Frequently asked questions
What is a site subsoil class and why does it matter?
NZS 1170.5 classifies sites from A, strong rock, through to E, very soft soil, based on the soil profile and its stiffness. The class directly scales the design earthquake actions, so getting it wrong can materially under or over estimate the seismic demand on the structure. It is determined from the geotechnical investigation, which is why the site subsoil class should be confirmed before the structural design is finalised rather than assumed.
What makes a building earthquake prone in New Zealand?
Under the Building Act, a building is earthquake prone if its ultimate capacity is less than 34 percent of the New Building Standard and it would be likely to collapse causing injury or damage in a moderate earthquake. Assessment follows the MBIE and engineering profession seismic assessment guidelines, usually as an initial seismic assessment followed by a detailed seismic assessment where the initial result is marginal or where a more accurate number is needed for a transaction or a strengthening design.
Does seismic design consider the ground as well as the structure?
Yes. Ground shaking is modified by the soil profile, and soft or liquefiable ground can amplify motion, cause settlement, lateral spread or loss of foundation support during shaking. Seismic design therefore covers both the structural response and the geotechnical response, including liquefaction triggering, seismic slope stability and the performance of retaining structures and foundations under earthquake loading.