AS 1170.4-2007 (R2018)
Structural Design Actions — Earthquake Actions in Australia
AS 1170.4-2007 (Reconfirmed 2018) sets out the procedure for determining earthquake actions for the design of structures in Australia. It defines hazard-mapped peak ground acceleration values across the country, classifies sub-soil conditions, specifies importance levels for different building classes, and prescribes the analysis methods (equivalent static, modal response spectrum, time-history) required at each combination of seismicity, structural form and importance level. The standard is the controlling reference for earthquake actions on Australian buildings and is referenced from AS 3600 (concrete), AS 3700 (masonry), AS 4100 (steel) and AS 1720 (timber) as the applicable seismic loading basis. AS 1170.4 is applied across new design and existing-asset assessment alike; in the existing-asset context, the standard provides the loading basis against which residual seismic capacity is assessed. The 2007 edition with 2018 reconfirmation remains the controlling Australian reference; a draft revision (AS 1170.4-202x) is on the Standards Australia work programme.
Australia is a low-to-moderate seismicity region by global standards, and the engineering tradition has historically treated earthquake actions as a secondary check against wind. That treatment is increasingly inadequate for two reasons. First, AS 1170.4-2007 quantifies seismic hazard non-trivially across populated regions: Sydney, Melbourne, Newcastle, Adelaide and parts of Perth all carry hazard values that produce design seismic actions exceeding wind on certain structure types — particularly low- and mid-rise unreinforced masonry, which has very low ductility. Second, for existing-asset assessment in heritage buildings (which are typically unreinforced masonry), seismic out-of-plane wall capacity is routinely the controlling failure mode, and the AS 3700-2018 / AS 1170.4-2007 combination is what the assessment must demonstrate. The Prince Consort Hotel investigation is a worked example: the boundary wall was assessed for out-of-plane bending capacity under AS 3700-2018, with seismic loading determined per AS 1170.4-2007 for the Brisbane sub-soil class. The check confirmed compliance, but only after the calculation was performed; the visual condition was insufficient evidence either way. Three application notes matter for existing-asset assessment. First, AS 1170.4 importance levels are critical — most heritage buildings fall into Importance Level 2 (the standard occupancy level), but post-disaster shelters, hospitals, schools and emergency-service facilities are higher, and the seismic action increases accordingly. Importance level must be confirmed in writing with the asset owner before the calculation is finalised. Second, sub-soil class drives the response spectrum amplification. Brisbane CBD is typically Class B (rock) at depth but commonly Class C or D (medium-soft soil) at the foundation depth of historic buildings; assuming Class B unconservatively reduces the design action by a factor of 1.5 or more. We require geotechnical confirmation of sub-soil class for any heritage seismic assessment that controls the structural decision. Third, the standard is silent on existing-structure assessment provisions. There is no formal 'assessment level' framework analogous to ASCE 41 or Eurocode 8 Part 3. TRSC's practice is to apply AS 1170.4 design loading to measured-property capacity, with a documented engineering basis for any reduction factor applied on the basis of low residual life, planned demolition window, or comparable considerations.
Form 15 RPEQ certification on heritage masonry, mid-rise concrete, and steel-framed existing structures routinely references AS 1170.4 as the seismic loading basis. The Form 15 declaration is conditional on the structure meeting the design seismic action; where measured capacity does not meet the AS 1170.4 design action under the current importance level, the Form 15 cannot be issued without either remediation, a documented Performance Solution under the NCC, or a written assessment basis that explicitly accepts a reduced loading. TRSC's standard practice is to issue Form 15 only against the full AS 1170.4 design action unless the asset owner has formally accepted an alternative assessment basis — typically because the building is on a defined demolition pathway with documented heritage consent.