‘Seismic gap’ on stairs had been filled
The gap allowing the Forsyth Barr building’s stairs to resist earthquakes was stuffed with rubbish, pipes and polystyrene, an inquiry has heard.
Evidence presented to the Canterbury earthquakes royal commission shows both flights of stairs in the 18-storey building collapsed in the 2011 February earthquake.
The collapse of all stairs below the 13th and 14th floor left occupants stranded for hours, with some abseiling to safety and others rescued by crane.
Commission lawyer Stephen Mills said it was only ‘‘good fortune’’ no-one had been on the stairs and plunged to their death during the quake.
‘‘This has been a very lucky result that no-one died,’’ he said.
The collapse has already prompted the royal commission to call for an urgent review of all multi-storey buildings using a similar stair design.
Evidence was presented yesterday of one tenant who was forced to edge his way from the toilet along what little remained of the stairwell landing in the pitchblack to return to his office.
Another group descended down two flight of stairs in darkness before being warned that they were about to step into an abyss.
Beca director Rob Jury, who co-wrote a Department of Building and Housing report on the stair’s collapse, said the stairs had relied on a 3-centimetre ‘‘seismic gap’’ where each flight connected to the landing below.
This gap would allow space for the stairs to move without damage during an earthquake.
However, when the quake struck on February 22 the building swayed by more than 10cm, squashing the stairs and causing flights to dislodge from their landings and collapse like ‘‘dominoes’’.
The problem could have been exacerbated because during construction it appeared some of the gaps had incorrectly been filled with mortar, polystyrene and piping, effectively reducing space for the stairs to move.
After the September 2010 quake the stairs also sustained damage, rousing tenants’ concerns.
Beca had inspected the building on behalf of its property manager soon after the September quake and recommended further investigation of the stairs.
However, when another firm, Holmes Consulting, took over assessing the building, no more stair inspections were made.
Jury said while damage in the September quake and the rubbish in the seismic gap could have lowered the stairs’ quake resistance, they would have collapsed regardless in February quake.