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Trucks tough it out with timber

No matter how hard they tried, they couldn't destroy it - the classic Kiwi 1960s timber-frame building really is as strong as we thought.

The Ministry of Education said the exercise to establish how much force its school buildings could withstand had saved it hundreds of millions of dollars in earthquake-strengthening costs.

A 1960s, two-classroom timber-framed building at South End School in Carterton was chosen for the experiment, carried out by BRANZ engineers in June.

By attaching cables to the building and pulling it in opposite directions, researchers were able to see how much force it could handle before it fell apart. It turned out to be more than three times that experienced in the Christchurch quakes, and even then it didn’t collapse.

Ninety per cent of the ministry’s 37,500 school buildings are timber-framed, which means the research has brought the potential bill for earthquake strengthening in schools down to between $10 million and $15m.

NZ Society for Earthquake Engineering executive officer Win Clark said the research reinforced what engineers already knew about timber-framing, and would give homeowners some confidence.

The research could be applied only to that specific type of building. Other timber-framed buildings would still need to undergo the usual assessments.

Registered Master Builders Federation chief executive Warwick Quinn was not surprised by the research. “Timber has been used in our housing, basically forever. Ninety-odd per cent of all homes are constructed of timber and we’ve always known that it performs well in an earthquake because it’s quite ductile – it can rack around and move without breaking.”

It was great to see the research done in school buildings in particular, and for parents that would be heartening knowledge, he said.

Certified Builders Association chief executive Grant Florence said the industry had always known that timber-framed buildings were strong.

“A stick of timber is a stick of timber, but when it’s put together with bracing calculations and engineering calculations, that’s what gives it additional strength. But there might be some learnings from this that the industry can pick up and use.”


Two trucks using cables connected to the building pulled in opposite directions.

The test was designed to put the building under the same kind of stress it might experience during an earthquake, and it was expected to splinter when 40 to 50 kilonewtons of force – equivalent to that experienced in the Christchurch quakes – was applied.

But it stood up, so engineers put further pressure on it and it wasn’t until the force was three times that of the Christchurch earthquakes that significant damage started to show.

Further testing showed that even when 242kN of force was applied, there were no signs of imminent collapse, although there was extensive damage.

However, the testing did not simulate the vertical accelerations of the Christchurch earthquake, or other factors that might affect it, such as soil conditions.


22 October 2013