Site Analysis

Understanding all the features of a site, using and protecting the best, and minimising the impact of the worst.

Earthquake risk

New Zealand is divided into four earthquake risk zones for the purpose of determining the bracing requirements of buildings to resist earthquakes.

 

Earthquakes can bring rapid, violent shaking both sideways and/or up and down, or slower rolling movements.

In some areas especially hillsides unstable ground may slide and rocks may fall.

On flat ground where there is a high water table and the soil is low-density sand or silt, liquefaction can take place. Liquid is forced to the surface, carrying sand and silt with it; land can slump; surface soil close to sloping ground (such as stream banks) can spread, with cracks opening up.

Depending on the location, earthquakes can bring other hazards such as tsunamis.

New builds

There are strong rules around house construction to help houses better cope with earthquakes, and some of these rules were strengthened following the Canterbury earthquakes of 2010/2011.

Under NZS 3604:2011 Timber-framed buildings, bracing must be provided for all buildings, with greater bracing being required for buildings:

  • in a higher earthquake zone
  • where heavy roof and/or wall claddings are specified.

Earthquake bracing demand is covered in section 5.3 of NZS 3604:2011. The section includes maps showing earthquake zones 14.

Earthquake bracing is provided by lengths of wall where the cladding and/or lining works with the timber frame to form a bracing panel. NZS 3604 ensures an even distribution of bracing elements, so the building is not expected to twist significantly in an earthquake.

Nowadays, many light timber-framed buildings are not designed completely to NZS 3604 because homeowners want large windows on one side of the house to enjoy a view. This often requires special bracing elements designed by a structural engineer. The rest of the building can still be designed and constructed to the standard.

Simple houses constructed to NZS 3604 performed well in the Canterbury earthquakes, but houses with a mixed bracing system often had significant damage.

BRANZ conducted a research project funded by the Earthquake Commission around houses with mixed bracing systems. Based on the study, a simplified approach has been proposed for engineers to reduce the likelihood of significant earthquake damage where there is mixed bracing. You can read more about the BRANZ study here.

Another key standard regarding earthquakes is NZS 1170.5:2004 Structural design actions Part 5: Earthquake Actions New Zealand.

Renovations

Depending on when and where a house was built, and what construction methods were used, there may be steps that can be taken to make it more resilient to earthquakes.

Houses with simple shapes and with timber frames and timber cladding tend to withstand earthquake shaking better, while unreinforced brick buildings are more likely to suffer damage.

Work that can be done to earthquake strengthen a house includes:

  • Fixing or replacing damaged piles
  • Making sure that piles are firmly fixed to bearers
  • Adding timber braces between piles and bearers or joists, or between piles and corner foundations
  • Adding new plasterboard lining over old sarking on interior walls
  • Removing or replacing unreinforced masonry chimneys
  • Making sure clay or concrete tiles are properly fixed

Removal of a building element such as an unsound chimney does not require a building consent (but the work must still meet Building Code requirements). This exemption is limited to any building up to 3 storeys high as long as the removal does not affect the primary structure, any specified system or any fire separation (which includes firewalls protecting other property).

Any repair work that is necessary for example, making good the gaps left in a roof after chimney removal can also be done without a consent.

Consult a chartered professional engineer over what may be possible with unreinforced masonry walls.

 

Updated: 06 May 2018