A report on the Forsyth Barr building in Christchurch, where stairs collapsed in the February earthquake, shows an earthquake-proofing mechanism failed.
The Department of Building and Housing on Friday issued a report into what caused the collapse of three buildings in the Christchurch earthquake on 22 February.
An expert panel employed by the department has been investigating the collapse of the Pyne Gould Guinness (PGC) building which killed 18 people, as well as structural failures in the Forsyth Barr and Hotel Grand Chancellor buildings.
The report says construction debris was found in so-called seismic gaps, built into the flights of stairs as part of the Forsyth Barr building's earthquake-proofing design.
Seismic gaps allow a building to move back and forth during a quake.
The report says a number of flights of stairs had seismic gaps that were either less than specified in the building's design or had construction debris in them. Two flights of stairs collapsed.
The department says the building, constructed in 1988, failed because the earthquake was extremely violent and building requirements at the time of construction were more lax.
The panel says the main reason the five-storey PGC building collapsed was because the ground acceleration was several times larger than the building, designed in 1963, was built to withstand.
The department has issued a directive to owners of multi-storey buildings throughout New Zealand to check their stairwells can withstand earthquakes and reinforce them if they can't.
The last person to be pulled alive from the PGC building after the Christchurch earthquake, Ann Bodkin, says it is easy in hindsight to say more could be done, but nobody anticipated such a destructive earthquake in Christchurch.
She says it is a good thing that buildings of a similar structure will be looked at, and people are now more aware of the risks and will seek out earthquake strengthened buildings.
Hotel wall collapsed
A wall in the Hotel Grand Chancellor building expected to carry substantial earthquake loads collapsed, the report said.
The failure of the wall, which supported one eighth of the building's mass, significantly increased the damage to the building.
The report says the concrete wall's collapse caused the failure of a major stairwell and columns and beams.
The loss of the wall meant one side of the building dropped 80 centremetres, putting the building on a lean.
The report says the strong shaking exceeded design standards in the 1980s, when the hotel was built, but it said the building was generally well-designed, which prevented a more catastrophic collapse.