'Significant weaknesses' in CTV design - engineerJOELLE DALLY
Last updated 13:46 31/07/2012
KIRK HARGREAVES/Fairfax NZ David Harding.
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A structural engineer who designed the Canterbury Television building has accepted the job was outside his level of competence and the design was not "best practice".
Evidence from David Harding continued this afternoon during a royal commission of inquiry into the CTV building collapse in the February 2011 earthquake.
Harding, who did the detailed design for the CTV building while working for Alan Reay Consultants in 1986, said earlier he had spent "much time" considering if there were deficiencies in his work, but could not identify any.
After being presented with contrary evidence, he conceded that there were in fact "significant weaknesses" in the building design.
Commission assisting counsel Stephen Mills, QC, referred to the findings of engineers who assessed the building before and after the collapse.
Professor Nigel Priestley described the connection of the north core of the building to the floor diaphragm as "very remarkable".
"That was not put in a positive way, I can assure you," Mills said.
In 1991, a review found that the lack of connection in those elements was not code-compliant even by 1986 standards.
A structural report prepared by Holmes Consulting Group in 1990 found the building had the potential to collapse in a moderate quake.
Opus looked at the building in 1997 with a view to taking up a tenancy, but did not take it up after a desktop survey check of the building identified the same issues as Holmes Consulting found.
United States engineering professor John Mander referred to the design of the diaphragm connection as "very troubling" and "not sufficient".
The latest non-linear time history analysis identified diaphragm connection as the likely initiator of the collapse.
Mander found the lack of reinforcement in the beam column joints was "definitely not best practice", and if interior columns had been made bigger the building would have performed "marvellously better".
Mills asked Harding whether he could accept the building had "a significant number of problems".
"Having had that pointed out to me, I accept that," Harding said.
He was pressed on how he used a computer modelling programme to test the CTV design.
He conceded he tested only the strength of the centre of the building, not the corners.
This was because of the limitations of the programme, called Etabs, at that time, so he was not aware he needed to test the corners separately.
Mills asked whether Harding agreed he was therefore working well outside his level of competence.
"Yes. That's the reason why I expected a review. I would not have taken this on on my own."
He said he was relying on his employer, Alan Reay, for guidance as he had no experience with high-rise buildings.
No deficiencies in design - engineer
Mills asked Harding if the fact that no other building in Christchurch collapsed it the way the CTV building did had caused him to question if there were any inadequacies in the design.
Harding replied that it had.
"As a result of that deep thought and contemplation, have you reached a conclusion?" Mills asked.
Harding replied that, based on the evidence he had seen, the CTV building collapsed because it faced unexpectedly high vertical forces in the quake that it was never designed, or required to be designed, to withstand.
The fact it survived the September 4, 2010, quake showed that it had performed as it was designed for in terms of horizontal loadings.
The February quake was ''different".
The epicentre was closer and there were horizontal and vertical loadings.
"The vertical loadings were over and above the code, and that is why it failed," Harding said.
Harding said he saw the building as a challenge that he wanted to take on and was relying on Reay for guidance as well as "accumulated knowledge" from other employees, including draughtsmen.
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March 13, 2012, 07:49:06 PM