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Sunday, May 22, 2011

Change to building standards for house foundations

New Building Code documents came into force on Thursday (19th), increasing the requirements for house structures and foundations. All new houses must comply with the new standards which are designed to make houses safer, and more likely to survive significant earthquakes. It is estimated that the additional cost for a new house will be the region of $2,000 to $9,000.

A rather technical information sheet covering the new requirements has been released by the Department of Building and Housing (DBH) and can be found here.

An outline of the changes is available on the DBH website (see link above) and is reproduced below. For those living in areas subject to liquefaction the bits about foundations on 'good ground', or not on 'good ground', are important. I have underlined these.

What is not clear is whether these requirements apply to houses where the foundations/slab have been damaged. Will the fully repaired house meet the old or new requirements? The cost of insurance may hinge on the answer.
Inside the Canterbury Earthquake Region, the following changes apply:
  • A new definition of ‘good ground’ excludes ground where liquefaction and/or lateral spread could occur.
  • For bracing demand, replace Section 5 of NZS 3604:1999 with Section 5 of NZS 3604:2011.
  • The Canterbury Earthquake Region shall be taken as Earthquake Zone 2.
  • Piled foundation details and perimeter concrete foundation wall details are unchanged.
  • All concrete slab-on-ground foundations on ‘good ground’ are to have reinforcing steel.
  • All perimeter foundations must be tied to the concrete slab with reinforcing steel.
  • All reinforcing is to be Ductility Class E, in accordance with NZS 4671.
  • Minimum slab reinforcing is to be 2.27kg/m2 welded reinforcing mesh sheets (1.15kg/m2 in each direction) lapped 225mm.
  • Once slabs exceed 24m, a free joint must be formed as required in the Standard, except that there shall be dowel bars placed to minimise the risk of differential settlement.
  • Brick veneer tie requirements are to be determined from NZS 4210 for Earthquake Zone A.
  • Foundations where ‘good ground’ has not been established, are outside B1/AS1 and need to be subject to specific engineering design. In particular:
    • Where liquefaction and lateral spread up to 50mm is possible and there is perimeter ground protection, designs may be based on the Department’s ‘Guidance on house repairs following the Canterbury Earthquake’.
    • Outside this, ie, where there has been severe land damage, the specific engineering design must include appropriate geotechnical investigations. These areas are defined by the Christchurch City Council, the Selwyn District Council and the Waimakariri District Council.
There is a ministerial release here.
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