WITNESS: Structural engineer Murray Jacobs gives evidence on code compliance.
KIRK HARGREAVES/Fairfax NZ
ANALYSED FAILURE: Structural engineers Clark Hyland, left, and Ashley Smith give evidence during the hearing.
The engineers who examined the Canterbury Television building collapse found the building did not meet building code requirements despite taking the "most lenient" view of the rules.
The Department of Building and Housing (DBH) commissioned a report on the building, which collapsed during the February 22, 2011, earthquake and killed 115 people.
The report found the building's design and construction fell short of 1980s standards.
Report co-author Ashley Smith this afternoon told the Canterbury earthquakes royal commission that experts could not reach a common interpretation of the code, but agreed the building was non-compliant.
"Although there were variations of interpretation ... none of the DBH expert panel members thought that the design of the [concrete] columns would have complied,'' he said.
"The variations of interpretation were about the extent of non-compliance only and which of the criteria in the codes was most critical."
Smith said he and report co-author Clark Hyland often disagreed on how the code should be applied, and had to compromise.
"It was necessary to come up with a report that everyone could live with that was not necessarily reflective of our individual opinions,'' Smith said.
"Dr Hyland's interpretation was a more lenient interpretation, [so] if we were saying things did not comply, we felt we had to go with the most lenient interpretation."
Despite his more conservative view, Smith agreed with the report's finding that substandard concrete columns were a key cause of the collapse and did not meet code requirements.
The columns should have had better steel reinforcement inside them, he said, so that if their outer "shells" fell away during a quake they would still have had adequate support.
"This is particularly important for the CTV building columns because they are relatively small ... The concrete core contained by the [steel reinforcement] is only around 56 per cent of the total section area,'' Smith said.
"Consequently, the strength lost when the concrete's shell spills off is a significant proportion of the total strength."
Other engineers are scheduled to testify on code compliance this afternoon.
'Never seen columns with such little reinforcing' - engineer
Structural engineer Murray Jacobs was asked by the commission to investigate whether the building was code-compliant.
He told the commission today that the concrete columns should have been reinforced with more steel.
"I've never seen a column like the columns in the CTV with such little reinforcing. In my experience, and I've had significant experience, I've just never seen a column like that."
The columns should also have had more ductility, he said.
"The columns in the CTV building were a risk to life if they failed, and they should have been designed to exhibit ductility. They were not,'' he said.
Ductility allows for key structural elements to stretch and return to their original position during a quake."As a consequence, they would fail when subjected to [force] such as would occur during an earthquake. The consequences outlined in [the building code] do not appear to have been heeded."
The code also outlined a structure like the CTV building should where possible be built "symmetrically about the centre of mass of the building".
"The CTV building does not comply with this instruction. The primary resisting elements in the structure are asymmetrical,'' Jacobs said.
"The main resisting element is a concrete core ... situated completely outside the main [support wall]."
Reinforcing steel in the concrete was also lacking, he said, the floors had limited connection to the major support wall and parts of a key support wall were "slender", which would have concentrated shaking forces there.
"I found in my engineering experience it does cause more rotation than a computer analysis would show,'' he said.
"The stresses that have to flow ... back out again to the wider part of the wall and that can often cause a concentration of cracking in that [slender] part of the wall."
Using computer modelling for building design, as the CTV designers did, had "limitations in representing an actual building in practice", he said.
"I did not rely on one computer analysis as being the absolute design basis for a building."
The CTV designers may have assumed the building's columns would never be subjected to "that little bit extra that would cause complete catastrophic collapse".
"This is despite several cautions ... in the code pointing out that the limitations of [such] assumptions,'' he said.
The commission will hear evidence from several experts on whether the CTV building complied with 1980s building codes.
The building was issued a building permit by the Christchurch City Council in September 1986.
Three codes of building practice were applicable.