Strengthening solutions for quake-prone buildings

Dunedin is home to a quarter of the most earthquake-prone type of buildings in New Zealand and its architectural history could be ''drastically affected'' by a big shake, a leading engineer says.

Dr Najif Ismail, a principal lecturer at the Otago Polytechnic's School of Architecture, Building and Engineering, has been working on finding solutions for hundreds of unreinforced masonry (URM) buildings in the city.

There were about 750 URM buildings in Dunedin, which was 25% of New Zealand's total and second only to Auckland, which had 27%. In Dunedin's CBD alone there were 226 URM buildings, which had a total value of $159 million.

Dr Ismail has worked in earthquake disaster zones around the world and played a big role in helping to fix and rebuild the city of Kashmir in Pakistan after a major earthquake in 2005.

He has developed three techniques for strengthening buildings in Dunedin, which he says would be much more affordable for building owners and would significantly reduce repair costs after an earthquake.

Those techniques have been tested on buildings, including a series of tests on one of the Speight's Brewery buildings before it was demolished earlier this year.

Step-by-step procedures had been developed for each technique and could be used with greater confidence by local engineers.

The techniques include post tensioning, which works like the straps around a load on the back of a truck, compressing the bricks to hold them together tighter.

Textile-reinforced mortar was a good way of strengthening a plaster-coated building.

It involves embedding a strong, polymer textile layer in new plaster, providing a rigid envelope around the building.

The third technique deals with cavity masonry walls where steel wall ties have corroded, leaving double-layer walls standing ''like thin veneers''.

Dr Ismail said those building types were very susceptible to collapsing outwards, posing a danger to people outside as well as inside.

Self-drilling stainless steel ties can be retrofitted in masonry and can be used to stabilise masonry arches, corners and door openings. Dr Ismail said retrofitted buildings should be able to survive more than one earthquake.

''I am hopeful that the majority of historic buildings will be saved.''

While the largest loss of life in Christchurch came from the collapse of two ''poorly designed'' but quite new buildings, the most injuries and largest financial loss was from the collapse of URM buildings.


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