Crushed cars may have sparked the blaze which engulfed the CTV building after its collapse, a hearing was told today.
Expert witnesses gave evidence on the third day of the royal commission hearing into the fatal collapse of the six-storey Christchurch office block, which collapsed and killed 115 people in the magnitude 6.3 quake on February 22 last year.
Urban search and rescue support engineer Graham Frost said six cars were removed from the building during the recovery stage in the immediate disaster's aftermath, all of them burnt out.
"I think it's possible the fire started in these vehicles," he said this morning.
It was previously believed that the fire was started by a nine kilogramme gas cylinder inside the building.
Mr Frost spent five days at the site collating evidence he knew would be crucial for investigating authorities to later establish why the building failed so "catastrophically".
While his primary role was to minimise the risk of danger to Urban Search and Rescue (USAR) and police teams working on the rescue and recovery, he spent five days taking photographs and making careful notes on what he saw.
Today, Mr Frost talked the hearing through many of his photographs, highlighting potential areas of failure.
Columns were a "very weak element ... very highly stressed", he explained, while the spandrels, connecting the floors to the walls, had "very light connections" and were unable to "transfer much load".
The concrete floor slabs did not stay intact in the collapse, while the beams had separated from the concrete columns "very early on" in the intensive shaking.
He concluded by identifying three potential areas of failure, which could have been triggered by the quake: beam to column connections, the concrete floor slabs, and a tension failure of the metal decking at "midspan" of the floor slabs.
His evidence was supported by another USAR worker, Brisbane-based forensic structural engineer Rob Heywood who described the building as being brittle like chalk.
Mr Heywood identified seven possible reasons for the "brittle nature" of the collapse.
The strength of the building's concrete, especially in the columns gave the Australian-based expert particular "concern", he said.
He was also concerned over the connections which tied the building together, especially the beam-column connections, connections between the floor slabs and the south wall, the edge beams and the north core, pre-cast elements, and the lack of continuous bottom reinforcement in the beams over columns in the slabs over the internal beams.
Concerns over the low ductility of the mesh reinforcement were also expressed.
"There's many questions in my mind," he said after reading through his 79-page brief of evidence at the Canterbury Earthquake Royal Commission of Inquiry hearing into the collapse.
"Brittle structures provide no warning of collapse and often no opportunity to redistribute the load.
"A stick of chalk, for example, is brittle when it is bent. One moment the chalk is carrying the load and the next moment it has failed without warning.
"Columns can also behave in a brittle manner, even if made from a ductile material."
Earlier today, a witness who saw the CTV collapse described the building as "swivelling" in the intense shaking.
Matthew Ross, who was driving a van through Christchurch city centre when the quake hit, clung to his steering wheel as he watched the building come down before his eyes.
"The shaking was long and vigorous," Mr Ross said.
"The building was turning either way. It was swivelling.
"I thought the building was going to collapse over Madras St so I was surprised it went straight down.
"I could see the top floors were intact as they collapsed into the dust."
The royal commission hearing into the failure of the CTV building started on Monday and is due to last eight weeks.
It continues tomorrow with witnesses giving evidence on how post-September earthquake assessments were carried out.