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Report – NZWood
9 February 2010
New engineering techniques allowing multi-storey buildings to be constructed of wood can mean buildings surviving major earthquakes with very little damage, according to a New Zealand engineering professor.
Andy Buchanan of Canterbury University says that if the presidential palace in Haiti had been built with a modern structural framework of wood, chances are it would not have collapsed so badly in the recent devastating earthquake.
While the palace is understood to have been built of reinforced concrete, Prof Buchanan says the worst buildings in earthquakes are built with un-reinforced masonry.
Many of those in Port-au-Prince are believed to fall into this category.
New Zealand engineers learned the lesson in the 1855 Wellington earthquake. The old Government Buildings, built in 1876, have proved that wooden buildings can last the distance. Although not put to the test by another earthquake of the 8.2 magnitude quake of 1855, or even one of the size of the Haitian quake, there is a high chance the building would still perform well.
Compared to wood, badly designed concrete buildings behave like un-reinforced masonry buildings. While modern concrete buildings can be excellent, Prof Buchanan says one disadvantage is their heavy weight.
Steel has the advantage of flexibility. Being a very ‘ductile’ material, it can perform very well in a well designed building and in fact, both wooden and concrete buildings rely on steel’s ductility in either their reinforcing or the connections between members.
“You need both strength and flexibility for a building to perform well in an earthquake, but weight is one of the most important factors.” The mass of the building can amplify the shaking.
Because of its weight to strength advantage combined with its natural flexibility, timber buildings can perform just as well as concrete or steel buildings, Prof Buchanan says.
One of the main misconceptions about wooden buildings, however, is their fire risk.
And fire can be a huge concern following an earthquake, if gas mains burst and water is scarce. Often fire trucks are unable to get through rubble filled streets to respond.
But contrary to the widespread belief, well designed heavy timber buildings can perform very well.
“Fire performance is only a disadvantage for wood if the buildings are poorly constructed with inadequate plaster-board protection of light timber framing and a large number of buildings very close to each other.
“But modern, well designed multi-storey timber buildings have more than enough fire resistance,” Prof Buchanan says.
Large wooden beams do not burn easily – they tend to char on the surface only and remain structurally intact (just think of all the tall tree trunks left standing after a devastating forest fire) and can actually perform better than unprotected steel, which can lose its strength very quickly.
“Mostly what burns in a building fire is not the wood or the wood structure, it’s the solid petrol in all the foam and plastic in the finishings.” This is a problem irrespective of whether the building is made from wood, steel or concrete.
In terms of earthquake resistance, in recent years six and seven storey wooden buildings have been tested on the world’s largest shaking table in Japan at simulations of magnitude 7.5 earthquakes and have shown almost no damage.
Leading research in the College of Engineering at the University of Canterbury has led to the formation of a new company to further research innovative timber building designs – the Structural Timber Innovation Company – STIC.
STIC is developing multi-storey timber post-tensioned buildings which incorporate tensioned steel cables together with laminated wood beams, a technique that is also used in some concrete building designs. The steel cables provide extra strength, elasticity and earthquake resistance.
Prof Buchanan says that over the last 30 years the guiding principle for earthquake engineering in New Zealand was ‘capacity design’ where the objective was to allow for small ‘controlled’ failures but to avoid collapse of the building.
The aim in capacity design was that in a small earthquake there would be no damage, in a medium-sized earthquake the damage would be reparable with the structural integrity of the building retained, and for the largest earthquakes the building may suffer non-reparable damage, but there would be no collapse.
“Now and only now are people thinking about being able to resist a large earthquake with no structural damage,” Prof Buchanan says.
“This is the way things are moving in the earthquake engineering world. Steel and concrete construction engineering is working towards this, and our new wood construction techniques are advancing right along with them.”
STIC is right at the leading edge of this research – with the multi-storey timber test buildings acquitting themselves very well in earthquake testing.
“These timber buildings not only have excellent earthquake resistance but the building structure will have no significant damage after the earthquake.”
