimneys, roofing components(tiles, metal sheeting), containers and miscellaneous components;
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• connections (fasteners) for wall attachments, curtainwalls, and exterior non-loadbearing walls;• partitions;• floors;• ceilings; and
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• architectural equipment, including storage racks and library shelves with a height over 2.0 meter

AS 1170.4 requires that the design of the mentioned parts and components be completed for earthquake actions using one of the techniques described in Section 8. This includes using established principles of structural dynamics or using general or simplified methods expressed in Clauses 8.2 or 8.3 of Section 8.3

RECENT REGULATORY CHANGES

In late 2023, Standards Australia’s BD-006 committee released a draft of an amended version of AS 1170.4 for comment. The amendment’s primary goals are to update the seismic hazard map and include general editorial changes to increase clarity and eliminate ambiguity. While changes to AS 1170.4 are still being considered, architects and designers should note the significant update that AS/NZS 2785 received in 2020. The previous version of the standard included general guidance on earthquake design for suspended ceilings, but many important details were lacking or not clear. This included how ceilings would interact with other components and the required clearances; methods for achieving adequate seismic restraint; and inter-storey drift and clearance requirements.

The updated 2020 standard provides clearer guidance and specifications covering a number of design aspects. This includes consideration of the primary structure to support ceiling loads; clarification on imposed actions; seismic clearance requirements for adjacent elements and penetrations; and serviceability requirements for square-set joints and cornices. Notably, the 2020 standard specifies partition wall restraint methods and requirements regarding when it is and is not appropriate to have ceilings fixed to partition walls.

In summary, partitions should not be attached to the ceiling unless the ceiling has been designed to provide restraint. If the ceiling is attached to the partition wall, the wall needs to be braced to the soffit and able to support the ceiling actions. If partition walls are fixed to the suspended ceiling, aseismic event may cause the ceiling to collapse due to the horizontal deflection of these components. Partition walls must have independent bracing and be supported through the ceiling with seismic gaps that accommodate the calculated loads.

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Ensure your structures are resilient and compliant with Australia’s rigorous seismic standards by partnering with our experienced seismic design team. With deep expertise in AS1170.4 compliance, risk assessment, and modular framing systems, we specialize in earthquake-resistant solutions that safeguard your projects. Trust ArchiEng to provide tailored, reliable seismic engineering services that protect both your investment and your peace of mind.

Contact us today to future-proof your build against seismic risks and achieve optimal safety and compliance!

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