CTV inquiry: Columns lacked sufficient reinforcing

Emergency services search the rumble for survivors of the collapsed CTV building after the earthquake. Photo / Brett Phibbs

Two engineering experts gave evidence today (Thur) to the Canterbury earthquakes royal commission that columns supporting the ill-fated CTV Building in Christchurch lacked sufficient steel reinforcing.

Auckland engineer Murray Jacobs told the commission he had never seen columns with as little reinforcing as those in the six-storey building that collapsed in February 2011, killing 115 people.

Dr Jacobs, who investigated the collapse, said the supporting columns that failed were not code compliant.

He said the concrete columns should have had more steel reinforcing.

They were "a risk to life" if they failed and were "not sufficient to provide ductile strength," he said.

Ductility allowed buildings to stretch and move in the event of an earthquake.

Dr Jacobs said light reinforcing may have contributed to the collapse.

His findings were that the CTV Building design did not comply with building codes relevant in 1985 when the building was constructed.

The building was not symmetrical, as it should have been according to codes current at the time.

"The CTV building does not comply with this instruction," he said. "The primary resisting elements in the structure were asymmetrical."

Dr Jacobs said the building's concrete core was "completely outside" the main supporting wall.

Engineer Ashley Smith, co-author of a Department of Building and Housing report that found the CTV Building's design and construction was not up to 1980s code standards, agreed that the lack of concrete reinforcement was a major cause of the collapse.

Concrete supporting columns should have more steel reinforcing to provide adequate support in an earthquake, he said.

It was "particularly important" in the CTV Building because the columns were "relatively small".

"Consequently, the strength lost when the concrete shell spills off is a significant proportion of the total strength."

The commission is seeking answers as to why the six-story concrete office block failed catastrophically in the 6.3 magnitude earthquake. It is expected to deliver its findings in November.