This report commisioned by Ministry of Business, Innovation and Employment (MBIE) from an Independant Panel of experts focuses on the design, construction and land influences on the performance of Wellington’s Statistics House during the 14 November Kāikoura earthquake in New Zealand.
Statistics House is a six storey reinforced concrete office building with a lightweight roof structure, built in the CentrePort Harbour Quays business park in 2005. As a result of the Mw7.8 Kaikōura earthquake on 14 November 2016, the building suffered the partial collapse of two floors.
The way a building responds in an earthquake is a result of a combination of factors, including the magnitude and distance from the epicentre of the earthquake, the size and design of the building, and the ground conditions at the site. In the case of Statistics House, these factors all combined to amplify the building response and cause the partial floor collapse.
The Chief Executive of MBIE commissioned an independent expert investigation into the performance of Statistics House in this earthquake in order to understand implications for the building regulatory system. This includes the Building Code, guidance published under the Building Act, and any other functions of the Chief Executive under the Building Act.
This report presents the conclusions and recommendations of this investigation. The investigation was not intended to consider the future use or reparability of the Statistics House building, nor to carry out a full review of the design and construction for compliance with the Building Code.
The Panel’s overall conclusion is that the partial floor collapses of Statistics House were caused by a combination of:
- a highly flexible ductile frame with two bays of frame per precast floor span, which effectively doubled the impact of beam elongation due to plastic hinging; and
- shortening of the precast double-tee flooring units as the ends spalled during the earthquake; and
- amplification of ground shaking, primarily due to basin-edge effects in the Thorndon basin area; and
- the duration of the earthquake.
The combination of these effects was not anticipated by the New Zealand design Standards recognised in the Building Code at the time of the design of Statistics House.
The Panel’s view is that the primary cause of the partial floor collapse was beam elongation in the transverse moment resisting frames that provide the building’s seismic resistance. This was exacerbated by the multiple bay frame arrangement in the east-west direction. This, combined with a sliding floor at one end, forced most of the effects of beam elongation to accumulate at one end of the frame, resulting in the loss of support to three precast concrete floor units on the first and second floors. The moment resisting frames performed as expected, and were not in danger of collapse.