Canterbury Earthquakes - The Performance of Residential Concrete Construction

The earth shaking during the Canterbury earthquakes, particularly the 6.3-magnitude quake which struck Christchurch on 22 February 2011, was more violent than designed for in the Building Code.

The seismic forces of the Canterbury earthquakes impacted on the performance of concrete across many forms of construction. In terms of residential homes, appropriately designed and constructed concrete slab-on-ground and reinforced concrete masonry, met the seismicity performance requirements of the Building Code.

Case Study - Cashmere Flat To better understand how residential concrete construction resisted the forces of the Canterbury earthquakes take a look at this property from Cashmere. Case Study

Concrete Slab-On-Ground

Some residential concrete slabs did suffer damage during the earthquakes, but these were predominantly unreinforced.  Although unreinforced slabs were allowed under the Building Code at the time, this is no longer the case. See Building Regulations section below.

Furthermore, the concrete slabs that developed cracks were, in the majority of cases, located in areas previously identified as at risk from liquefaction. The decision to place an unreinforced concrete slab on land with geotechnical problems was unwise.

As a foundation for residential properties, cost effective reinforced concrete slabs will continue to offer outstanding durability, along with low maintenance, fire resistance, thermal comfort and an array of surface finishes.  However, design and construction must be fit for purpose and fit for the site on which it is being used.

Reinforced Concrete Masonry

Concrete masonry is a staple material of the construction industry; a silent workhorse that includes reinforced concrete block and concrete block veneer, along with concrete block paving and flagstone paving.

During the earthquakes correctly reinforced and constructed concrete masonry met all modern Building Code requirements. There were no reported structural reinforced concrete masonry failures.

It is important to note that concrete masonry, since its introduction into New Zealand during the 1950s, has always been reinforced. Reinforced concrete masonry must also not be confused with unreinforced masonry, predominantly clay brick masonry.

Concrete block paving and flagstones also performed extremely well. Even where concrete paving was disturbed as a result of the earthquake forces, it is able to be reused following remedial earthwork. 

Building Regulations

The Department of Building and Housing (DBH) has recently made changes to the Building Code's supporting documents for Clause B1 Structure to address increased awareness of seismicity risk.

Amongst various changes, B1/AS1 now references NZS 3604:2011 instead of the 1999 version.  In addition, all concrete slabs must be reinforced with ductile steel and tied to perimeter foundations. You can no longer use unreinforced slabs anywhere in New Zealand.

The definition of ‘good ground’ in the Canterbury region now excludes ground subject to liquefaction or lateral spread. The DBH has published Revised Guidance on House Repairs and Reconstruction Following the Canterbury Earthquake, which in effect provides specific engineering designs for housing built on 'poor ground' as defined in NZS 3604.

Visit the DBH website for more information.