The exact cause of the Canterbury Television building's collapse in last year's earthquake may never be known, even if more sophisticated modelling could be done, an inquiry has been told.

William Holmes, a structural engineer from California, peer-reviewed a Department of Building and Housing report into the CTV building collapse that was released in January.

The expert panel that carried out the investigation for the report found the Christchurch building's collapse was "almost certainly" initiated by the failure of one or more columns on the east wall, facing Madras St.

Giving evidence yesterday at the inquiry into the collapse, Holmes said he found the most important seismic deficiencies in the building were the brittle gravity frames and the poor diaphragm, particularly the connections to the north tower walls.

He disagreed with evidence from panel member Rob Jury, a structural engineer, that the building's columns might have withstood the failure had they been reinforced by a type of steel known as confinement.

Holmes believed the beam column joints were more important. If they had been improved, the collapse might have been partial or localised, whether the beams were reinforced or not, he said. Determining the "exact set of deformations that instigated the collapse" might never be known because many important parameters could only be estimated, Holmes said.

These included the exact ground motion sustained by the building, the drift at which the joints would start to degrade, the strength and stiffness of the connections of the building diaphragm to north tower walls, the strength and stiffness of the diaphragm, the extent of interaction of the block wall and the resulting torsion, and the effects of vertical ground motion on critical components.

Holmes said lessons from the CTV building failure could be applied to seismic evaluations on older buildings.

Engineers could look for brittle building framework, building diaphragm issues and how structural elements connected with non-structural elements, he said.

- © Fairfax NZ News

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