Christchurch earthquake: Collapse of stairwells concerns professor

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The failure of high-rise stairwells - normally considered a lifeline to safety - during the Christchurch earthquake should be a focus of a high-level inquiry, an Auckland University academic believes.

Workers in the 17-storey Forsyth Barr building had to abseil to safety after the stairs collapsed, even though the building didn't. The Hotel Grand Chancellor's stairwells also disintegrated into rubble.

Charles Clifton, an associate professor of civil engineering, was in the city during the quake giving a seminar on assessment and retrofit for quake-affected buildings. In the aftermath, he has helped to assess buildings for civil defence.

Stairs in multi-storey buildings have a fundamental design philosophy, Professor Clifton said.

Because building levels move relative to each other in a quake, stairs are typically fixed at the top, however, they are supported on something that allows them to slide at the bottom.

While investigations would ultimately reveal the causes of the individual collapses, Professor Clifford said there were generally two reasons for failure.

First, the stairs could have moved off their supports - of which he was doubtful. More likely, stairwells were fixed at both the top and bottom.

"If, for any reason, the bottom is prevented from sliding then what happens is that you've got two floors moving with a lot of force - if you've got a stair that's rigidly fixed between them and is not able to slide, then the stairwell will break its back.

"Stairs are particularly vulnerable unless they're properly detailed to allow the sliding to occur."

Professor Clifton said the Building Code requires that people be able to exit buildings in the event of a quake without specific reference to stairs.

"I've seen the search and rescue people picking through the stairs. There will be an inquiry and that'll be one of the areas to focus on."

Hotel Grand Chancellor owner, Frank Delli Cicchi, said he could not comment on the building's stairwells until he had received a report from structural engineers.