Quake safety cost must relate to risk

Last updated 09:59 21/06/2012
Clarendon Towers
JOHN KIRK-ANDERSON/Fairfax NZ

STILL STANDING: The earthquake-damaged Clarendon Towers in Christchurch, photographed in January. While property damage was great in February 2011, the occupants of most buildings got out of them alive.

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Department of Building and Housing deputy chief executive Dave Kelly says the risk of harm from earthquakes remains low, relative to other risks in our everyday lives, and there needs to be much better public understanding about what defines a safe building.

The tragic loss of life caused by the Canterbury earthquakes has, naturally, raised awareness and concern about building safety standards.

This has prompted strong reactions, possibly even over- reactions in some cases. Tenants have walked away from older buildings, and unions reportedly want to make employers provide quake-safe workplaces.

The term "earthquake- prone" is a technical term that is used to describe buildings that are less than one-third of the earthquake strength required for new buildings.

Being "earthquake-prone" doesn't necessarily mean that the building should not be occupied, but it does mean that the owner should get immediate expert engineering advice and work out a plan to fix the problems.

Building owners are responsible for ensuring that their buildings are safe. New Zealand has a sizeable stock of old buildings that do not meet the new building standard (NBS) for earthquake design, and there's no doubt that repair or demolition is long overdue for the some of them.

Before the Canterbury earthquakes, there may have been some complacency about getting on with this work.

While large, destructive earthquakes can devastate communities, they are, fortunately, very rare. The risk of harm from earthquakes remains low relative to many other risks we face in our everyday lives - and outside of Canterbury, the risk has not changed since the earthquakes.

Since records began in 1843, 483 people are recorded as having died in New Zealand as a result of earthquakes. The vast majority of our earthquake fatalities - 447 people - resulted from the two terrible earthquakes in Christchurch in 2011 and Napier in 1931.

By comparison, more than twice as many people (1125) died on our roads in the three years from 2008-10. The risk of dying in a road accident is around one in 10,000 each year, compared with a one in million risk of dying from an earthquake while in a new building.

Concerns about earthquake risk are high because the consequences of a major earthquake are so devastating. We must, of course, take sensible steps to protect our buildings from life-threatening collapse in earthquakes. But we must also keep the investment required in proportion to the risk.

It is hard for everyone involved - government, owners, engineers and users - to make good judgments about how much earthquake resistance is enough for our buildings. Realistically, it is impossible to contemplate strengthening our entire building stock to withstand a Christchurch-scale event due to the massive cost and resources that would be needed.

Also, the February 22 earthquake was an extremely rare, one in 2500-year event, according to GNS (Institute of Geological and Nuclear Sciences). The odds are that none of us will ever experience such a devastating earthquake again.

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Therefore, it's necessary to strike a reasonable balance between protecting people from harm in earthquakes and the economic cost of strengthening buildings.

Current seismic resistance standards for new building design are based on a one in 500-year earthquake, taking account of the building's location in New Zealand. This is why, for example, Wellington buildings are required to be stronger than buildings in Auckland, as Wellington has higher seismic risk. The standard also requires high-importance buildings like hospitals to be designed to resist stronger earthquakes.

It is estimated that less than 5 per cent of New Zealand's commercial and multistorey residential buildings are "earthquake- prone". Councils can require the owners of these buildings to reduce or remove the dangers, including by strengthening or demolition.

There are other buildings across New Zealand that, while they may not be deemed to be "earthquake-prone", present significant safety risks.

Some of these can be improved with relatively low- cost fixes like removing parapets or chimneys, or tying back facades to buildings. In some cases strengthening work may be uneconomic.

Building owners need to decide themselves how to manage their buildings, aided by expert engineering advice on the individual circumstances and risks in each case.

We must recognise that there is no such thing as a fully earthquake-resistant building - all buildings are prone to damage or collapse if the earthquake is big enough, as Christchurch has shown us.

The lessons of the Christchurch earthquakes are now being considered by the Canterbury Earthquakes Royal Commission. At the same time, the Department of Building and Housing is conducting its own review of policy and practice in this area.

The department's review will help the Government respond quickly to the royal commission's findings.

It already seems clear that, given the massive scale of the Christchurch quake, most the city's buildings actually performed relatively well in terms of protecting life.

A total of 133 of the 185 people who tragically lost their lives were in one of two large buildings, CTV and PGC, that collapsed due to the intense ground shaking and critical structural weaknesses. Most of the other fatalities resulted from falling debris, striking people who were outside buildings. While the property damage was great, the occupants of most Christchurch buildings - even the older structures - got out of them alive.

For the future, it is clear that there needs to be much better public understanding about what constitutes a safe building, and on how individual buildings will perform in earthquakes.

We do not have good information nationally about the seismic performance of our buildings and where they are located. Wellington is one of the few local authorities with good data on the seismic performance of buildings in its region, and it has published a list of "earthquake-prone" buildings on its website.

Work is already under way on how to collect better information about the seismic performance of our buildings and to make this information more readily available to the public.

In the meantime, the department has prepared guidance for engineers, specifically aimed at the Canterbury rebuild, and it will soon release general guidance for building owners on earthquake risk. This will give them useful practical advice to help them make decisions.

- The Press

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