Site classification methodology for TS 1170.5 design spectra

The Technical Specification (TS) 1170.5 has been developed to incorporate the output of the 2022 New Zealand National Seismic Hazard Model revision (NSHM2022) [1] and update Clause B1 Verification Method 1 (B1/VM1) of the New Zealand Building Code. In this paper, we discuss the proposed site classification methodology based on Vs(30) (i.e., the time-averaged shear-wave velocity from the ground surface to 30 m depth) which is used to incorporate site effects in the TS 1170.5 design spectra. The reasoning for the use of Vs(30) for site classification, a significant departure from New Zealand Standards NZS 1170.5 [2], is first elaborated. Based on detailed scrutiny of uniform hazard spectra obtained from NSHM2022, seven site classes are proposed, with associated design spectra for six of the site classes. Multiple objectives were considered in the definition of TS 1170.5 site classes, with the principal goal being to represent relevant site conditions in a robust yet practical manner, appropriate for engineering design practice. As Vs(30) is the principal parameter in the site classification scheme, the establishment of the Vs profile at the site is a critical step. Several methods for obtaining a Vs profile, measured or inferred, and subsequent calculation of Vs(30) are recommended. Each method is associated with a different uncertainty factor that affects both site classification and consequent design spectra. In this context, a multiple site class definition must be adopted with an envelope design spectrum in cases where the range of Vs(30) values span several site classes. Importantly, the variation in design spectra due to uncertainty in the site class is relatively small compared to the uncertainty in the uniform hazard spectra themselves (due to uncertainties in NSHM2022 and PSHA). For sites with ground conditions not well-represented within the PSHA performed for NSHM2022, site-specific (special) studies are recommended.