Building Performance in the 2010/2011 Canterbury Earthquake Swarm

Abstract eng:
Christchurch is the largest city in the South Island of New Zealand and is the second largest urban region of New Zealand. The city was first settled in 1848 and received a Royal Charter in 1856, making it officially the oldest city of New Zealand. Christchurch is located in the province of Canterbury. On 4 September 2010 the city was struck by a M7.1 earthquake having an epicenter located approximately 40 km from the city’s central business district (CBD) at a depth of 10 km, near a town called Darfield (after which the earthquake was named). The Darfield earthquake resulted in spectral accelerations recorded in the CBD of between 67-200% of the design 500 year event, depending on fundamental period. Extensive damage occurred to unreinforced masonry buildings and many had to be demolished, but there were no fatalities and no major damage to concrete or steel buildings. One reason for the lack of fatalities was that the earthquake struck at 04.35 am local time. Because this earthquake had characteristics that were similar to the 12 January 2010 M7.0 Haiti earthquake, it is interesting to compare the absence of fatalities in the Darfield earthquake with the large number of fatalities in the Haiti earthquake. Extensive building inspections were conducted after the Darfield earthquake, with a number of international teams also visiting the city. Hence a comprehensive data set was obtained of the condition of the city’s building stock. The city continued to be struck by aftershocks of significant magnitude, with a notable aftershock on 26 December 2011 also causing damage to unreinforced masonry buildings. On 22 February 2011 at 12.51 pm a M6.3 aftershock occurred that had an epicenter located 10 km from the CBD at a depth of 5 km. This aftershock generated spectral accelerations in the CBD that were in excess of 200% of the 500 design level, and approximately corresponded to the 10,000 year event. This time, extensive damage was caused to buildings of all forms and materials, with a significant proportion of all the heritage masonry buildings in the CBD being partly or totally damaged. However, most significant was the damage to concrete buildings, with 2 buildings completely collapsing and several others being close to collapse. Eccentrically braced steel frames were also damaged in the earthquake. Currently the death toll is projected to exceed 200 people. Because of the data sets that were obtained after the September 2010 event, and because of the similarities between the Christchurch building stock and that of many other countries, the learning lessons to come from this earthquake are both extensive and internationally relevant. The author flew to Christchurch on 5 September 2010 to assist in building inspections and by chance was in the Christchurch CBD on 22 February, about to commence a 1.00 pm seminar to structural engineers. This presentation will outline the nature of damage to buildings in the various earthquakes, with particular attention given to the unreinforced masonry buildings that performed poorly, and to the reinforced concrete buildings that suffered significant damage. Details will be given of the key learning outcomes to have been identified, and plans for the recovery of the city.

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