Christchurch Earthquake 2011
The Pyne Gould Corporation building was doomed to fail.
That might seem like a harsh conclusion, given that similarly worded assessments have been made on the Canterbury Television building collapse, on which a damning royal commission report was released this week, but it is an entirely different case.
The commission's findings on the PGC building failure, which claimed 18 lives in the February 2011 earthquake, showed the structure had little chance of surviving the shake.
Built in 1966, the five-storey structure needed much lower seismic strength than today to comply with building standards.
It lacked the "ductility" that allows modern buildings to bend and absorb force in a quake.
The building stood around a shear core - its strongest part - that ran in a north-south line, meaning it was stronger if shaken in that direction than east-west.
The commissioners found that on February 22, 2011, east-west shaking was "appreciably greater" than north-south shaking, and "failure of the eastern wall initiated the collapse".
They noted there were "considerably more" structural walls on the ground floor, but stopped short of endorsing two assessments of the collapse cause, one by the Department of Building and Housing and one by engineering firm Beca, that the eastern wall gave way just above this, between levels one and two.
The report listed six structural weaknesses that, on reading, give weight to Professor Nigel Priestley's observation during the commission's hearing that the PGC was "not a happy building".
One of the weaknesses was a lack of strength in the perimeter columns. When the shear wall failed in the quake, the columns on the east side bore the brunt of the pressure. They collapsed and the building came down.
This problem was picked up in 1997 when a Holmes Consulting Group engineer noted "the potential failure of the columns is a life-safety issue as it could result in the loss of support and consequential collapse of all or part of the building".
Steel props were installed behind the columns a year later, which "partially overcame" the problem, the report said, but another problem identified by the Holmes engineer, that the shear walls would likely "rock" in only a moderate quake, went unaddressed.
By 2007, the building's age meant the Christchurch City Council's new quake-prone building policy came into play when upgrades were considered, but an assessment by Holmes that the building should perform "reasonably well" in a quake satisfied this.
The building was inspected five times between the September 2010 and February 2011 quakes - four by Holmes engineers - and declared safe each time.
Their inspections were "damage-based", standard in Christchurch after the September quake, meaning Holmes' long history with the building and its problems were not taken into account.
The commissioners found: "Despite references to seismic weaknesses . . . the advice of [Holmes] at the relevant times was that the building was not earthquake-prone. We have no reason to doubt the correctness of that advice."
Small cracks in old buildings, like those found by the Holmes engineers at the PGC building, could be wrongly interpreted as insignificant, their report said.
"The reinforcement crossing the crack might have either extensively yielded or completely failed at the crack. After the earthquake the crack, which might have opened to an appreciable width during the earthquake, might close," it said.
"This indicates that the visual inspection procedures after an earthquake for buildings such as the PGC building need to be reviewed."
The Holmes engineers "did not consider the cracks observed were significant".
"The evidence before the royal commission would not justify a finding that these conclusions were incorrect," the report said.
Similarly, building manager Harcourts was "entitled to rely on the advice received and convey the advice to the building's tenants that the building could be safely occupied", it said.
- © Fairfax NZ News
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