An engineer has defended the "general nature" of findings on the Canterbury Television building collapse, saying different experts could "come up with their own scenarios" for the cause of the disaster.

The Canterbury earthquakes royal commission has this week heard evidence from a Department of Building and Housing (DBH) investigation into the CTV building, which collapsed during the February 22, 2011, earthquake, killing 115 people.

An expert panel that considered the findings of structural engineers Clark Hyland and Ashley Smith agreed that the failure of concrete columns started the collapse, but not what led to that trigger.

DBH panel member Rob Jury said many scenarios for what caused the collapse were looked at, and differing opinions were inevitable.

"We have presented a number of collapse mechanisms and I guess any engineer looking through the evidence has the ability to come up with their own scenarios of what might occur,'' he said.

"We might come under some criticism for the very general nature of this conclusion, but I think it is a direct result of the number of scenarios that have been hypothesised already."

The exact sequence of the collapse was "not absolutely clear", he said.

"In some respects it's like the fact that you die when your heart stops, but anything can cause that to happen."

The DBH report, released in January, found design and construction flaws in the building but was dismissed as "technically inadequate" by Alan Reay, director of Alan Reay Consultants Ltd (ARCL), which designed CTV in the 1980s.

ARCL's counsel, Hugh Rennie QC, pressed Jury on an October 2011 email exchange with Hyland on the possibility of a cracked concrete slab causing the collapse.

Hyland's response to Jury's opinion that the slab theory warranted consideration was that he was "stunned [Jury] should rely on his instincts regardless of what the evidence showed".

"There were differing views, and I think in terms of [those] emails ... some of those were quite forthright," Jury told the commission this afternoon.

"I think they indicate the robust discussion among the panel and also with the investigating consultants about the conclusions that they were coming to in terms of collapse sequences, but these differing views included what was the most likely initiation of the collapse."

'Current standard may be short'

The CTV building may not have withstood the February 2011 earthquake even if had been built to current standards, Smith said.

He told the commission that east-west shaking during the quake far exceeded what the CTV building could endure.

"Certainly the 1984 design standard well underestimated these forces ... The current standard may be short."

He stopped short of concluding that current standards would definitely not have saved it as further analysis was still being done.

The report, done for the Department of Building and Housing (DBH), concluded that concrete column failure was a key cause of the collapse.

The robustness of connections between the north shear core, a key support wall in the building and the only one to remain standing after the collapse, and floor diaphragms is also a focus of the commission's inquiries.

A floor diaphragm is a horizontal brace designed to pass lateral force, such as earthquake shaking, to key support walls like the north shear core.

Smith and Hyland ran tests on the likely effect of east-west shaking on the connections, but the results ran into the hypothetical.

"The peak demand was never reached because things fell over before that happened," Smith said.

DBH investigation panel member Rob Jury said panel members agreed on the report's general conclusions, but there were differing opinions on the exact cause and nature of the collapse.

"There is a lot of evidence around that could attest to any of these scenarios [for collapse]," he said.

'No impact' from nearby demolition

The demolition of a nearby building had no impact on the Canterbury Television building, the inquiry was told today.

Hyland and Smith defended a conclusion in their report at the commission today that the demolition of the adjacent Les Mills building in Cashel St before the quake had no impact on the CTV building's strength.

The report concluded that the Les Mills building demolition was "unlikely [to have caused] demolition sufficient to affect the earthquake resistance of the CTV building''.

"This is because it is common practice to use such equipment for demolition work [and for it] not to cause any significant structural damage to adjacent buildings," it said.

Counsel assisting the commission Marcus Elliott asked the pair if their conclusion was based solely on the "common practice" theory.

"The sort of energies and effects that causes is generally accepted not to cause significant damage," Hyland said.

"You can [cause] damage when people are digging out underneath or adjacent to buildings, but it's not normal to expect serious damage to occur."

Former CTV tenants have testified to increased "liveliness" and "bounciness" in the building during demolition in early 2011.

Smith said that description was a "recurring theme" when the pair gathered witness evidence.

"It was obviously a big issue for them. They may have [taken] exception that we didn't give that adequate coverage in our report [but] they weren't discounted, those tenants' views."

He said CTV tenants were likely to have experienced "heightened sensitivity" to shaking in early 2011 because major tremors, such as the September 4 and Boxing Day quakes in 2010, had made them more conscious of it.

The commission heard last week that the Les Mills building demolition breached consent by using a wrecking ball, but Smith said this did not change their conclusion.

"We are aware of [Christchurch City Council resource consents and building policy manager] Steven McCarthy's evidence that they used procedures in the demolition that weren't approved,'' he said.

"We still have the view that that did not impair the seismic resistance, but it would have certainly led to big vibrations in the building."

The report's findings, including that the building did not meet 1986 design or building standards, were not universally accepted.

They were dismissed as "technically inadequate" by Alan Reay, director of Alan Reay Consultants Ltd (ARCL), which designed the building.

''Some of the assumptions made in the reports are highly questionable. As a consequence, the report's findings are not conclusive. In fact, in many areas they may be flawed,'' he said.

ARCL will call its own expert, John Mander, who will testify that the building complied with all design and building codes when it was constructed in 1986-87.

Mander is scheduled to give evidence on Thursday.

- © Fairfax NZ News

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