Earthquake-prone buildings: how to reduce the risk

By Rebecca Macfie In Letter from Christchurch

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Unreinforced masonry buildings

photo Rebecca Macfie

The Canterbury Earthquakes Royal Commission of Inquiry report into the performance of unreinforced masonry buildings looked in detail at the failure of old brick and masonry buildings that failed in the February 2011 earthquake, killing dozens of people. It proposes a raft of policy changes aimed at reducing the risk posed to the public by the 15,000 to 25,000 similar buildings scattered around New Zealand’s towns and cities.

Here’s a short summary of the commission’s key points:

# Forty-two people died in the February 22, 2011 as a result of building failures other than at the Canterbury Television and Pyne Gould Corporation sites.

Among those who died were five-month-old Baxter Gowland, who was killed by a falling brick fireplace in his home, and five-week-old Taneysha Prattley, killed along with her mother Kelsey Moore when the front of a hairdressing salon collapsed on them in Lichfield St.

Of the 42, all but one death was caused by the failure of older unreinforced masonry (URM) or brick/block structures. Thirty-five died as a result of the failure of facades or walls of URMs; four were killed inside URM buildings; one died running out of a building and was crushed by a free-standing wall; one died when she was crushed by a concrete spandrel falling from a car parking building onto her car.

The fact that 70% of those killed by URM buildings were in public places outside the buildings – in vehicles or on the streets – highlights that this is a community problem, says the commission. It also shows the inadequacy of a passive policy approach to earthquake-prone buildings, such as that pursued by the Christchurch City Council prior to the September 2010 earthquake, which imposed no timeframes for the strengthening of earthquake-prone buildings.

# Most of New Zealand’s unreinforced masonry buildings (URM) were built between 1880 and 1935 – usually one, two or three story brick buildings built for commercial purposes. The URM category also includes stone masonry churches and some important public buildings. They were designed to resist gravity wind loads and incorporate materials that are subject to deterioration with age – timber and lime mortar. Many have been poorly maintained. They are generally rigid structures with little capacity to flex when subjected to high accelerations created by earthquakes. The can collapse in moderate earthquakes. The structural elements of such buildings are often poorly interconnected and detach from each other, causing catastrophic collapse in an earthquake.

 

# Research conducted for the commission showed that buildings retrofitted to 100% of new building strength performed well in the February earthquake; those strengthened to 67% performed moderately well; and those strengthened to less than 33% did not perform significantly better than those that had not been strengthened at all. The forces exerted by the February earthquake were, however, 1.5 – 2 times the design level of the building code.
The research also showed it was generally safer to be inside a URM building than to be outside it, because walls are likely to collapse outwards and parapets and gables fall onto adjacent property.

 

# There are three broad approaches to managing the risk posed by URMs:

    1. Do nothing and live with the risk on the basis that damaging earthquakes occur infrequently;
    2. Demolish, URMs, which would have an impact on the heritage and character of New Zealand towns and cities;
    3. Install some level of strengthening.

The commission says option 3 is the intention of the current law, and is not an approach that should be abandoned. But the characteristics of URMs that perform poorly in earthquakes have to be recognised – they are stiff, heavy and brittle. They have little ability to deform once the strength of their elements has been exceeded, leading to abrupt failures. Unrestrained parapets, chimneys, ornaments, and gable end walls are usually the first elements to fall in an earthquake. Inadequate connections between walls and floor diaphragms are also a “significant deficiencies”. Generally, URMs are made of weak mortar and strong bricks – bricks that fell from a considerable height in the February quake were often not broken, whereas the mortar could be crushed between the fingers.

 

# The commission wants URMs to be graded for their seismic risk from A (best) to E (weakest), in a system that would be easily understood by local bodies, owners, tenants and the public.

Based on the estimate that New Zealand has between 15,000 and 25,000 earthquake-prone buildings, it calculates the cost of strengthening over different time frames and to different standards. To reach 33% of code over an average of 28 years would cost $1 billion; to reach 33% over 15 years $1.7 billion; and to reach 67% over 15 years $7.6 billion.

Overall, the commission concludes the current threshold for defining a building as earthquake prone at 33% of the new building strength should remain, and does not need to be lifted. But local bodies should have the power to adopt a high threshold for buildings of high importance or high occupancy and where public funding is to be contributed.

It recommends territorial authorities be required to assess all URMs within two years, with owners then required to bring the building up to 34% within five years; however for parapets, gable ends and facades the requirement would be to go to 50%. Owners who didn’t comply would have their buildings demolished at their expense.

It also recommends that owners doing strengthening work should not be required to incorporate access and facilities for disabled people as a condition of their building consent – a provision in the current regime that many building owners claim has imposed extra cost and worked as a major disincentive to retrofitting their buildings.

 

# While the Royal Commission has been considering evidence on the Christchurch earthquakes the Government has been running a parallel process developing proposed reforms to deal with the problem of New Zealand’s unreinforced masonry building stock. The result of that work was released at the same time as the Royal Commission’s report.

Produced by the Ministry of Business, Innovation and Employment, the consultation document proposes a softer set of rules than those recommended by the commission. Specifically, it proposes:

  • All non-residential and multi-unit, multi-storey residential buildings be seismically assessed within five years
  • Information on whether a building is above or below the earthquake prone building threshold be available on a public register
  • All earthquake prone buildings be strengthened or demolished within 15 years of the changes taking effect (ie, five years for assessment, and ten years for owners to take action).
  • The definition of earthquake-prone building remain as it is at present, at less than 33% of new building standard.

MOBIE has costed the various scenarios for improving the standard of New Zealand’s earthquake prone buildings:

  • Under the current system (with local councils allowing 28 years, on average, for owners to reach 34% of code) the net cost is $933 million;
  • MOBIE’s proposed regime (15 years to reach 34% of code) – $1.68 billion
  • To achieve 67% of code in 15 years would cost $7.6 billion
  • To achieve 34% of code in 10 years – $ $2.1 billion
  • To achieve 67% in 10 years – $9.7 billion
  • To reach 34% in five years – $2.7 billion
  • To reach 67% of code in five years – $12.3 billion

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