New foundation rule recognises seismic hazard

Changes are being made to Building Code documents to increase the seismic hazard factor for Canterbury and to require stronger foundations for buildings, Canterbury Earthquake Recovery Minister Gerry Brownlee says.

The Department of Building and Housing has decided to increase the seismic hazard factor for the design of all buildings by 35 per cent from 0.22 to 0.3 based on the best scientific and structural engineering advice available.

In addition, concrete floor foundations for housing will need to be tied and reinforced. This change was signalled in guidance on foundation repairs provided to the construction sector after the September 4 earthquake.

These changes are likely to increase costs for new residential building work in the order of $2,000 to $9,000 per property, depending on the size of the house, the foundation option chosen and the state of the ground.

"Residential homes built in Christchurch will be required to have increased bracing and foundations and be more resistant to bowing or cracking," Mr Brownlee says.

“These changes will mean new buildings are constructed better to withstand any future severe earthquakes.”

The changes will take effect from Thursday 19 May 2011 in the three local authority areas in greater Christchurch: Christchurch City, Waimakariri, and Selwyn District Councils. The seismic hazard factor for buildings in much of the Selwyn District, closer to the Alpine Fault, is already greater than 0.3.

"The aim is to improve safeguards for people occupying the buildings and reduce personal harm and damage to property from any future earthquakes.”

Mr Brownlee says the changes to the seismic hazard factor and foundation requirements needed to be made promptly so that commercial building owners and homeowners could get on with rebuilding or repairing properties as quickly as possible.

Building and Construction Minister Maurice Williamson says the changes are based on the best available science and the advice of recognised seismic and structural engineering experts.

The Department of Building and Housing also worked closely with the Christchurch City Council in developing the new seismic hazard factor.

"Ground shaking in Christchurch on 22 February was very violent and exceeded Building Code design requirements for buildings in the area," Mr Williamson says.

"The scientific assessment of the earthquakes and how buildings performed in them does not support an increase in the seismic hazard factor above the 0.3 level.”

The Department of Building and Housing is one of a number of organisations working with the Canterbury Earthquake Recovery Authority (CERA) on rebuilding greater Christchurch and its surrounds, and supporting the welfare of its residents. For more information visit www.cera.govt.nz