Quake-proofing: How far to take it
Getting earthquake-damage prone buildings up to the present building threshold will go a long way towards saving lives and making buildings safer, writes risk management expert Tony Taig.
New Zealand has an estimated 15,000 to 25,000 earthquake-prone buildings. New Zealand's Building Act empowers local authorities to require building owners to reduce or remove the danger presented by buildings defined as earthquake-prone.
An earthquake-prone building is defined in the Building Act 2004 as being likely to collapse in a moderate earthquake. A moderate earthquake is defined in the act as being one-third as strong as what a new building at the same site would be designed to withstand.
My report, A Risk Framework for Earthquake Prone Building Policy, written in collaboration with GNS Science, addresses, among other things, the factors the New Zealand Government needs to consider in developing policy for earthquake-damage prone buildings.
The report was commissioned by the former Department of Building and Housing, now the Ministry of Business, Innovation and Employment (MBIE). It forms part of a review on earthquake-prone building policy settings and its implementation.
Proposals resulting from the review, and questions about them, were released by MBIE in early December as part of a consultation document.
This document aims to help the public and technical experts comment on proposals to improve the earthquake-prone buildings policy system. The MBIE consultation is open for submissions until 5pm on March 8.
Based on evidence gathered from the February 22, 2011, earthquake in Christchurch, I believe that the immediate focus should be on bringing vulnerable buildings up to the present earthquake-prone building threshold rather than on raising that threshold. This threshold has become shorthanded over time to "33 per cent of New Building Standard", or NBS.
Many Christchurch buildings that were strengthened to more than 33 per cent NBS were badly damaged, though most did not collapse and kill or injure people inside them.
The February 22, 2011 earthquake produced some of the most severe shaking recorded in any urban area in the world. In parts of Christchurch everything normally held down by gravity was effectively thrown up into the air, including buildings, people, cars and roads.
Had the earthquake happened at night when everyone was at home, there would have been considerably more fatalities in areas like the Port Hills. Several homes there were destroyed by collapsing slopes, and more than 50 homes were struck by boulders, one-metre-plus in size.
As a result of the earthquake, new evidence became available to help understand how buildings impacted on the survival of people inside and outside shaking buildings.
With the notable exceptions of the Canterbury Television (CTV) and Pyne Gould Corporation buildings, those who died were mainly pedestrians hit by falling facades and masonry while passing by or escaping from buildings.
My report concludes that raising the earthquake prone building threshold from 33 per cent to 67 per cent of NBS would be of questionable benefit. No building that had been strengthened to the 33 per cent threshold or better collapsed and killed anyone in the Christchurch February 22, 2011 earthquake.
Buildings strengthened over and above the present threshold sustained less damage than those strengthened just to the threshold, but many such buildings in Christchurch were pulled down anyway as engineers were reluctant to vouch for their structural integrity after the earthquake.
When it comes to saving lives, Christchurch gives us confidence that upgrading all vulnerable buildings to the present earthquake-prone building threshold (33 per cent) would be of major benefit. Other important measures we can be confident would save lives include bringing high-risk buildings not yet identified into the earthquake-prone building framework; and paying equal attention to other hazards from earthquakes such as landslides and collapsing cliffs.
My report provides further background on earthquake risk in comparison with other hazards facing New Zealanders as well as discussing the relative merits of different strategies to reduce earthquake risk. There should be a debate about whether the 33 per cent threshold should be raised, but that debate needs to be informed by better evidence about what the benefits would be. That research should be progressed vigorously, but in the meantime I favour measures that we can be confident would save many lives over ones where we can't be sure they would.
Consultation will run until 5pm Friday, March 8. A series of regional public information meetings will be held this month across New Zealand. To view the consultation document, meeting schedule or to make a submission visit: dbh.govt.nz/consultingon- epbp.
As a specialist consultant with UK-based company TTAC Ltd, Tony Taig has more than 30 years' experience in dealing with risk and uncertainty in natural and man-made hazards. Previous TTAC Ltd projects involving New Zealand include a report on lahar on Mt Ruapehu.