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International experience provides pointers for rebuilding Christchurch

22 March 2011

The experience of rebuilding the Italian town of L’Aquila after its devastating earthquake in 2009 showed people who have been through such disasters have a preference for timber, according to Italian engineers.

Engineer Paolo Lavisci from the Society of Engineers in Italy said the people in L’Aquila “were asking for no more concrete” immediately after the earthquake. The substantial wooden buildings erected as a result have met with people’s “full satisfaction”.

“This has projected a very good image of timber construction everywhere in Italy,” Mr Lavisci says.

Since then, the use of wood in construction has taken off in Italy. “Now we have completed designs for six and eight storeys and are designing for 12 storeys for large ‘traditional’ building companies who just a year ago would never even commission a timber house.”

The shallow 5.8 magnitude earthquake killed 308 people, destroyed up to 11,000 buildings and left 65,000 of the medieval town’s population of around 100,000 homeless.

Mr Lavisci’s company won a tender, along with several other contractors, to build a number of three storey, 27-apartment blocks using timber. Their construction took only 72 days – only 14 days for the erection of the watertight outer shell.

Made from cross-laminated timber sections, the completed block weighed around 430 tonnes compared with what would have been more than 2,000 tonnes in concrete.

The lower mass (weight) of a building, can lessen the damage sustained from earthquakes.

Dr Geoff Thomas from Victoria University’s School of Architecture says earthquake loads or forces on a building are also proportional to its weight.

“A timber building, typically lighter than a brick, concrete or steel building, therefore induces lower earthquake loads.”

Being inherently lighter and more flexible, timber buildings need not be as strong as a more rigid structure in order to resist the same level of earthquake shaking.

“In the event of a collapse, survival is much more likely in timber buildings as the lower weight of any falling debris from the structure is less likely to cause serious injury than that of heavier materials,” Dr Thomas says.

“Understanding these facts has resulted in better building design which saved many lives in the Christchurch and other earthquakes around the world.”

Following the Kobe earthquake in 1995 which killed 6,300 people, Makoto Watabe, chairman of the Earthquake Disaster Committee of the Architecture Institute of Japan did a comparison of mortality rates from people killed in wooden or concrete structures.

Because of the pre-dawn timing of the Kobe quake, most people were in their own homes at the time. More than 80,000 houses – almost 10 percent of all homes in the area – either collapsed totally or were substantially destroyed.

Mr Watabe said that in Kobe, while the number of wooden house collapses was very high largely due to their light frames and very heavy tile roofs designed to resist typhoons, they killed relatively fewer people than the reinforced concrete buildings which collapsed. Approximately one person was killed for every 16 collapsed wooden houses.

“Assuming an average of three people lived in each house, that is about a two percent mortality rate,” he said.

"In the case of reinforced concrete construction, while there weren't as many collapses, that percentage went up to 15 percent - a big difference."

In short, while in Kobe there was less chance of a reinforced concrete building collapsing than their poorly designed houses, if you were in one that did, you had a greater chance of being killed.

Dr Thomas said that timber buildings of six stories or more are now being designed and built to resist earthquake loads.

“Such buildings are common in earthquake prone regions such as Vancouver, Canada and Seattle.

“A full size seven storey cross-laminated timber building survived testing at levels of up to 0.8 times the force of gravity, on the E-Defense earthquake shaking table in Japan in 2007.”

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