Seismic Focus Part 2: What are Earthquake Design Categories, and how do they affect design?

Our last blog covered why Australia has seismic requirements and why our earthquake standard is getting more attention lately.

We also covered some of the basic requirements of the NCC around seismic compliance, in particular, the need for conformance with AS 1170.4-2007: Structural Design Actions Part 4: Earthquake actions in Australia.

In this blog, we'll review how these requirements play out for internal wall and ceiling linings and framing, and their associated fixtures.

What are Earthquake Design Categories, and how do they affect design?

Australia has three Earthquake Design Categories (EDC), covering low level, low-risk commercial buildings up to multi-level, higher-risk buildings that contain many people.

The relevant EDC is determined by referring to Table 2.1 of AS1170.4-2007.

Briefly, the implications of these three categories are summarised below:

EDCI: Earthquake Design Category I requires an engineering analysis of a simple lateral load applied at each level. This means the structure of a building must be able to withstand the sideways force of an earthquake. (refer Section 5 of AS1170.4)

EDCII: Earthquake Design Category II requires a static engineering analysis. This means the building must be able to handle the force of an earthquake in all directions. (refer Section 6 of AS1170.4)

EDCIII Earthquake Design Category III requires a full engineering design with dynamic analysis. This is required for the highest hazard levels and tallest structures. (refer Section 7 of AS1170.4)

This third category involves calculating the effect of the force of an earthquake in all directions, including inertia and vibration.

At the end of the day, to stop framing moving or breaking in the event of an earthquake, seismic restraints or bracing gets specified by the engineers.

What is Australia's standard for seismic bracing?

Australia's standard for structural design for earthquakes is AS 1170.4-2007.

This includes the requirements for seismic bracing and earthquake resistance of all components.

The National Construction Code (NCC) requires buildings to comply with AS1170.4 with reference to Section 8 of the standard for non-structural components.

Seismic bracing has long been in the building code, but rarely thought necessary in Australia to enforce it. As a result of the lack of enforcement, builders and contractors have often overlooked the bracing requirements of the seismic standard.

Seismic Focus Part 2: What are Earthquake Design Categories, and how do they affect design?

In line with more attention being paid to the seismic standard, a new suspended ceilings standard was released in June 2020, namely, Suspended Ceilings – Design and Installation AS/NZS2785:2020.

ANZ Sylvia Park project – Seismic bracing detail

What do architects need to know about AS1170.4?

A common misconception is that only the primary structural frame needs to be considered for earthquake actions, but this is not the case. Non-structural components and fastenings need to be designed for earthquake forces as required by AS1170.4. This applies to all Earthquake Design Categories.

AS1170.4 Clause 8.1.4 provides a comprehensive list of non-structural components that require consideration for earthquake loads as follows:

The Australian Standard also stipulates that several mechanical and electrical components and their fastenings commonly found in high rise buildings also require consideration of their capacity to accommodate earthquake loads. For example:

What this means is that lightweight steel framing such as partition and suspended ceiling framing require a new structural assessment for every project.

A structural engineer is required to ascertain if a project's partitioning and ceiling systems require seismic bracing and ensure the framing system is safe in the remote possibility of a seismic event.

A structural engineer does this by running the required seismic bracing load calculations according to the relevant Earthquake Design Category.

What is a Fix and Float floating ceiling?

Part of the seismic restraint design begins at the architectural stage, in consultation with structural engineers and installers to allow suitable spacing between structural and non-structural elements in the building. This reduces or prevents collision or damage in the event of an earthquake.

For ceilings, a common seismic design is called ‘fix and float’, where the ceiling framing is only fixed to one of four walls, with the other three walls being floating. Supawood's recommended 15mm perimeter joint can still be used, but the framing is not locked to the other three walls. Usually, the most stable wall is chosen as the anchor wall. This allows the ceiling to move in an earthquake without being torn apart.

Note that walls are usually braced anyway, so the main effect of seismic engineering are changes to the ceilings and the fixtures and services that run through them.

What do builders and contractors need to know about AS1170.4?

Building contractors are ultimately responsible for ensuring that the buildings they construct comply with the requirements of AS1170.4.

Builders need to ensure that all non-structural elements such as the architectural, mechanical and electrical components referred to above have been properly checked for earthquake loads in their design through to installation.

It may be best to request the structural engineer to provide effective floor acceleration and the allowances for inter-storey drift to manufacturers of architectural, electrical and mechanical components to make sure the selected systems have the needed performance characteristics to accommodate the expected earthquake actions.

In summary...

For your peace of mind, most Supawood systems are available with seismic engineering calculations for no extra charge.

Images: ANZ Sylvia Park, Auckland NZ