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There are over a million earthquakes annually but most are too small to be felt. Although earthquakes can occur anywhere, there are certain locations where the likelihood of strong earthquakes is particularly high. Around the world, earthquakes claim many lives each year – many from damage to buildings. There have been relatively few deaths in recent North American earthquakes. This can be attributed to North American building practices, including the widespread use of wood framing for housing.

The 1964 earthquake in Prince William Sound, Alaska was one of the most powerful earthquakes ever recorded in North America. Considering the magnitude of the earthquake relatively few lives were lost. Measuring 8.4 on the Richter scale, the earthquake claimed only 131 lives and 122 of these resulted from the tidal waves caused by the earthquake. By Contrast, 15,000 people were killed in the 1999 earthquake in Turkey measured 7.4 on the Richter scale.

The Geographical Institute of the University of Alaska explains the relatively low losses in the 1964 Alaska Earthquake as follows:

“The number of deaths from the earthquake totaled 131; 115 in Alaska and 16 in Oregon and California. The death toll was extremely small for a quake of this magnitude due to low population density, the time of day, the fact that it was holiday, and the type of material used to construct many buildings (wood).”

In California, there are over 400 million square feet of public schools and 80% of this area is wood-frame construction. An assessment of the damage to school buildings in the 1994 Northridge Earthquake was summarized as follows:

“Considering the sheer number of schools affected by the earthquake, it is reasonable to conclude that, for the most part, these facilities did very well. Most of the very widespread damage that caused school closure was either nonstructural, or structural but repairable and not life-threatening. This type of good performance is generally expected because much of the school construction is of low-rise wood-frame design, which is very resistant to damage regardless of the date of construction.”

In 2002, the State of California Department of Government Services (DGS) completed a legislated inventory and earthquake worthiness assessment of schools. School buildings that were constructed of steel, concrete, reinforced masonry or mixed systems, designed between 1933 and July 1, 1979 were required to be evaluated. Older wood-frame schools were exempted from the assessment on the basis that, “Wood-frame buildings are known to perform well in earthquakes.”

These endorsements of the ability of wood-frame construction to perform well in the face of earthquakes are based on several researched and documented wood building system characteristics.

1. The attachment of sheathing and finishes to the numerous wood joists and studs in a typical wood-frame house provides redundant load paths for the earthquake forces. There are numerous small connections. If one connection is overloaded, its share can be picked up by adjacent connections.

2.Wood has high strength to weight ratio and therefore wood buildings tend to be lighter than other building types. Lightness is an advantage in an earthquake.

3. The nailed wood connections in wood-frame systems allow the building to flex thereby absorbing an dissipating energy during an earthquake.

4. In engineered wood-frame buildings, structural panels (plywood or OSB) acting in combination with studs and joists, create shearwalls and diaphragms – very effective lateral force resisting building assemblies.

This Building Performance bulletin is intended to improve the understanding of earthquakes and their effects on wood-frame buildings. Except for a few exceptional cases, hundreds of thousands of wood-frame buildings have provided protection for their occupants when exposed the devastating effects of severe earthquakes. The traditional North American wood-frame house provides the fundamental elements for seismic resistance and wood-frame building practices are continually evolving. New wood-based materials have been introduced, building research has provided better details and lessons learned in past earthquakes are being used to build better houses.

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