As 1170.4 Commentary !full! -

The hazard factor ($Z$) represents the seismicity of a specific region. In Australia, $Z$ values range from 0.03 (low hazard) to 0.20+ (higher hazard areas like parts of Western Australia and South Australia).

| Criterion | Score (out of 10) | | :--- | :--- | | Accuracy (for its era) | 8 | | Clarity & readability | 7 | | Practical worked examples | 6 | | Up-to-date relevance (2025) | 4 | | Value for seismic region III | 9 | | Value for seismic region I | 2 | | | 6/10 |

This is the variable that accounts for the building's natural period ($T_1$) and the spectral as 1170.4 commentary

Where:

10. Alex McFadyen - Australian Earthquake Engineering Society The hazard factor ($Z$) represents the seismicity of

Essential reference for structural engineers, but not a standalone design code.

This document is not the Standard itself but the explanatory companion published by Standards Australia. The main standard references ductility capacity ((\mu)) and

Perhaps the most misunderstood concept in Australian seismic design is ductility. The main standard references ductility capacity ((\mu)) and structural ductility factors. The commentary devotes significant space to this because Australia uses a unique approach compared to the US (IBC) or Europe (EC8).

The determination of $Z$ is probabilistic. It is derived from a 1/500 annual probability of exceedance. A critical commentary point here is the granularity of this data. While capital cities have well-defined values, regional areas can sometimes suffer from a lack of granular seismic data, requiring engineers to exercise judgment or refer to specific geological reports. The standard provides a map, but local site-specific hazard analysis can sometimes yield results that deviate from the broad zoning.

Section 4 of the Commentary provides invaluable soil mechanics context. It explains the (average shear wave velocity to 30m depth) methodology, which is only briefly mentioned in the main Standard. For engineers dealing with deep soft soil sites (Class Ee, Ee/De), the Commentary warns about the limitations of the simplified site response spectrum—specifically regarding long-period amplification.