29 Sep 2017

'Pigtail' construction in firing line after Kaikōura quake

9:43 am on 29 September 2017

The Ministry of Business, Innovation and Employment (MBIE) wants to ban a construction technique that proved catastrophic during last November's Kaikōura earthquake.

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Statistics House has been unusable since the November earthquake. Photo: RNZ

The technique was used in Statistics New Zealand's waterfront building in Wellington, where two floors partially collapsed during the midnight quake.

If staff had been in the building at the time, they would have been killed.

The technique involves binding heavy concrete floors to supporting pillars with a thick band of steel known as a "pigtail".

These steel bands failed to keep the two floors in place in Statistics House. The building is now unusable.

The Society of Earthquake Engineering welcomed MBIE's proposed ban, and said the use of the steel bands was being phased out anyway.

"Our response is favourable," society president Peter Smith said.

"This detail did prove unsatisfactory during the Kaikōura earthquake, and existing buildings which have that detail need to have modifications so the buildings do not rely on the pigtails for support of a floor system."

It is not known how many buildings use the pigtails, which were introduced in the late 1980s.

Mr Smith estimated about 10 percent of high-rise buildings constructed at that time would have used them.

He said the aim of the change would be to add another layer of support for heavy concrete flooring where a collapse could be disastrous.

The ministry said its plan was a proposal only and it was open to counter-submissions.

It did not want to ban all steel bands, just those used in special cases: extra-long pre-cast concrete flooring with a distinctive double-T shape and a flange at the end.

It also said not all buildings using the same method as Statistics House would fail in the same way, as individual buildings performed differently in a quake.

MBIE has released a report into the performance of Statistics House in Wellington during the Kaikōura earthquake.

An image from MBIE's report on the performance of Statistics House in Wellington during the Kaikōura earthquake Photo: Supplied / Ministry of Business, Innovation and Employment

These qualifications to the reform plan were welcomed by Wellington engineering expert Adam Thornton of construction firm Dunning Thornton.

He said any ban would need to be undertaken with caution.

"Not all floors with the pigtail detail would be problematic," he said.

"Any such ban needs to be justified as to why it should not be used in a new building and what its effects might be in an existing building."

Mr Thornton said other forms of flooring also caused concern, and he wanted moves to outlaw the steel bands to proceed carefully.

"It is good that the pigtail devices are being considered but they are potentially one of the lesser problems that the industry faces."

MBIE said there were other ways of binding pre-cast concrete beams together besides the method being proposed for the chop.

It added research was constantly under way to better understand how buildings performed in a quake, and neither the ministry nor anyone else had solved this problem yet.

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