'Significant weaknesses' in CTV design

JOELLE DALLY
Last updated 14:01 31/07/2012
David Harding
KIRK HARGREAVES/Fairfax NZ
David Harding.

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A structural engineer who designed the Canterbury Television (CTV) building has accepted the job was outside his level of competence and the design was not "best practice".

Evidence from David Harding has continued this afternoon during a royal commission inquiry into the CTV collapse during the February 2011 earthquake.

Harding, who did the detailed design for the CTV building while working for Alan Reay Consultants Ltd in 1986, said earlier he had spent "much time" considering if there were deficiencies in his work, but could not identify any.

However, after being presented with a host of contrary evidence, he has now 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 both 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 CTV building had the potential to collapse in a moderate earthquake.

Opus looked at building at one point 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.

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 also found the lack of reinforcement in the beam column joints was "definitely not best practice" and if interior columns had been made bigger, it would have performed "marvellously better".

Mills then asked Harding if he could accept the building had "a significant number of problems".

"Having had that pointed out to me I accept that [it did]," Harding said.

Harding was also pressed on how he used a computer modelling programme to test the CTV design.

He conceded he only tested the strength of the centre of the building, not the corners.

This was due to the limitations of the programme, called Etabs, at that time, so he was not aware he needed to test the corners separately.

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Mills asked if 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 again that he was relying on his employer, Alan Reay, for guidance as he had no prior 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.

- The Press

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