6 Apr 2017

Engineer defends assessment of quake-damaged building

10:57 pm on 6 April 2017

An engineer is defending his assessment of a Wellington building damaged in November's earthquake, even though it has floor supports the industry had been warned to avoid.

A concrete floor support unit fell in the 14 November earthquake.

A toppled concrete floor support unit toppled in Statistics House shortly after the November earthquake Photo: Statistics NZ

Parts of the flooring on two levels of Statistics House on Wellington's waterfront collapsed in the 7.8 earthquake last year.

The five-storey office block was constructed in 2005.

Dunning Thornton engineer Alistair Cattanach, the building's structural designer, assessed it in 2013 and 2016 as more than 90 percent of the new building standard under the current Building Code.

In 2008 the Structural Engineers Society had told engineers not to use steel loops, also called pigtails, at the end of concrete panel floors in buildings.

It said the detailing of typical pigtail hangers did not comply with the concrete design standards updated in 2006.

And in 2009, engineers writing in the Structural Engineering Society's journal identified many failure mechanisms "unique" to concrete floors with pigtails.

The design standard says adequate support must be provided to the floor units to take account of the building moving in an earthquake. Statistics House was designed to be very flexible and, in the November 2016 earthquake, moved 50 percent more than it should have.

Mr Cattanach said his assessment took full account of the pigtails.

"In reflection we still feel it is robust," he said.

"Yes, this is an absolutely critical issue. The performance of this falling out of the ceiling is unacceptable but we need to not overstate or understate the risk. We've got to try to portray it accurately and that's what we feel we've done."

He said the industry was missing some crucial information about how much buildings could move in a earthquake, but the study of Statistics House's failure would help fill in those gaps.

Mr Cattanach did not ask for an independent engineer's opinion of his assessment of the building, as he said the gaps in research meant this would not have been useful.

The building's owner, CentrePort, asked for the assessment straight after the 2013 Seddon earthquake, aware that advances in earthquake science were coming out of the Canterbury Earthquakes Royal Commission. It freely went over this report with RNZ News, even though it declined to provide a copy of it, and Mr Cattanach provided a long interview.

In March 2016, Mr Cattanach repeated his rating to CentrePort. He had recommended strengthening the floor supports with extra brackets within five to 10 years. That work began in September 2016 as part of an interior makeover and was a third of the way through when the earthquake hit.

He said that was the right time-scale to finish the strengthening work, and that there were plenty of other buildings more dangerous than Statistics House that currently have been given much longer to do strengthening work. He stressed the 7.8 earthquake exceeded the 7.5 allowed for in the building code.

Mr Cattanach did not accept the pigtails were non-compliant as a blanket rule, though Dunning Thornton no longer used them.

In response to RNZ's report about the pigtails, Government Statistician Liz MacPherson said it served to strengthen her view of the "critical importance" of independent technical advice and peer review.

"If I had known in 2013 what I know now, I may have made different choices."

She did not see the 2013 Dunning Thornton report, and earlier said if she had she would have had it independently reviewed.

"What is most important is that we all learn from this experience."

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