|†||Session 3.1 - Seismic Loadings Standard||†|
The development of the New Zealand seismic coefficients of the draft Australia/New Zealand Loadings Standard from a probabilistic seismic hazard analysis (PSHA) is presented. The hazard factors Z arise directly from a scaling of the 500-year return period estimates of the 5% damped response spectral accelerations SA(T) for period T of 0.5s. Truncation of Z to the range corresponding to rock peak ground accelerations of 0.13g to 0.6g is justified on deterministic grounds. Normalisation at the intermediate period T of 0.5s avoids the need for the U.S. practice of mapping multiple spectral values to construct spectra. The code spectral shapes for the various site classes give near upper bounds across New Zealand to mapped 500-year ratios of SA(T)/SA(0.5s) resulting from the PSHA, leading to code spectra that are near-envelopes of the hazard spectra. Hazard curves for most locations are fitted well by the risk factor R that scales the 500-year spectra to other return periods. The new near-fault factor N(T,D) caters for systematic near-fault directivity effects for longer-period structures at distances D less than 20 km from New Zealandís most active faults. This factor is based on results calculated from published empirical near-source modification factors.
Keywords: loadings code, seismic hazard, spectra
The development of the joint Australian/New Zealand earthquake loadings standard has been underway for the past 5 years as part of the development of the joint Structural design actions (Loadings) standard, AS/NZS 1170. Comments within this paper are based on committee draft 8 (December 2002) which, although expected to still be subject to further change, is considered to be approaching the final standard technically. The paper is structured to follow the layout of the new standard. Important changes are emphasised along with discussion of the background upon which the committee decisions were concluded.
Keywords: earthquake loadings, Australia New Zealand joint standard, capacity design, performance engineering