Fears over high-rise floor strength

By ANNE GIBSON and SIMON COLLINS

The head of the country's biggest building firm, Fletcher Construction chief executive Ralph Waters, has questioned the widespread use of precast concrete floors in New Zealand high-rise buildings.

The 54-year-old Australian, who took the helm at Fletcher two years ago, said Australian builders did not cast floors in off-site factories as they did here.

"The only place I've seen this is in New Zealand and South America."

A report on Wednesday by engineers Sinclair Knight Merz expressed "significant concern" at the performance of some precast flooring systems, as well as at other high-rise building techniques such as a trend towards very thin concrete walls.

The Government ordered the report after Auckland structural engineer John Scarry said in March that precast concrete "has gone from a bad state to an appalling state".

Canterbury University tests on a common combination of a precast concrete floor and steel columns and beams found that, even when built correctly, the floor collapsed under a simulated earthquake.

"The floor literally fell through, which is a worry if you are sitting at a desk below," said Canterbury engineering professor John Mander.

He said precast concrete became common, in Europe as well as New Zealand, in the 1980s.

"But in Europe the earthquake hazard is not so pronounced as in New Zealand."

Mr Waters said he was not criticising construction techniques, or the people who used them.

"I'm not saying standards are lower, they are just quite different.

"In most places around the world, you pump the concrete up 50 or 100 levels into place and the concrete floor would be six to eight inches thick and full of reinforcing steel."

Fletcher Building owns businesses that supply both precast concrete floors and concrete that can be pumped into buildings on site.

Professor Mander said he began his research after the Los Angeles earthquake in 1994, when some high-rise buildings collapsed even though they had been built according to the latest building codes.

Most US research on the issue had focused on steel construction, which was more common in California. But until now, no one had tested the connections between steel frames and reinforced concrete floors.

"Certain parts of the buildings have performed very, very well, such as the frames - the columns and beams that hold the floor up," he said.

"The floors themselves perform okay. But it's the connections between the two that are not so good."

The Canterbury tests have been done on precast concrete floors, but Professor Mander said concrete cast on site would have problems too.

He has just won a six-year, $3.2 million grant from the Foundation for Research, Science and Technology to develop better connections using steel caps, which could be retrofitted into buildings at a cost of $5000 to $50,000 a connection.

Meanwhile, research done for a new draft Australian and New Zealand standard for earthquake engineering shows that the risk of a big earthquake in Auckland is slightly less than previously thought.

A scientist at the Institute of Geological and Nuclear Sciences, Andrew King, said the new standard would set the earthquake risk in Auckland at about one-third of that in Wellington, compared with a ratio of about half under the old standard.

Christchurch's risk remains half that of Wellington. The committee aims to set building standards that produce an equal risk of collapse in each city so that the cost of insurance is roughly the same.