Resources

Purpose

This notice updates and supersedes TAN #22-10, published 4th October 2022.

This note is to advise the sector of the direction of the NZ Transport Agency regarding the adoption and implementation of updated seismic hazard models in the design of state highway bridges.

General information

The understanding and estimation of seismic hazard across New Zealand has evolved significantly throughout the past two decades, and an updated National Seismic Hazard Model (‘NSHM’) was published by the Institute of Geological and Nuclear Sciences (GNS) in 2022. The 2022 NSHM is expected to be the basis for determination of seismic hazard on new capital improvement projects through the development of a Site-Specific Seismic Hazard Assessment (‘SSSHA’) or other means.

The seismic hazard presented in the 2022 NSHM represents a general increase over previously codified values used in structural design, particularly for regions of moderate-to-high seismicity. The risk thresholds set out in the NZTA Bridge Manual (the ‘Bridge Manual’), when applied to the increased seismic hazard presented in the 2022 NSHM, may represent a significant challenge to the affordability of new structures due to the onerous resulting seismic demands.

A comprehensive review of current seismic design procedures in the Bridge Manual and associated documents is underway, in collaboration with the wider sector. This review encompasses seismic risk settings and associated performance criteria. Whilst this review is in progress, and until further direction is provided through updates to relevant Standards, the following approach shall be adopted in the design of new NZTA structures.

The processes and procedures outlined in this update must be adopted for NZTA projects commencing subsequent to the date of this TAN.

Key changes

Basis of seismic hazard

The 2022 NSHM shall form the basis for the definition of seismic hazard on new projects. The following approaches are acceptable for adoption of the 2022 NSHM:

• The draft TS1170.5 published by MBIE in 2024, or
• Direct adoption of the GNS output of the 2022 NSHM, particularly for PGA and for structures with a period greater than 1 second noting that the 2022 NSHM does not apply a plateau in the short period range as is typical for design spectra, or
• Site specific assessment, bounded by +/- 30% of 2022 NSHM.

The provisions of the Bridge Manual, including the definition of Importance Levels and return periods for design events, shall form the initial basis for design and reviewed with the below considerations:

• Importance levels: The Importance Level assigned to each structure should primarily be determined with respect to the route classification, post-disaster function and project outcomes. Where the construction cost of a structure results in an uplift of IL which does not align with the adjacent road corridor and/or the project outcomes, this should be identified and an alternative proposed where appropriate.

• SLS performance: The performance of a structure subject to a SLS-level earthquake shall be appropriate for the route designation (ONRC), the resilience of the adjacent corridor, and for the level of redundancy in the surrounding local transport network to support network operability following moderate earthquakes. A relaxation of SLS performance requirements may be warranted for design solutions which enable fast repairs and minimise network disruption following a design-level earthquake.

• CALS performance: The performance of a structure subject to a CALS-level earthquake shall be conditioned to safeguard life safety principles, noting that significant structural damage may be expected at typical structures following a CALS earthquake. The outcome of the Bridge Manual provisions for defining the CALS event should be checked against the thresholds for life safety in the NZ Building Act, noting that a 1-in-2,500 year return period event is considered an appropriate "upper bound“ for most structures.

• Performance criteria: Performance criteria limits in the Bridge Manual, such as settlement and displacement limits. which are identified to result in considerable additional cost to the design solution should be reviewed and the potential efficiencies provided by the adoption of performance-based criteria considered and presented.

• Baseline performance: a 1-in-500 year return period earthquake for the DCLS/ULS represents a consistent historic benchmark for seismic risk for most structures. Performance and risk should be tested against this baseline where amendments to Bridge Manual risk metrics are proposed.

Opportunities for efficiency

Design should be cognisant of opportunities for efficient solutions through consideration of the following early in the design process:

• Significant step changes in demand and performance should be identified to inform understanding of risk in decision making.
• The design process should thoroughly consider the advantages and potential cost savings presented by the adoption of low-damage solutions. This is of particular relevance at sites where inertial, rather than kinematic, effects are of primary concern.
• Single-span bridges designed to slide on shallow foundations present an efficient solution that minimises structural damage and allows quick reinstatement. Such solutions should allow for the expected level of movement at CALS-level earthquakes to avoid loss of seating.
• Capacity design principles and good detailing provide confidence in performance beyond assessed levels and with inherently uncertain conditions.
• Advanced analyses, where appropriate, may provide significant efficiencies in assessing the seismic performance of bridges, particularly where high PGAs and significant liquefaction coincide.

Implementation

Seismic Resilience Philosophy

A Seismic Resilience Philosophy (SRP) shall be developed for each project identifying appropriate Importance Levels, return periods and performance criteria with consideration of context and consequence. The SRP shall be included within the Design Philosophy Report as required by the Bridge Manual and subject to the approval of the NZTA Lead Advisor Structures at structures@nzta.govt.nz

Further opportunities and clarifications

For clarification and discussion of further opportunities, please direct queries to the Lead Advisor Structures at structures@nzta.govt.nz.

Document review

This technical advice note will be withdrawn when a future amendment of the Bridge Manual replacing these requirements is implemented. Cancellation will be confirmed at that time.

Publication details

Publication title: Seismic resilience of new state highway bridges

Published: 17 December 2024

Reference: #24-01

Location: https://www.nzta.govt.nz(external link)

Further information

If you have any questions or feedback, please contact the Lead Advisor Structures at structures@nzta.govt.nz

For more information visit the Highways Information Portal