Design & Land Use in Question - Not Concrete

02 Nov 2010


Design & Land Use in Question - Not Concrete

In a recent Press article, Engineer Favours Piles Over Concrete (16/10/2010), the Professor of timber design Andy Buchanan from Canterbury University suggested that raised timber floors suffered less damage than concrete slabs during the September 4 Christchurch earthquake.  The strong, and incorrect, implication was that all concrete slabs are unsuitable for buildings in earthquake zones.

As a foundation for residential properties concrete slabs offer outstanding durability, along with low maintenance, fire resistance, thermal comfort and an array of surface finishes.  BRANZ research confirms that concrete slabs are the preferred flooring option for new homes built in New Zealand, with around 80% market share.

The concrete slabs that did fail as a result of the Christchurch earthquake did so for two reasons, neither of which detract from the advantages of an appropriately designed concrete slab.

Firstly, those concrete slabs that suffered damage were predominantly unreinforced.  Although unreinforced slabs are allowed under the New Zealand Building Code, it is always preferable to reinforce a concrete slab.  For minimal cost a reinforced concrete slab offers enhanced performance. 

Secondly, the concrete slabs that developed cracks were, in the majority of cases, located in areas previously identified by Environment Canterbury as at risk from liquefaction.  The decision to place an unreinforced concrete slab on land with geotechnical concerns was unwise.  To prevent the continuation of this practice regulatory amendment may be required.

Furthermore, during significant liquefaction most foundation and floor systems, including raised piled floors, will incur damage, requiring assessment and repair.  On properties where lateral spreading (the shifting or cracking of the ground beneath) occurred the entire house suffered damage, not just the foundation and floor.

In summary, those concrete floor slabs with observed cracking following the September 4 earthquake were not opitimally designed and were principally located in liquefied areas where damage occurred across the entire property, including foundation and floor systems regardless of type.

Concrete slabs have grown to become the favoured flooring option for New Zealand homes on merit. Anyone planning to build can be assured concrete floors will continue to offer great insulation, sound proofing and value for money, while providing peace of mind under foot.

But the key is, as with any building material and any structure, design and construction must be fit for purpose and fit for the site on which it is being used.