Engineering and Construction

It is generally recognized that single-family wood frame structures that include shear walls in their construction are very resistant to collapse from earthquake shaking. This is due to several factors, such as their flexibility, strength, and light dead loads, which produce low earthquake-induced inertia loads. These factors make the wood-frame construction much better at resisting shear forces and hence more resistant to collapse.

There are exceptions to the general rule that wood frame structures are resistant to collapse. For example, in the 1995 Kobe earthquake, the vast majority of deaths were due to the collapse of one- and two-story residential and commercial wood-frame structures. More than 200,000 houses, about 10 percent of all houses in the Hyogo prefecture, were damaged, including over 80,000 collapsed houses, 70,000 severely damaged, and 7000 consumed by fire. The collapse of the houses has been attributed to several factors, such as (EQE Summary Report, 1995):

• Age-related deterioration, such as wood rot, that weakened structural members.
• Post and beam construction that often included open first floor areas (i.e., a soft first floor), with few interior partitions that were able to resist lateral earthquake loads.
• Weak connections between the walls and the foundation.
• Inadequate foundations that often consisted of stones or concrete blocks.
• Poor soil conditions consisting of thick deposits of soft or liquefiable soil that settled during the earthquake. Because of the inadequate foundations, the wood-frame structures were unable to accommodate the settlement.
• Inertia loads from heavy roofs that exceeded the lateral earthquake load-resisting capacity of the supporting walls. The heavy roofs were created by using thick mud or heavy tile and were used to resist the winds from typhoons. However, when the heavy roofs collapsed during the earthquake, they crushed the underlying structure.