From hazard factor to base shear: AS 1170.4 in five steps
A walk through the equivalent static method: hazard factor Z and probability factor kp, site sub-soil class, the spectral shape factor, structural ductility and the vertical distribution of forces.
Australia's earthquake loading standard turns a regional hazard into storey forces using a small number of factors. The equivalent static method is the usual starting point for regular low-rise buildings.
- 1. Choose the importance level and annual probability of exceedance (from AS/NZS 1170.0). This gives the probability factor kp, which is 1.0 for a 1/500 event.
- 2. Read the hazard factor Z for the site from the standard's map or table. Always check the current edition and amendments. The toolkit asks you to enter Z rather than assuming a value.
- 3. Classify the site sub-soil from Ae (strong rock) to Ee (very soft soil). Softer sites amplify longer-period shaking.
- 4. Estimate the fundamental period, T1 = 1.25 kt hn^0.75, and read the spectral shape factor Ch(T1).
- 5. Divide by the structural ductility factor μ, multiply by the structural performance factor Sp, and get V = kp Z Ch(T1) Sp/μ · Wt. Then distribute it up the height with kF,i.
Two things in this picture are easy to miss. First, soft-soil sites (De, Ee) keep high spectral values out to much longer periods, so a flexible building on soft ground is not automatically safer. Second, for low-hazard sites kpZ has a minimum value, so even remote locations get some design action.
Geotechnical side: the same Z feeds the ground
The peak ground acceleration used in liquefaction and retaining-wall checks is related to kpZ and the site class. Mononobe–Okabe adds a dynamic earth-pressure increment to a wall with kh (often taken as a fraction of PGA), and pseudo-static slope analysis applies kh to every slice. These checks are linked and should use one consistent design basis.