Questions and Answers - Changes to Structure and External Moisture Building Code documents (1 August 2011)
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What are the changes for Structure (Building Code clause B1)?
- NZS 3604 has been updated. B1/AS1 now references NZS 3604:2011 instead of the 1999 version. There are modifications. All concrete slabs must be reinforced with ductile steel and tied to perimeter foundations; you can’t use unreinforced slabs anywhere in New Zealand.
- The definition of ‘good ground’ in the Canterbury earthquake region excludes ground subject to liquefaction or lateral spread.
- Two new documents are referenced in B1/VM1:
- A Standard for designing seismic restraints for equipment in earthquakes, NZS 4219:2009 - Seismic performance of engineering systems in buildings. The referencing modifies NZS 4219:2009 by increasing the seismic hazard factor for buildings in Canterbury to 0.3 minimum.
- A standard for light steel framing. NASH standard “Residential and Low-Rise Steel Framing Part 1 2010 Design Criteria”.
- B1/AS2, the Acceptable Solution for timber barriers, is being withdrawn. A design guide for barriers using timber, glass and metal will be published shortly.
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What are the changes for External Moisture (Building Code clause E2)?
- Acceptable Solution E2/AS1 has been clarified and updated to include new information.
- Verification Method E2/VM1 has been expanded.
- A new Acceptable Solution E2/AS3 for concrete and concrete masonry construction references CCANZ CP 01:2011.
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Are there any modifications to NZS 3604:2011 in the new B1/AS1?
Yes.
- Modifications for the Canterbury earthquake region
On 19 May 2011, the Department (now part of the Ministry of Business, Innovation and Employment) made changes to Acceptable Solution B1/AS1, which apply to the Canterbury earthquake region. These changes took effect immediately. The definition of ‘good ground’ was changed to exclude ground subject to liquefaction and /or lateral spread, and stronger foundations were required for that region. These modifications to the referencing of NZS 3604:1999 have been carried forward to the referencing of NZS 3604:2011.
The changes for Canterbury were made to allow homeowners in the region to quickly progress with their repairs or rebuilding. Details on the changes for Canterbury can be found in the information sheet Immediate changes to seismicity and foundation details for Christchuch.
- Modifications for all New Zealand for concrete slab floors and foundations
On 1 August 2011 the Department extended the requirement for stronger foundations to the rest of New Zealand. It modified its referencing of NZS 3604:2011 to exclude unreinforced slabs. All concrete floor slabs on ‘good ground’ are required to have reinforcing steel mesh and all perimeter foundations are required to be tied to the concrete slab with reinforcing steel.
This modification has been made to provide the rest of New Zealand with the same readily administered, effective and robust Acceptable Solution requirements as those already made in the Canterbury earthquake region.
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What is the purpose of the reinforcing mesh and of tying the perimeter foundation to the slab reinforcement?
The purpose of the mesh is to provide crack control, some flexural capacity to allow the slab to span local weak spots, and to provide some resistance against differential settlement by dowel action. Mesh will stop or limit the spread of a crack if it opens up. Without mesh, a crack in an unreinforced slab is likely to widen and spread across the whole slab, and differential settlement can occur, resulting in loss of amenity.
The requirement to tie the perimeter foundation to the slab reinforcement is to provide a more robust composite foundation/floor, and limit movement and damage between these elements that might occur during earthquake shaking and subsequent ground settlement/displacement.
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Won’t it cost more to use ductile reinforcing mesh, and to remove the option for unreinforced slabs?
There is little cost difference between ductile and non-ductile mesh. However, there is a cost of reinforcing slabs as opposed to using unreinforced slabs. The estimated cost of about $14.50/m2 (including supply and placement) brings considerable benefits in terms of dealing with slab cracking during shrinkage and limiting damage that may result from earthquake shaking.
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The reinforcing mesh required in B1/AS1 is not currently available in New Zealand. How is the Department addressing this?
The reinforcing for concrete slabs-on-ground, required in the Acceptable Solution B1/AS1, is a minimum of 2.27kg/m2 of Grade 500E reinforcing mesh fabric which complies with AS/NZS 4671.
Currently Grade 500E reinforcing mesh fabric of this specification is not available from any of the suppliers in New Zealand. Therefore, the Department of Building and Housing (now part of the Ministry of Business, Innovation and Employment) has published guidance on an alternative solution for the Canterbury earthquake region to provide time for manufacturers to produce Grade 500E reinforcing mesh fabric.
The Department has amended this guidance on alternatives to ductile steel mesh, to cover the whole country and not just the Canterbury earthquake region.
The alternative solution allows for lower yield strength steel than Grade 500E to be used if an equivalent tensile capacity in the slab can be achieved. The amount of steel (kg/m2) necessary to achieve the equivalent capacity is determined as 2.27 x 500 / Strength Grade of steel (where the strength grade of the steel is the verified lower characteristic yield strength of the steel bar in MPa). The uniform elongation Agt (refer Table 2 of AS/NZS 4671) must equal or exceed 10%.
For full details consult the Guidance on Reinforcement for Concrete slabs-on-ground.
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Why is the Department not extending to the rest of New Zealand the new definition of ‘good ground’ it introduced for the Canterbury earthquake region in May this year?
The defining of ‘good ground’ in terms of liquefaction and/or lateral spread potential is complex and new knowledge is still being gained from the Canterbury experience. In the case of Canterbury, there was a need to take steps immediately with the best available knowledge, so that homeowners could progress with necessary repairs and rebuilding. However, the Department considers it prudent to consider this matter further on the basis of fuller learnings from Canterbury, before taking steps to address the issue of ‘good ground’ for the rest of the country.
In the meantime, the Department has prepared guidance for homeowners building where there may be a potential for liquefiable soils and who may choose to use foundation details which are even more robust than those provided by the Acceptable Solution. For full details consult the Guidance on using NZS 3604 construction on ground with potential for liquefaction.
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Are there any modifications to NZS 4219:2009 in the new B1/VM1?
Yes. The Standard NZS 4219:2009 is referenced with modification to incorporate the new seismic hazard factor for Canterbury of 0.3 minimum. (The seismic hazard factor for the Canterbury earthquake region was increased to 0.3 minimum in B1/VM1 on 19 May 2011.) This reflects Canterbury’s increased seismic risk for the next few decades. In addition, the component risk factor Rc in the Canterbury earthquake region must be taken no less than 0.33.
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Where can I obtain…?
- the new versions of the Structure (B1) and External Moisture (E2) documents?
You can download the new versions of the B1 and E2 documents (contained in the B1 and E2 Compliance Documents) at: www.dbh.govt.nz/compliance-documents.
- copies of NZS 3604:2011 or NZS 4219:2009?
Copies can be purchased from Standards New Zealand through its website: www.standards.co.nz
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- a copy of the CCANZ Code of Practice for weathertight concrete and concrete masonry construction (CP 01:2011)?
Copies of the Code of Practice for Weathertight Concrete and Concrete Masonry Construction can be downloaded (or hardcopies purchased) from the CCANZ website: www.ccanz.org.nz.
- A copy of the NASH standard “Residential and Low-Rise Steel Framing Part 1 2010 Design Criteria?
Copies can be purchased from NASH through its website: www.nashnz.org.nz.
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