Seismic performance of wooden structures

The magnitude 8 earthquake that struck Sichuan province in China on May 12, 2008 has so far caused about 40,000 deaths and millions of people have been made homeless. This is the strongest and most destructive earthquake since the founding of the People's Republic of China. The earthquake caused a large number of buildings to collapse and destroyed. Thousands of people were buried. Because of the disruption of traffic and communication and the lack of manpower, many difficulties were encountered in the rescue work.

In the Ludian County earthquake in Yunnan, all the old buildings in the Longtoushan Old Street in the epicenter were almost completely destroyed. Only the three-storey wooden building of the Longtoushan Supply and Marketing Cooperative, built in 1949, stood upright and suffered only a few “skin injuries.” The two children sleeping on the ground floor escaped the disaster.

As we all know, earthquakes are a common natural phenomenon. Earthquakes in the world each year have to reach more than a million earthquakes, but most of them are relatively minor and people are not aware of them. Most earthquakes are caused by crushing, breaking or sliding of the crust. So most of the seismic activity is concentrated on the edge between the plate and the plate. Due to the constant movement between the plates, stress and deformation occur. When the stress is greater than the strength of the crust, a sudden sliding force is generated, energy is released, and a seismic wave is formed on the surface. Where the energy is released is called the source, and the surface point directly above the source is called the epicenter.

Due to the process of earthquake generation and its complexity, and the occurrence of strong earthquakes is a very small probability event, earthquake prediction is a worldwide problem. So far no country in the world can successfully predict the occurrence of earthquakes. In order to prevent the sudden destruction of earthquakes to human life, in some earthquake-prone countries and regions, a kind of architectural form with excellent anti-seismic performance—modern wooden architecture system is often used.

Recent seismic studies around the world have shown that modern wooden structures are better than any type of building in their seismic resistance. Wood-framed houses provide the most basic seismic elements, and past seismic experience has been used to improve and practice seismic materials. The reason why wood construction has such excellent seismic performance is mainly due to the following points:

- Modern wooden structures use nailed joints, so when sudden loads occur or earthquakes fall, their structural system is very flexible, so it can absorb and dissipate energy in earthquakes.
- Wood has a high strength-to-weight ratio, and thus wood construction is lighter than other types of building structures. This is a major advantage in earthquakes.
- Typical wood-framed buildings have panels and finishes bound to a large number of joists and studs. This provides additional force transmission paths for seismic forces. Once a connection point is overloaded once, other overloaded parts can be shared by adjacent connection points.
In addition, wood structures can effectively reduce human casualties in the event of an earthquake and facilitate the escape of people living in it. Their light weight characteristics also facilitate the subsequent rescue work. Many factors make the structural performance of wood frame structures perform well in seismic destructive forces, which also makes it possible to even withstand the side loads caused by hurricanes, tornadoes and typhoons.
In North America, the design standards for wood structures have also been updated through field surveys and studies of the impact of earthquakes on wood buildings. Countries (and regions) with high-frequency seismic hazards, including: Taiwan, Japan, China, and Turkey, are developing and improving building standards to allow the construction of more wooden structures.

Let us look again at the Kobe earthquake that occurred in the south of Hyogo Prefecture in Japan on January 17, 1995. The earthquake was one of the largest earthquakes in recent times that caused damage. It is estimated that the loss of property was more than 100 billion U.S. dollars and nearly 6,000 residents were killed. Since earthquakes of this magnitude have never been expected in the region, local architectural design and construction are neglected. Various forms of buildings such as highways, elevated roads, bridges, ports, public service facilities, railways and various concrete buildings have been severely damaged and collapsed. However, many new buildings have survived the disaster safely. These buildings include high-rise buildings, most of which are steel structures and some modern wooden structures. What is worth mentioning is that the two demonstration projects of wood structure at the earthquake center, "Seattle-Vancouver Village" and "Nishinomiya Hemisphere Residential Park", did not have any visible damage except some tiles damaged during the earthquake.

The earthquake once again showed us the devastating forces caused by the earthquake hitting many areas of the population. All weaknesses in design, construction and maintenance cannot be realized at this moment. The wooden structure building is a time-tested building system, which can save the loss of life in the most earthquake-prone areas.