Christchurch earthquake: Pre-70s buildings are 'at risk'

Rescue workers on the collapsed Pyne Gould Guiness building in central Christchurch. Photo / Mark Mitchell

Earthquake engineers are calling for tougher standards for strengthening old buildings after the catastrophic collapse of many pre-1970s buildings in the Christchurch earthquake.

The Pyne Gould building, Canterbury TV and other buildings where most lives were lost are all believed to have been built before a succession of increasingly tough earthquake-resistant standards began in the mid-1970s.

A 2004 change to the Building Act required all local councils to adopt policies to strengthen such buildings to at least one-third of the current building standard.

But engineers say that almost all councils, except Wellington, have adopted merely "passive" policies which do not require strengthening old buildings except when their owners seek new building consents for alterations.

And Auckland University Associate Professor Jason Ingham, a management committee member of the NZ Society for Earthquake Engineering, said the society advocated raising the requirement to two-thirds of the current standard.

"If you improve your building to only one-third of the building standard, it is still 20 times more likely than a modern building to fall down in an earthquake," he said. "At two-thirds, it's only something like three times as likely to fail."

Ironically, Dr Ingham and other international experts were in Christchurch when the quake struck on Tuesday holding a seminar aimed at getting councils to give more urgency to strengthening old reinforced masonry (brick, stone or concrete) buildings.

He said that, even after the first Christchurch quake in September, most councils had not identified buildings at risk in a major earthquake, contacted the owners or developed any plans for how to strengthen them.

"Everybody has to learn from every earthquake. Anyone who feels there is nothing to be learnt is just being naive," he said.

Another Auckland University engineer, Dr Quincy Ma, said Tuesday's building collapses did not necessarily show that the structures were not strong enough, because the quake was so violent.

"The ground motion in Christchurch was over what we expect for a 1000-year return period," he said. "We design for a 475-year return period."

This means the ground shaking from Tuesday's magnitude 6.3 quake was even more violent than Christchurch can expect from a magnitude 8 jolt on the Alpine Fault, which is expected every 300 to 400 years and was regarded as the test that buildings had to be designed for. The fault last moved 290 years ago.

Dr Ma said the fault that shook the city this week had not been identified as a major risk before.

"From the last [September] quake we thought all the lessons had been learnt, but that was just a dress rehearsal for the real test," he said.

Wellington City Council requires any building built before 1976 to be strengthened to a third of the current standard within 20 years if it may contain, or risk damaging, crowds of people. A previously tighter timetable was pushed out to 20 years in 2009 after building owners complained about the costs. Former mayor Kerry Prendergast urged the council to revisit the issue "with a view to tightening the deadlines again" after the September quake.

Auckland Council building policy manager Bob de Leur said all previous councils in the Auckland region had only passive policies requiring strengthening when new building consents were sought. The new combined council is expected to review that policy this year. Home Owners and Buyers Association president John Gray said virtually all "leaky buildings" had been found to have structural defects as well as leaks.

But he said a Building Amendment Bill now before Parliament would weaken controls even further by removing an existing requirement for consent authorities to physically inspect new buildings before issuing code compliance certificates - now to be called "consent completion certificates".