NZS 3604 review preparation

By - , Build 113

The current technical review of our timber-framed building standard is limited by the rapid rate of change still occurring in the industry.

The change to the seismic map of New Zealand – NZS 4203 (left) and NZS 1170.5 (right).

New Zealand’s timber-framed building standard, NZS 3604:1999, was last revised 10 years ago and amended in 2006 to take account of the new timber properties resulting from changes to the grading and verification of standards for timber.

In the 10 years since, a number of significant changes have occurred within the New Zealand building industry. For example:

  • a new Compliance Document has been published for NZ Building Code clause B1, which changed the cited loading standard from NZS 4203:1992 to AS/NZS 1170
  • Acceptable Solution E2/AS1 has been introduced, superceding many weathertightness details that are currently included in Section 11 of NZS 3604
  • there have been major changes to insulation requirements resulting from revision of NZ Building Code clause H1 in 2007
  • a new Building Act in 2004 introduced BCA accreditation, practitioner licensing, product certification and other more minor changes
  • the ‘leaky homes crisis’ has, in conjunction with the new Act, considerably changed the building consent process and the level of documentation required from designers
  • the ‘weak wood crisis’ resulted in the introduction of new timber grades to NZS 3604 in 2006, almost tripling the size of the standard overnight
  • changes to timber treatment processes and methods have big implications for the durability of metal fixings in high moisture environments.

Meanwhile, there have been many gradual changes to the way we build timber buildings in New Zealand. These include more off-site prefabrication, increasing use of proprietary components and systems, and building in more exposed situations as coastal sites become more desirable and flat sites more scarce.

After a long preparatory and planning phase, including consultation with key stakeholders, surveys of users and a scoping workshop, Standards NZ has begun the formal process to review the document.

BRANZ research focus

To expedite the review, BRANZ undertook a research project to consider feedback received on the standard over the years since its last publication and provide technical back-up.

The project focused on collecting together previous work (by BRANZ and others) and remoulding it into a form required for inclusion directly in the standard, or for consideration by the drafting committee. There were three main areas of interest: durability, bracing and loading.

The change to the seismic map of New Zealand – NZS 4203 (left) and NZS 1170.5 (right).

DURABILITY

The focus here was the application of BRANZ research findings on corrosivity of metal fixings in copper treated timber in high moisture environments.

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BRACING

The wall bracing testing procedure was re-evaluated, and ways to reduce holding down forces on timber floors from increasingly high capacity bracing panels were achieved with a recommendation to place a maximum limit on bracing ratings and minor changes to floor connection details.

The distribution of wall bracing required for good seismic performance of irregular building layouts was assessed. Such layouts have become more common as designers stretch the boundaries.

Recommendations were made for details of cast-in and post-fixed concrete anchors, especially at slab edges formed by masonry header blocks (this work also included recommendations to reduce the load criteria for anchor bolts currently in NZS 3604).

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LOADING

A cost-benefit study investigated the effect of the major changes to the seismicity map of New Zealand resulting from the citing of NZS 1170.5. This part of the study also highlighted the considerable influence of soil type on the seismic load demand on timber-framed buildings. Earthquake ground motion is amplified by weaker or less stiff soils overlying bedrock, such as in wide river valleys or marine deposits, and there is a strong correlation between ground motion and building damage.

Although there are changes to the way wind loads are determined by AS/NZS 1170.2, it was concluded that the minimum impact on the industry (particularly on prefabricated components) would be to keep the zone wind speeds the same as they are currently. However, consultation with a number of industry players indicated a wish to include a higher wind zone to reduce the need for specific design as buildings are constructed on more exposed sites.

Recommendations were made for adjustments to the snow zones and limiting altitudes to account for the major changes introduced with the citing of AS/NZS 1170.3, in the aftermath of the South Canterbury snowstorms of 2007.

The project also collated together 10 years of feedback, received by the BRANZ Helpline, on errors, inconsistencies and ambiguities in NZS 3604. Comments by users proved an invaluable source of corrections to the text, tables and diagrams.

Three-tier structure to carry out review

Standards NZ has set up a three-tier structure to carry out the review.

A leadership group, consisting of chief executives of a number of major industry organisations, provides strategic advice and guidance to the technical committee. It is also intended as the focus for political and industry lobbying.

The main technical committee, consisting of 22 representatives of interested industry groups, design practitioners, builders, local and central government, research organisations (including BRANZ), material suppliers, and users is ultimately responsible for the content of the new standard.

Working groups covering specialty areas, such as durability, bracing, loading, roofing and presentation, are carrying out the ‘nuts and bolts’ of the review. Their drafting recommendations are fed to the technical committee for incorporation in the new document.

Minimum update while many changes still in pipeline

The need for a technical update to the standard is clear, and the final outcome must be the best the country can produce, given the resources available. But with all the impending changes to the building delivery process and control system still in the pipeline, only the minimum necessary update or ‘limited technical review’ will be undertaken.

The committee is now well into its task and aims to have the first draft available for public comment in October this year, in preparation for publication later in 2010.

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For more

This research was funded by the Building Research Levy.

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Articles are correct at the time of publication but may have since become outdated.

The change to the seismic map of New Zealand – NZS 4203 (left) and NZS 1170.5 (right).

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