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BC Update 172: Updating the seismicity hazard factor for Canterbury

Posted: 28 September 2015

BC Update
This notice, issued as Building Act section 175 guidance, is to notify structural engineering designers and reviewers of minor amendments to the Canterbury seismic hazard values for use in structural design.

This Building Controls Update is a periodic news bulletin for the building sector.

In May 2011, on advice from seismologists and engineers, the Department of Building and Housing made immediate changes to the seismic hazard factor in the Canterbury earthquake region (Christchurch City, Selwyn District and Waimakariri District) because of a heightened risk of seismic activity over the next few decades (refer to BC Update 114). The Z factor, as described in NZS 1170.5, was amended to be not less than 0.3 (previously 0.22 in Christchurch) and the return period factor Rs for the serviceability limit state to be increased from 0.25 to 0.33 in the Canterbury Earthquake region. The site hazard spectra using the Z = 0.3 minimum factor applied to all structures with periods up to 1.5 seconds. Pending further research, structures with periods greater than 1.5 seconds were to be subject to specific investigations (special studies).

Following additional research and consultation with experts, the Ministry is further clarifying the seismic hazard to be used for building work design for the Canterbury region.

The limitation on using the site hazard spectra using Z=0.3 minimum factor in Canterbury only for structures of 1.5 seconds and less has now been removed. Designers can now use the specified Z factor as amended in May 2011 (ie not less than 0.3 in the Canterbury earthquake region) along with the Spectral Shape Factor Ch(T) per Table 3.1 and Figures 3.1 and 3.2 of NZS 1170.5 for the full period range.

In addition, the return period factor Rs for serviceability limit state in Canterbury shall now revert to 0.25, i.e. the same as that provided in Table 3.5 of NZS 1170.5.

Note: This does not change the Peak Ground Acceleration, PGA, values for liquefaction assessment (Class D – 0.13g for SLS event and 0.35g for ULS event, per the Residential Guidance Appendix C2) as these geotechnical design parameters are derived on a different basis to the structural values.

It is anticipated that the clarifications that this update relates to will be incorporated into the revision of NZS 1170.5 within the next few months. Referencing the revised standard in an updated Building Code Verification Method B1/VM1 will follow.

Note: This Building Controls Update was amended on 31 March 2017.

This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: