Bulletin of the New Zealand Society for Earthquake Engineering

Background to proposed changes to NZS 1170.5:2004

2025-03-01T18:59:42+13:00

Seismic hazard with deterministic maximum limits: Considerations in a New Zealand-specific context

2025-03-01T18:59:58+13:00

This paper outlines the consideration of deterministic limits on maximum ground motion levels within seismic design codes and standards. The specific motivation is to outline the basis for the exclusion of such a limit in the 2024 draft Technical Specification for NZS1170.5 [1], despite the presence of such limits in NZS1170.5:2004 [2]. An overview of the historical consideration of so-called ‘deterministic’ and probabilistic seismic hazard analysis methods is provided, as well as how they have translated into contemporary seismic design codes and standards in New Zealand (NZ) and internationally. The fundamental issues with deterministic maximum limits are outlined through the use of examples in a NZ-specific context. The underlying reason ‘well above average’ ground-motion intensity levels (for a given earthquake scenario) are prevalent in regions of high seismicity is discussed, as well as other common misconceptions that lead to the use of deterministic limits to achieve apparently realistic design ground motion intensities. Finally, in the vein of the hazard-risk separation principle, sentiments are expressed for achieving economic and resilient seismic design in regions of high seismicity without resorting to implementing deterministic limits.

Site classification methodology for TS 1170.5 design spectra

2025-03-01T19:00:14+13:00

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 V_s(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 V_s(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 V_s(30) is the principal parameter in the site classification scheme, the establishment of the V_s profile at the site is a critical step. Several methods for obtaining a V_s profile, measured or inferred, and subsequent calculation of V_s(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 V_s(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.

Development of provisions for simplified design of rocking foundations

2025-03-01T19:00:46+13:00

A simplified design procedure has been developed for potential inclusion within NZS1170.5 to allow rocking shallow foundations for low- to mid-rise buildings without special study. Rocking foundations allow a nonlinear uplift and soil yielding mechanism to form at the soil-foundation interface, and can significantly reduce the required size of foundations (or avoid requiring deep foundations) and reduce seismic demands on a building. The procedure has been used to design a series of buildings and the performance of these buildings has been evaluated using a displacement-based assessment procedure that accounts for soil-foundation-structure interaction. Given that the equivalent static procedure in NZS1170.5 contains several conservative assumptions for multiple storey buildings, and the displacement-based approach is a first mode approximation, the majority of designs and assessments were based on a single degree-of-freedom (SDOF) system. Variations in the design and soil property assumptions were considered, as well as different thresholds for proposed limitations on the applicability of the simplified procedure. Seven performance measures were used to evaluate and demonstrate that the limitations in the proposal result in adequate behaviour for ultimate limit state and serviceability limit state actions.

A displacement correction, to conventional NZS1170.5 displacement procedures, is proposed as a concentrated rotation at the underside of the foundation. This correction accounts for foundation rotation prior to reaching the moment capacity.

Recommendations for the revision of the approach for seismic design of parts and components in New Zealand design standards

2025-03-01T19:00:30+13:00

This work makes recommendations for revision of the design provisions for the seismic demands on non- structural elements, parts and components in the New Zealand seismic loading standard. The proposed approach seeks to incorporate new and international knowledge of the factors affecting seismic demands on non-structural elements, but also maintaining simplicity to facilitate adoption. The most significant changes include new expressions for the influence of floor height and building nonlinear response on floor acceleration demands; the amplification of demands on flexible parts due to dynamic amplification associated with the response of structural modes; and potential reductions in part strength requirements by permitting nonlinear part response. The proposed revisions are supported by data from instrumented buildings, numerical modelling and experimental testing. Comparisons between the recommended and existing NZS 1170.5 approaches are presented that show that the proposed approach will lead to reduced design actions in many cases and increased loads in others. Greater demands are prescribed for flexible parts with limited ductile capacity and some parts at the serviceability limit state design level. Conversely, design forces reduce for rigid parts; parts and components on lower levels of buildings; ductile flexible parts; parts in ductile buildings at the ultimate limit state design; and parts with long periods. Finally, the proposed approach is compared with the most recent updates to the code design approaches in Europe and the United States to provide an international context of the state of the art.