8 April 2005
The disasters caused by the Asian Boxing Day 2004 earthquakes and subsequent tsunamis sent shockwaves around the world. For New Zealand, apart from the huge outpourings of grief and sympathy, they have had the impact of making many more people aware of the very real threat posed by these natural phenomenons.
Seismically, New Zealand is among one of the most active places on earth. On average there are around 15,000 earthquakes shaking the country every year. These are caused by New Zealand being caught between two of the earth’s plates, the Australian plate to the west and the fast moving Pacific plate to the east. Their straining and movement, of about 40 mm per year, means that New Zealand is constantly being compressed and pulled sideways at the same time. This movement results in the majority of earthquakes that occur.
Only about 150 of the earthquakes are strong enough to be felt and even fewer cause any serious damage or injury. However, this does happen. Two of the most famous cases include the Napier earthquake of 1931 and the Edgecumbe earthquake of 1987. Measuring 7.8 on the Richter scale, the Napier earthquake resulted in devastation of the city and its buildings as well as similar impacts in many of the surrounding towns. A total of 256 people were killed while many more where injured. In Edgecumbe, no people were killed, but there were many injuries and some buildings suffered structural damage. Many of the roads spilt open and the railway line buckled in the earthquake which measured 6.3 on the Richter scale.
These, and many other earthquakes, confirmed to New Zealanders the damage earthquakes can cause and the building practices that were better at minimising such damage. This reaffirmed historical initiatives to improve building design and construction methods. It was as a result of the Napier earthquake that the Government set up a Building Regulations Committee that recommended a national uniform building code be prepared and applied. The outcome was that in 1932 the New Zealand Standards Institution, which later became Standards New Zealand, came into being. This resulted in the first earthquake loading Standard for New Zealand, being NZSZ No 95. This remained current until it was superseded by NZSS 1900 in 1965.
SNZ involvement in the development of design and construction Standards remains as strong as ever. The latest addition to the library of building Standards was published at the end of 2004 and is entitled Structural design actions, Part 5: Earthquake actions – New Zealand (NZS 1170.5:2005). This Standard is the final, part that completes the structural design actions series for New Zealand. The other parts are:
- Part 0: General provisions
- Part 1: Permanent, imposed and other actions
- Part 2: Wind actions
- Part 3: Snow and ice actions
Parts 0 to 3 of the Standard where developed and published jointly with Standards Australia (SA) and it was originally planned that a Part 4 would be the joint earthquakes part of the series. However, during the development process it was decided that, due to differences in the professional practices for the two countries, SA would develop a Part 4 to cover earthquake loadings for Australia only and SNZ would develop a New Zealand only Part 5. As such there will be no Part 4 in the series for the New Zealand market.
Andrew King, chair of the Standards technical committee that has been developing the Standard explained the impact of Part 5 being specifically for New Zealand. “The separation permitted a New Zealand focus to be reintroduced. This focus permitted several changes such as the performance objectives being clarified, and more closely aligned to the NZ Building Code stability requirements of Clause B1. A more direct link to the material Standards has also been sought. Perhaps the greatest change for the design profession will be in an updated seismicity section where a significantly greater spread of seismicity across New Zealand has been recognised with the ratio of lowest to highest seismicity zones increasing from 2 (in the old Standard) to 5 in the new. Near fault amplification effects are also included for areas adjacent to our most active faults. Other significant changes are the development of a more comprehensive section relating to inelastic time-history analysis and changes to the acceptable interstorey drift limits. The sections relating to the design of parts of buildings and their contents have been completely re-written into a simpler form with more uniformity being achieved.”
Commenting on the benefit to industry of the development of the Standard, Andrew King said, “The new standard is the culmination of 8 years of effort by the committee and industry leaders. It provides a good basis for the New Zealand building industry to continue to provide buildings that are economical and provide an appropriate level of safety even under rare but extreme events such as earthquake attack. That it coincides with the implementation of the new Building Code of New Zealand, with its earthquake retrofit requirements, is also convenient as the design basis will now be able to be applied to both new and retrofit structures in New Zealand.”
At the end of March 2005, Standards New Zealand will be publishing all New Zealand relevant parts of the 1170 series as one document, supported by a second commentary volume for support.
The statistics used in this article are from the Institute of Geological and Nuclear Sciences. More information on them can be gained by visiting www.gnz.cri.nz
The first quote from Andrew King is an extract from a paper he co-wrote with G.McVerry, R.Fenwick, D.Bull, A.D.O’Leary, R.Jury, C.Clifton and I.Brewer on the development of Part 5 of the Standard for the 2004 New Zealand Society for Earthquake Engineering Conference.