Rethinking Seismic Risk in the Building Control System

- Ross Roberts

Today MBIE released a report developed by a group of experts including NZGS members. You can read the full report here: Seismic Risk and Building Regulation in NZ – For Release

Along side the release of the document, Jenni Tipler (MBIE Engineering Manager) noted,

The Seismic Risk Work Programme as outlined in the attached report is still at a very early stage, but we are now starting to ramp it up.  There will be significant consultation processes as part of this work, which will offer you a chance to share your feedback with us, including your thoughts on this report.

We will continue to keep you informed as work progresses, and as opportunities to have your say come about.

The NZGS management committee is regularly discussing these activities with MBIE. If you have any comments or questions that you would like us to pass on to MBIE please comment below.

Background

The National Seismic Hazard Model (NSHM) is currently being updated by GNS Science. The updated NSHM will incorporate advances in earthquake science and experience gained from earthquakes that have occurred over the last decade.

MBIE pulled together a group of experts, working through Engineering New Zealand, to discuss the current building regulatory system and identify possible improvements ahead of the Seismic Model’s release. This Seismic Risk Working Group were asked to focus on how NSHM results should be used within the Building Code to support the design and construction of future buildings.

The group made a number of recommendations across a broad range of topics covering
seismic performance objectives and expectations, current design practices, gaps in the existing
system and seismic hazard considerations.

MBIE summarised the key themes in the working group feedback as:

  • Clarity of performance objectives: The provisions in the Building Act and Building
    Code are generally appropriate but are not stated with sufficient clarity or
    transparency to inform the development of Verification Methods or Alternative
    Solutions.
  • Policy/risk settings: Policy/risk settings should sit above (in terms of the legal
    hierarchy) the technical means to achieve them. These settings should reflect both
    Government intent and societal expectations. In the past, Standards’ committees have
    at times made policy decisions based on industry consensus that may or may not
    reflect the intent of Government policy.

    • Certain design provisions result in inconsistent building performance: Inconsistent
      performance may be exacerbated due to;

      • Design provisions allowing for trade-offs between strength and ductility
      • The treatment of irregular structures
      • Liquefaction that is triggered at shaking levels between SLS and ULS
  • Geotechnical considerations: Geotechnical provisions for seismic design should be
    substantially overhauled so as to appropriately incorporate ground conditions,
    foundation performance and soil-structure interaction in the design process.

Working Group Members

MBIE selected the Seismic Risk Working Group members from both engineering practice and academia, including people with seismological, structural and geotechnical expertise. The working group comprised Misko Cubrinovski, Ken Elwood, Matt Gerstenberger, John Hare, Rob Jury and Rick Wentz. The lead author was Hugh Cowan.

Recommendations

The recommendations section of the main report (Appendix A in the attached document) is repeated below:

The recommendations have been thematically categorised, with those in bold the most important, but are not listed in priority order. They are not mutually exclusive, nor is this an exhaustive list: it will evolve with further discussion, analysis and collaboration between the sector and with the regulator.

PERFORMANCE OBJECTIVES

1. Review current Building Code clauses (including consideration of seismic risk settings) to ensure they articulate societal expectations and are reflected in the Building Act.
2. Review whether NZS 1170.5 and supporting Standards provide sufficient means and criteria, including limit states, to enable design that is fully consistent with the performance objectives outlined in the Building Act and the Building Code.
3. Develop a better understanding of ‘amenity’ as it applies in the Building Code and ensure consistent clarity in supporting documentation.
4. Assess the required performance of fire-resisting/protection systems following earthquake and how this may be achieved in design.
5. Reassess whether the secondary effects of earthquake such as fire-following, tsunami and slope instability are adequately reflected in performance objectives and/or should be addressed in seismic design provisions.

STRUCTURE OF THE BUILDING CONTROL SYSTEM

6. Aspects of NZS 1170.5 – specifically regarding Importance Levels (ILs) for buildings – need to sit above the technical means by which those are achieved.
7. Ensure the Building Code (i.e. clause B1), relevant compliance documents and supporting documents provide clear commentary with sufficient quantitative clarity around intent for seismic performance.
8. Ensure the means by which Alternative Solutions can be demonstrated to meet the performance objectives, are clearly stated.

ASSESSING AND IMPROVING PERFORMANCE

9. Review current practice that allows additional strength to be offset by reductions in ductility. Consider whether the requirements for elastic strength and stiffness are sufficient to deliver desired objectives under moderate levels of shaking, as well as performance in aftershocks or future events that may follow large earthquakes (e.g. ULS levels of shaking) during the life of a
building.
10. Require irregular structural forms or potential stiffness incompatibilities to be considered in the selection of overall building performance requirements, not just control the analysis method that must be used to design them.
11. Review the entire approach to the incorporation of geotechnical information for seismic design to ensure a consistent and robust approach that will enable designers to achieve target performance objectives. This includes challenging the basis for retaining B1/VM4, which does not provide appropriate guidance for foundation design of engineered structures.
12. Consider incorporating the assessment of performance of the structure / ground / foundation system at shaking levels between SLS and ULS.
13. Facilitate a challenge relating to the current provision of education and training for seismic design.
14. Review assurance processes for construction monitoring to ensure seismic design is being executed and realised as intended.

ADDRESSING GAPS IN THE SYSTEM

15. Develop provisions for the treatment of damaged buildings in the Building Act, noting the absence of guidance following recent earthquakes was problematic for regulators and building owners alike.
16. Incorporate considerations of ground conditions, foundation performance and soil-structure interaction effects in the design process. Such provisions could include reference to ground failure (e.g. liquefaction), non-liquefaction-related foundation settlement and consideration of ‘off-site’ geotechnical hazards.
17. Relevant key Standards should be clearly aligned, accessible and able to be efficiently applied by practitioners across the industry.
18. Develop a process that assures the quality and continuity of revision for selected technical Standards (e.g. by establishing standing committees for timely review of design and material standards).

SETTING THE LEVELS (CONSIDERING THE NATIONAL SEISMIC HAZARD MODEL)

19. Consider appropriate and alternative ways (e.g. different engineering parameters) for using the seismic hazard output in design. This recommendation recognises the need to use different engineering parameters in structural and geotechnical design of buildings and infrastructure.
20. Provide guidance enabling the Seismic Model team to provide the required parameters in a manner that adequately reflects inherent uncertainties and design needs. This will need to commence early but will require an iterative process as the results are received and evaluated.
21. Consider incorporating region-specific hazard characteristics for selected regions (or urban centres), where justified by benefits and data availability.
22. Address how to practically achieve risk-based design through the Building Code. In parallel to the Seismic Model, develop a decision-support framework, explicitly based on appropriate risk targets, that uses the National Seismic Hazard Model to support the selection of design factors, such as zonation Z, importance factors, and consideration of collective risk in cities.

Ross Roberts
Email - ross.c.roberts@gmail.com

Leave a Reply