A range of possible collapse triggers have been identified in closing submissions for the royal commission hearing into the ill-fated office block's catastrophic failure in the killer earthquake last year.
But commission lawyers have concluded after eight weeks of evidence that the "principal and critical failings" for the concrete tower lay with the structural design work carried out by Alan Reay Consultants Ltd in the mid-80s.
Stephen Mills QC, counsel assisting the Canterbury Earthquakes Royal Commission, says that the Christchurch design firm principal Dr Alan Reay, and David Harding, his inexperienced engineer who he gave "sole responsibility" for the "dangerously vulnerable" and non-compliant design, must both "carry the responsibility".
"The decisions that they made about the structural design of the building are, in my submission, the primary cause of the building's collapse," Mr Mills said today.
"In critical respects, the building they designed was not code compliant and was dangerously vulnerable to any earthquake that took the building any distance beyond its elastic response state and into the inelastic range.
"There was, in my submission, either no or inadequate margin of safety provided for in the event that this occurred."
Dr Reay acknowledged during one of his many visits to the witness stand during the hearing that his firm was ultimately responsible for any failings in the work carried out by Mr Harding.
He apologised to bereaved families for a building that "did not meet my standards".
But Mr Mills today said Dr Reay's responsibility went further than that.
"It was his decision to give Mr Harding virtually the sole responsibility for carrying the structural design of the CTV Building, and in my submission that was done in circumstances where Mr Harding on any objective view was not competent to be given this level of sole responsibility.
"Dr Reay then made a deliberate decision not to provide no active supervision or mentoring for Mr Harding."
He said it also emerged that Dr Reay himself had "insufficient experience and competence" in the design of complex multi storey structures.
Once his experienced structural engineer John Henry left the firm shortly before the CTV Building job came in, he should've rejected the job, the commission lawyers concluded.
Mr Mills summed up: "The result was a building with numerous design defects, that was not compliant with bylaw 105, nor with the applicable code provisions, and as a result was extremely vulnerable when struck by the very strong earthquake forces of February 22."
The commission lawyers said there were a number of candidates for the actual trigger for the collapse, but Mr Mills said it was not possible to identify one and say, "That's the one."
More than 80 witnesses were called during the hearing designed to establish just why the CTV Building failed to catastrophically in the magnitude-6.3 quake, and why it collapsed so totally when no other building Christchurch did.
Mr Mills said the evidence was now there to "enable a clear understanding of what went wrong here", from the initial design through to its collapse.
Conclusions can be made on its critical failings and who is responsible for them, he said.
A total of 10 lawyers for the various affected parties will sum up over today (Wednesday) and tomorrow.
The Royal Commission has until November 12 to produce its final report.
