A renewed investigation into why Statistics House on the Wellington waterfront was badly damaged in the 2016 Kaikōura earthquake has found the floors were not built as planned.
Some of the overlap where the precast concrete floor units sat on ledges around the perimeter, was not as wide as it should have been.
Officials say the discovery does not change the original conclusion about why parts of two floors collapsed, or the recommendations of what to do about that.
Demolition workers at 12-year-old Stats House discovered the fault earlier this year.
This prompted the Ministry of Business, Innovation and Employment to reopen its engineering investigation, but it did not make public until now why it had done so.
"Observations made by the engineering consultant who monitored the demolition indicated that the seating provided for some of the precast concrete floor units was less than what was shown on the original design documents," the ministry said in a statement today.
The minimum width of the overlap, or seating, where the concrete sits on the ledge, was set by Standards at 60mm, but Stats House had seating overlaps ranging from 38mm to 120mm wide.
"The demolition engineers ... opinion was that around 50 percent of the measured seatings were less than 50mm," the new report said.
This was probably due to the way the floors were designed and made, which made it difficult to make sure that concrete beams that were precast offsite, fit inside the actual building properly, it said.
The standards for concrete structures at the time had since been updated, to iron out inconsistencies in how much "tolerance" was allowed on-site, it said.
But the floors would have failed anyway, irrespective of the narrow ledges, the report said, adding the failure had more to do with supporting loops of steel called 'pigtails' breaking through the concrete.
The ministry last year warned engineers not to use pigtails, and councils must factor in this warning if faced with a consent application that specifies pigtails.
Centreport declined to comment on who, if anyone, might be liable for how the floor seating-ledges were built.
The ministry said only that the investigation panel "was not engaged to look at issues around liability".
"The presence of the short seatings of the precast concrete floor units does not change the [engineering] panel's conclusions that the partial floor collapses were caused primarily by a combination of a highly flexible ductile frame, beam elongation, shortening of the precast concrete floor units, amplification of ground shaking and the duration of the earthquake," the new report summary said.
Sources indicated to RNZ that cracks in the concrete floors of Stats House may have been evident before the earthquake.
However, the Statistics Department said its records showed no communication about such cracking between its project manager, the contractor, or Centreport, in the months before the quake when a contractor was working on strengthening the floors.
It was "not aware of any concerns of this nature being brought to the Stats NZ's attention prior to the quake", it said in an Official Information Act response to RNZ.
The reopened investigation makes three new recommendations.
One of them addresses the issue of Stats House's floor ledges not being built as planned.
It said the ministry and industry bodies should review building standards so there was better coordination between the designers and builders, especially around seating-ledge requirements for concrete floors; and look at the steps taken to install floors.
The other two new recommendations were that the ministry:
- Bring in experts to advise on how to get the industry to build precast concrete floor systems properly.
- Tell the industry about what the failure of the floors at Statistics House means for them.
CentrePort said the whole investigation had provided useful information for policymakers, regulators, building designers, owners and tenants.
The original investigation found the combination of factors that led to floor failure was not anticipated by the design standards in place when Stats House was built.
That has led to the ministry working to update the Building Code system, and to research into how to retrofit existing buildings so their floors are made stronger.