Resources

Purpose

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 (SH) bridges, and to provide associated recommendations for the design of non-SH bridges.

General information

The Bridge Manual is primarily tailored to the design of bridges on the state highway network, with a focus on ensuring continuity of operation of the state highway network for connectivity and freight productivity. Whilst the requirements of the Bridge Manual are based on general engineering principles, the performance requirements are set in accordance with risk tolerance deemed appropriate for the state highway network.

Recent developments in the understanding of natural hazard risk in New Zealand, including seismic and flooding, have led to a general increase in design demands. The increased demands require sharper critical thinking to be applied in setting performance requirements for resilience and in designing efficient solutions for new structures.

Key changes

Basis of seismic design for non-state highway bridges

The NZTA has published a Technical Advice Note (TAN #24-01) to support the delivery of consistent outcomes and efficient solutions in seismic design of State Highway bridges. Local authority bridge owners may choose to refer to this TAN and the principles laid out within it for the delivery of new bridges on the local road network.

Context and level of service of the receiving road network are important factors in setting appropriate levels of risk tolerance for new bridges. Local authorities should consider both in setting risk thresholds and acceptable design approaches. This may include one or more of the following considerations:

• The use existing codes for determining seismic hazard, such as NZS 1170.5:2004, noting that a quantification of equivalent risk based on current seismic hazard models is recommended in such cases,
• A 1-in-500 year DCLS/ULS event is embedded within the Building Code framework as a benchmark appropriate for a significant proportion of structures, and this can be considered an appropriate benchmark for most low-to-medium volume roads. Adoption of a more stringent design event should be considered where necessary for critical structures on priority lifeline routes.
• Amend performance criteria to accept more frequent damage where this presents significant cost savings, and repair and reinstatement is speedy and low cost, and/or
• Focus on opportunities for major cost savings, including where major step changes in cost and risk arise.

TAN #24-01 provides further guidance to some of the above.

Flood resilience for non-state highway bridges

Similarly to seismic design, the design of new bridges to accommodate updated flood modelling is posing challenges to the affordability and feasibility of established designs. The NZTA is promoting a similar adaptive approach to the design of new bridges for flooding, with context of the receiving road network key in achieving fit for purpose outcomes. The key opportunities in this space include:

• Allow inundation of bridge decks when subject to major flood events, particularly where providing clearance and/or freeboard would necessitate significant raising of bridge and approach levels, and
• Challenge the Importance Levels and associated return periods used for design to match the local network context, including allowance for redundancy of the local network, ability to access the bridge during flood events, and consequence of inundation and localised damage during major flood events.

Implementation

Process

Deviations from the requirements of the Bridge Manual to enable the adaptive approach detailed above shall not necessitate a formal departure process, subject to the approval of the proposed seismic and flooding resilience philosophy by the asset owner.

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: Recommendations for an adaptive approach to resilience of non-state highway bridges

Published: 17 December 2024

Reference: #24-02

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