Design firm blunders were the "primary cause" of the CTV Building collapse in the February 22 earthquake which claimed 115 lives, an inquiry has heard.

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 lawyers assisting the commission 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, said the Christchurch design firm principal Dr Alan Reay, and David Harding, the inexperienced engineer who was given "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.

"There was... either no or inadequate margin of safety provided."

Dr Reay has acknowledged that his firm was ultimately responsible for any failings and 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 since his worker was plainly not up to the job.

Dr Reay himself also had "insufficient experience and competence" in the design of complex multi storey structures, Mr Mills said.

Once his experienced structural engineer John Henry left the firm shortly before the CTV Building job was offered, he should've turned it down.

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."

Lawyers for both Dr Reay and Mr Harding will give their submissions tomorrow.

Outside the hearing, Brendan Baker, 45, who lost his fiancee Deb Roberts in the collapse, welcomed the conclusions of the commission lawyers.

He was due to marry his long-term partner on her 40th birthday four months after the disaster.

They'd just had lunch together and accounts manager Miss Roberts had just returned to her desk on the third floor at King's Education when the massive jolt struck.

Mr Baker has been trying to follow the royal commission as much as he can while juggling a fulltime job, raising his three teenage children alone and battling his personal grief.

Like many of the bereaved families, he just wants answers.

"We're not going to blame anyone for it, we just want the truth," he said.

"There have been a few lies, and a lot of denials earlier on. But as more evidence has come out, a lot (of witnesses) have had to accept their part.

"Today shows what a lot of us knew - they there were serious design flaws that led to it coming down.

"There were construction issues too, and for us who have lost people, the fact there were no structural checks after September (2010) and it wasn't red-stickered is the most gut-wrenching of it all."

He has taken unpaid leave from his factory job over the last three months to follow the hearings in person.

While it's brought back many painful memories of his beloved partner of seven years, he wants to make sure the right questions are being asked.

"We have to get it sorted out once and for all," he said.

"Not just for me - it won't bring Deb back - but I want to make sure future generations, and my kids, don't have to go through this again."