How soil affects structural response during earthquakes?

Earthquake-driven resonance phenomena inevitably cause calamities, and it is important to consider natural vibration periods of structures in seismic design. This study shows the effects of soil on the natural period of structures and suggests a way to reflect those effects in the seismic design.

Article | Spring 2022

Earthquakes cause loss of human lives and huge damage to infrastructure, leading to both immediate and long-term impacts on society. As infrastructure such as bridges, ports, and pipelines are important to support the functioning of society, the stability of these infrastructures is paramount after big earthquake events. One crucial point of seismic design is to avoid resonance occurring in structures during earthquakes. Resonance is the amplification of vibration that occurs when the period of an earthquake is close to the natural period of a structure. This implies that not only strong but weak earthquakes can cause resonance in and collapse of structures. Accordingly, investigation of the natural periods of structures is the key to for seismic design to avoid resonance and prevent sudden collapse of structures.

In conventional seismic design, the natural period of a structure is evaluated without considering the effect of soil. However, most of infrastructure is constructed on soil. Earthquakes can readily soften soils, thereby significantly altering the natural period of the structure from that used for the conventional design; this effect is referred to as soil-foundation-structure interaction (SFSI). The main objective of this study is to evaluate the natural periods of structures by considering SFSI during strong earthquakes (Figure 1).

 

Dr. Kil-Wan Ko of the KAIST Geo-centrifuge Center, in collaboration with Professor Dong-Soo Kim, has performed a series of earthquake physical modeling experiments by simulating structures and soil conditions. In the area of earthquake engineering, geo-centrifuge testing using an earthquake simulator has been widely implemented as an effective physical modeling tool to replicate soil dynamic field conditions (Figure 2).

 

The results show that the natural periods of structures are not fixed values but increase with seismic intensity because soil stiffness degrades during an earthquake, in what is known as the period lengthening phenomenon (Figure 3). For the first time, this research group has suggested a coefficient that can be used for seismic design practice to reflect period lengthening. This study contributes to the enhanced seismic resilience of infrastructure against earthquakes. The results were published in prestigious journals relevant to geotechnical earthquake engineering.

 

Reference
Ko, K. W., Ha, J. G., & Kim, D. S. (2020). Structural inertial interaction effects on foundation behavior. Soil Dynamics and Earthquake Engineering, 136, 106238.
Ko, K. W., Ha, J. G., Park, H. J., & Kim, D. S. (2021). Investigation of period-lengthening ratio for single-degree-of-freedom structures using dynamic centrifuge test. Journal of Earthquake Engineering, 25(7), 1358-1380.