Earthquake Design Requirements In Australia

Australia sits away from the plate boundaries that produce the largest earthquakes, which leads to a common assumption that seismic design does not apply here.

It does. Every building is designed for earthquake actions, and in a low seismic region what that mostly changes is the detailing rather than the member sizes.

What governs the load

Four inputs.

The hazard at the site, which varies across the country and is not uniform. Some areas carry a meaningfully higher hazard factor than others, and it is read from the standard for the actual location rather than assumed.

The site subsoil class, which is frequently the largest factor. Soft or deep soil amplifies ground motion substantially compared with rock. A building on deep soft sediment can see several times the acceleration of the identical building on rock a kilometre away.

Which means the geotechnical investigation determines the seismic load, and a site classification is an input to the structural design rather than only a footing matter. It sits alongside the work in [site classification and soil investigation basics](/blog/site-classification-soil-investigation-basics).

The importance of the structure, because a hospital, an emergency facility or a building holding large numbers of people is designed for a less frequent and larger event than an ordinary one.

The structure's own period and ductility, meaning how it responds and how much it can deform without failing.

Why it rarely governs member sizes

In most Australian buildings the wind load exceeds the earthquake load, so the lateral system is sized by wind.

What earthquake design then adds is the requirement that the structure behave acceptably when it is pushed beyond its elastic limit. Wind is a load you design not to be exceeded. An earthquake is an event where the structure is expected to deform, absorb energy and survive, which is a different design philosophy.

That is why ductility is the central concept, and why the detailing is the real output.

What ductile detailing actually means

A member that fails gradually, with warning, after significant deformation, rather than suddenly.

In concrete that means:

Confinement. Closely spaced closed ligatures in columns and at the ends of walls and beams, holding the concrete together so it can deform without disintegrating. Ligature spacing is a seismic requirement as much as a shear one, which is part of the congestion problem in [reinforcement detailing and congestion](/blog/reinforcement-detailing-and-congestion).

Anchorage and lap lengths developed properly, and laps kept away from the locations where the greatest deformation is expected. A lap that slips converts a ductile member into a brittle one.

Avoiding brittle failure modes, meaning the member is designed so bending governs rather than shear. A beam that fails in shear before it yields in bending has no ductility at all, and designing so the ductile mode happens first is the whole intent.

Continuity, so the structure holds together. The tying provisions overlap almost entirely with the integrity requirements in [disproportionate collapse and tying requirements](/blog/disproportionate-collapse-and-tying-requirements).

In steel it means connections that can develop the member's capacity and sections that will not buckle locally before they yield.

The parts that actually fail in Australian earthquakes

This is the practical point, and the Newcastle event in 1989 is the local reference.

The damage in a moderate earthquake in a region like ours is concentrated in the non-structural elements and the older unreinforced masonry, not in modern engineered frames.

Unreinforced masonry, particularly parapets, chimneys, gable ends and facade elements. Heavy, brittle, often poorly tied to the structure, and positioned where they fall onto footpaths. This is the single largest life safety issue in Australian seismic risk, and it is an existing-building problem rather than a new-design one, which is why it turns up in [heritage building assessment](/blog/heritage-building-assessment-guide) and [cracking in brickwork](/blog/cracking-in-brickwork-what-it-means).

Facade panels and their fixings, where the panel has to accommodate the structure's movement without its fixings failing. The anchor capacity question in [anchors into concrete and why they fail](/blog/anchors-into-concrete-and-why-they-fail) and the substrate question in [facade fixings and substrate capacity](/blog/facade-fixings-and-substrate-capacity) are both seismic issues.

Rooftop plant and services, which are heavy, mounted high, and frequently fixed with whatever was available. Restraint of plant, tanks, ducts and pipework is a design requirement, not a mechanical trade decision, and it overlaps with [rooftop solar and plant structural adequacy](/blog/rooftop-solar-and-plant-structural-adequacy).

Internal partitions and ceilings, where a suspended ceiling with no lateral restraint comes down on the occupants.

Stairs, which have to keep working after the event because they are the egress route, and which are frequently detailed without allowance for the building's inter-storey movement.

Separation and pounding

Two buildings close together move independently and can strike each other.

Which makes the separation between structures, and across movement joints within a structure, a seismic dimension rather than only a thermal one. A joint sized for thermal movement is usually far too small for the relative displacement in an earthquake.

The same applies internally to anything that bridges a joint: a pipe, a duct, a walkway or a cladding panel crossing a movement joint has to tolerate the movement or it becomes the connection that transfers the load.

For existing buildings

A change of use that increases the importance level, an addition that changes the mass or the period, or a modification that removes bracing all bring the seismic case back into play.

Which means it forms part of any assessment under [change of use structural assessment](/blog/change-of-use-structural-assessment), and on an older masonry building the parapet and facade restraint is usually the first thing worth looking at, because it is the highest consequence item and generally the cheapest to fix.

ACSES provides structural engineering and structural assessment across Sydney. Talk to us about a project.