An earthquake struck Melbourne late on Sunday night, shaking the city and waking residents. “A magnitude-3.8 earthquake has been recorded with an epicentre near Sunbury, Victoria,” the Victoria State Emergency Service said.

“The earthquake occurred at 11.41pm, with widespread reports (of people feeling it). No injuries or damage have been recorded at this time.”

Sunbury is located about 35 kilometres northwest of the Melbourne CBD. Melbourne residents swiftly hopped on social media to check that what they’d felt really was an earthquake. Many said they’d been woken up by the event. Half an hour after the earthquake, more than 1300 people had reported feeling it to Geoscience Australia.

A resident of Sunbury, Corey Lainez, told The Age he’d felt “one very big, violent shake” and it had left a crack in his kitchen wall. “I thought a car or truck had hit the house, and before I could even stand up the dogs were running around the house barking,” Mr Lainez said. Adam Pascale, a scientist with the Seismology Research Centre, posted a video on social media in the immediate aftermath.

“Woke me up, got me out of bed,” he said. “Potentially there could be some minor damage at the epicentre. I haven’t heard any reports yet.”

This quake was stronger than the 2.8-magnitude one that struck Melbourne’s eastern suburbs earlier this month, though that one was still strong enough to shake people’s homes.

In September of 2021, Victoria was rocked by a record-breaking 5.8-magnitude quake, whose epicentre was near the small town of Mansfield. That one was felt as far away as Sydney and Tasmania. It shook buildings and knocked down walls.

It was followed by two 4.0 and 3.1 magnitude aftershocks 18 and 39 minutes later - both within 10km of the original tremors.

So, how safe is your house?

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. 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.

So, if you live in a domestic house, including landed property, townhouse, unit that is categorised as Class 1 buildings in accordance to National Construction Code (NCC), and less than 8.5meters tall, AS 1170.4 provides for simplified design for domestic houses  are excluded from requiring specific detailing. Read more from HIA's website here. In another word, 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.

Ok, so what type of house requires 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.

Does Your Project Need To Be Accessed For Seismic Design?

Short answer is yes. All buildings and their parts need an assessment of their seismic requirements, in accordance with the NCC. However, it is their ‘Level of Importance (LOI)’ which determines the extent of the seismic design requirements.

The NCC defines the LOI for the different building types, ranging from 1 through to 4, with LOI 1 being the lowest level. The LOI is typically established through occupancy levels and function of the building within the community or broader. If you wish to understand more about seismic design for your projects other than domestic Class 1 houses, refer to this blog for more detailed explanation.

Victorian Building Authority has also issued a industry statement regarding earthquake design and construction, you can read the updates here.

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:

• Walls that are not part of the seismic-force-resisting system

• Appendages, including parapets, gables, verandas, awnings, canopies, chimneys, roofing components (tiles, metal sheeting), containers and miscellaneous components

• Connections (fasteners) for wall attachments, curtain walls, exterior non-loadbearing walls

• Partitions

• Floors

• Ceilings (see also Fix and Float ceilings below)

• Architectural equipment, including storage racks and library shelves with a height over 2.0m.

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:

• Lighting fixtures

• Ducts, cabling and piping distribution systems

• Fire suppression and sprinkler systems.

• For a comprehensive listing of all components which require consideration for earthquake loads, refer to the Australian Standard.

What this all 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 ceilings systems require seismic bracing and ensures 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 two of four walls, with the other two walls being floating. 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.

Can you summarise all this for me?

A summary of what we've covered in these two blogs are as follows:

1. All commercial buildings in Australia need to allow for earthquakes, no matter where they are located.

2. Depending on the building use, engineering calculations are done to determine if the structure of the building can withstand the force of an earthquake.

3. Non-structural components also need to allow for earthquakes both in the way they are fixed and in the way they are located adjacent or to other structural or non-structural components.

4. A structural engineer will assess a building against AS 1170 Part 4. Although there are common or generic earthquake designs for ceiling and wall bracing, etc., this is required for each project.

5. Builders are ultimately responsible for constructing a building according to AS1170.4, the Earthquake Standard. However, architects need to ensure their designs and materials specified can be certified accordingly.

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