Some of the columns holding up Christchurch's CTV building had needed more reinforcement, the royal commission inquiry into the building's collapse has been told.

Dr Clark Hyland of Hyland Consultants and Ashley Smith of Structuresmith, who wrote a report for the Department of Housing and Building, said the drag bars connecting floor plates and columns were not the first thing to go in the collapse in the February 22, 2011 earthquake.

Their evidence pointed to weakness in the columns.

Earlier on Friday, US expert Brian Kehoe upheld the actions of the engineer who inspected the CTV building after the first September 2010 Christchurch earthquake.

The building collapsed in the city's subsequent devastating quake in February last year, killing 115 people.

In response to repeated questioning about a range of inspection issues, Mr Kehoe said there was little he would have done differently to David Coatsworth of CPG NZ, the structural engineer who inspected the CTV building after the September, 2010 earthquake.

Mr Kehoe said that no structural damage had been seen and therefore the building did not require analysis that would involve removing linings or ceilings.

The building had stood up well and it was fair of Mr Coatsworth to assess it as being able to withstand a similar shake.

Nor was it absolutely necessary to obtain structural drawings although this might have been desirable, Mr Kehoe said.

Structural drawings had not been obtained from the local council and many documents were unavailable because a facility had been significantly damaged for weeks after the first earthquake.

Mr Kehoe said that training of inspectors was better in the US.

He agreed that the New Zealand green, yellow and red sticker regime could be improved for clarity.

Sub-contractor Daniel Morris told the inquiry that he estimated he had cut up to 200 holes in concrete beams to accommodate electrical wiring for various CTV building tenants.

Other sub contractors had done similar work over the years.

At least once he complained to the main contractor that he thought the work could structurally compromise the building.