Researching housing resilience

, Build 141

A research project to help improve the resilience of the types of homes that performed poorly in the Christchurch earthquakes has been launched.

Contemporary homes in Sumner, an area that suffered heavy earthquake damage.

ENGINEERS FROM BRANZ, with support from EQC, will use computer modelling to assess the impact of simulated seismic action on modern homes with more complex configurations.

The research has been prompted by findings that homes with less complex design performed better in the Christchurch earthquakes than those built beyond the limits of NZS 3604:2011 Timber-framed buildings.

How the bracing systems of different stiffness interact during a seismic event will be examined, and learnings from the research will be used to develop new guidelines for houses built outside the scope of NZS3604:2011 to mitigate potential earthquake damage.

Findings from Christchurch

Project leader Dr Angela Liu, part of the BRANZ team that inspected residential properties in Christchurch, says that one phenomenon observed was the damage at the junctions where bracing systems changed or where the layout of the building changed.

‘We found that many older buildings, pre-NZS 3604, light timber-framed houses with small windows and smaller rooms, had performed better than many modern buildings with complex, seismic-resisting systems,’ says Dr Lui.

More severe earthquake damage was often observed in homes that had a mixture of different seismic-resisting systems, either because of bigger rooms at one end of the dwelling or larger windows included on one side for a better view.

‘According to the current regulations, seismic design of buildings mainly focuses on preserving life, and our residential houses performed very well in this regard.

‘Builders and designers are guided by NZS 3604 unless architectural and structural designers are employed. Bracing elements as per NZS 3604:2011 are often sheathed timber walls.’

Nowadays, many people want bigger spacing between bracing lines so they can have bigger rooms or windows, and quite commonly, the spacing of bracing lines becomes too large to comply with NZS 3604.

‘The designer will bring in an engineer to design that part of the house, and often the engineer will design a bracing system that is not sheathed light timber walls as introduced in NZS 3604,’ says Dr Lui, ‘or, an owner may want to take out supporting walls in an older house to make larger rooms, and the engineer may, for instance, recommend installing a steel portal to brace that wider space.

Contemporary homes in Sumner, an area that suffered heavy earthquake damage.

‘It appears there is often a mismatch between the mixed stiffness bracing elements resulting in some parts of the house moving more than others in an earthquake. We repeatedly found that cracking had appeared where the stiffer and the more flexible bracing met.’ Also, the more flexible bracing used to provide views often resulted in so much damage that the house was uninhabitable.

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Looking forward

‘We aim to find better ways to improve the performance of more complex houses,’ says Dr Liu. ‘We will look at individual areas, and the computer model will allow us to adapt the bracing elements to ensure all the elements of bracing work together.

‘That will enable us to provide guidance around areas where engineers need to pay more attention in future designs in order to mitigate problems with potential differences in stiffness between the specifically designed bracing and NZS 3604 bracing elements.’

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Contemporary homes in Sumner, an area that suffered heavy earthquake damage.