CTV Building — Non-Ductile Columns That Pancaked Six Storeys and Killed 115

At 12:51 p.m. on 22 February 2011, a magnitude 6.3 earthquake struck Christchurch, New Zealand, and the six-storey Canterbury Television (CTV) building at 249 Madras Street collapsed in seconds, killing 115 people — about 60 percent of the entire earthquake’s death toll of 185. The Royal Commission of Inquiry into Building Failure Caused by the Canterbury Earthquakes and the parallel Department of Building and Housing (DBH) technical investigation reached the same conclusion: the ground shaking was severe, but the building came down because its own structure was under-designed. Its reinforced-concrete columns were non-ductile, starved of confinement steel, and unable to sustain the displacements demanded of them.

The mechanism was brittle, not exotic. Thin reinforced-concrete floor slabs sat on slender columns and were braced by an off-centre pair of shear walls — a north service core and a south wall — that left the structure torsionally unbalanced. When the earthquake drove the floors sideways, the columns had to follow. Lacking the close-spaced transverse ties that let a ductile column bend without bursting, they reached the limit of their cover concrete at very small drifts, lost their ability to carry vertical load, and failed. Once the columns on one side let go, the floors above dropped one onto the next in a classic pancake collapse. Only the north core was left standing.

The building had been designed in 1986 and completed in 1987 by Alan Reay Consultants Ltd. The structural design was carried out by David Harding, an engineer who had not previously designed a building taller than two storeys, working without adequate supervision from the firm’s principal. The Royal Commission found the design was deficient and should not have been approved, that Harding had worked beyond his competence, and that the building did not comply with the loadings and concrete codes in force when it was built. The structure that fell in 2011 had carried a latent overload trap in its columns since the day it opened. A junior engineer was assigned work above his experience, a developer pressed for minimum-cost design, a peer review and a council consent passed a deficient design, and a non-ductile load path was built into a country that had already moved decisively toward ductile detailing. The earthquake supplied the demand; the building had never had the capacity.