Napier reminder that timber houses are "safe as"

3 Feb 2006

The outstanding performance of timber houses was one of the major lessons of the Napier earthquake says Forest Owners Association chief executive David Rhodes.

"Experience in Napier in 1931 and in large earthquakes since have confirmed that properly constructed timber-framed homes are one of the safest places to be when the big one hits."

Timber framing has energy-absorbing properties which mean it can withstand the effects of powerful earthquakes. Structural failure in wooden houses is unusual and deaths are rare, he says.

"The biggest risks to occupants in timber-framed homes are from falling objects and unsecured furniture and appliances. Falling masonry from poorly designed chimneys is also a risk with older houses.

"When structural failure occurs in older buildings it's usually because they are not properly anchored to their foundations or because the ground floor in a multi-storey building has been ‘hollowed out ’ with big picture windows or garaging. Both flaws can be corrected through retrofitting."

The 1964 Alaskan earthquake measured 8.4 on the Richter scale, one of the most powerful ever recorded. Yet very few lives were lost and most of these were in the resulting tsunami.

"The Geophysical Institute of the University of Alaska says this is because most people were at home on holiday and the material used to construct most of their homes was wood."

An official report on the 1994 Northridge quake in California attributes the low level of casualties to the fact that it occurred at 4:31 a.m. when "most people were sleeping in their wood frame single family dwellings, generally considered to be the safest type of building in an earthquake."

Although only 6.7 on the Richter scale, the peak ground accelerations in the Northridge quake were among the highest ever recorded and significantly higher than those specified in the building codes at the time.

Mr Rhodes says similar conclusions have been reached by the engineers who studied the San Fernando quake in California (1971), and quakes at Edgecumbe in the Bay of Plenty (1987) and Kobe in Japan (1995).

"Most of the residential buildings in these study areas were built according to conventional practice and were not specifically engineered to withstand earthquakes. While many commercial buildings collapsed in these quakes, a high percentage of timber-framed houses performed well. Most damage was non-structural and easily repairable."

Timber-framed buildings perform well in earthquakes for several well-documented reasons, including their high strength to weight ratios, Mr Rhodes says.

Typically the sheathing and finishes in timber-framed buildings are attached to numerous joists and studs which share the extreme loads. There are numerous small connections, rather than a few large-capacity connections, so if one of these is overloaded its share can be picked up by others.

Nailed wood connections in timber-frame systems allow the building to flex, thereby absorbing and dissipating energy. Also the structural panelling acts in combination with studs and joists to resist the lateral (sideways) forces which often flatten the un-reinforced masonry buildings typical of many under-developed countries where earthquakes are accompanied by high casualty rates.

Mr Rhodes says traditional New Zealand timber-framed homes have a high level of seismic resistance. In addition, building codes and practices are being constantly improved as engineers apply lessons learned from the earthquakes of the past.

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For more information, please contact: David Rhodes, chief executive, NZFOA.Tel: Mobile 0274 955 525