Building Design to Enhance Societal Resilience

Masayuki Kohiyama (Professor, Department of System Design Engineering)

Many people gather in cities to engage in social, economic, and cultural activities. The buildings that make up a city provide a safe and comfortable indoor environment that shelters us from rain and wind. Each building type serves as a venue for specific activities: residences for living, hospitals for medical care, school buildings for education, and office buildings for business. At the same time, they are also important elements that shape the urban landscape. These buildings that support our activities sometimes encounter major external forces and disturbances. Various phenomena such as fires, floods, strong winds, heavy snow, earthquakes, and tsunamis can affect them, but they must not be frequently damaged and rendered unusable, nor should they collapse and harm the people inside. So, what kind of design should we implement to ensure our human activities can be sustained?

For phenomena that cause buildings to shake, such as earthquakes and wind, designs are implemented that focus on the building's vibration characteristics. Buildings have a period at which they are prone to swaying, called the "natural period," and if an external force or disturbance with the same period is applied, a "resonance phenomenon" occurs, causing the building to shake violently. A "base-isolated building" is a structure designed to be less susceptible to shaking by installing seismic isolation devices—such as members made of laminated layers of rubber and steel plates—in the foundation. This lengthens the building's natural period beyond the period range where earthquake waves carry strong energy. The two major factors that cause building damage are "deformation" (displacement) and "acceleration." In a base-isolated building, the deformation of the layer containing the seismic isolation devices (the seismic isolation layer) is somewhat large, but the displacement and acceleration of the floors above it are significantly reduced. This minimizes damage such as cracks in walls due to deformation and harm from ceiling collapses or overturned furniture due to acceleration. On the other hand, different methods are used in skyscrapers. A common method is to install devices that dissipate vibration energy, such as oil dampers, on each floor to suppress shaking during resonance. Another frequently used method is to install a large pendulum, a "tuned mass damper," at the top of the building. This modifies the vibration shape, or "natural mode," during resonance so that the pendulum sways significantly while the building itself sways very little, thereby suppressing the deformation and acceleration of the main structure. Furthermore, there are also "active mass dampers," which actively move the pendulum's weight to control the building's sway with the resulting reaction force. These methods are collectively known as "vibration control." "Base isolation" and "vibration control" are design approaches that differ from "earthquake resistance," which suppresses deformation with strong walls and braces. Their major advantage is the ability to reduce acceleration more effectively than earthquake-resistant designs.

Sosokan (Fig. 1) on the Yagami Campus is a base-isolated building, but it is also a vibration-controlled building that incorporates a semi-active seismic isolation system based on an idea by the late Vice-President Kazuo Yoshida. This building is equipped with semi-active oil dampers (Fig. 2) in its seismic isolation layer, which have four switchable levels of damping performance, allowing the building's sway to be controlled. The Kohiyama Laboratory conducts seismic observations of Sosokan (Fig. 3) and has analyzed its vibration characteristics. Then, in March 2017, in collaboration with the Masaki Takahashi Laboratory, also in the Department of System Design Engineering, and Obayashi Corporation, which designed Sosokan, we updated the parameters for controlling the devices to improve control performance. The design of these parameters involves an optimization that considers the probability distribution of the intensity of shaking from earthquakes the building may encounter in the future.

To create a "resilient" society that is less prone to disasters and can recover quickly when they do occur, building design continues to evolve. As mentioned at the beginning, buildings make up a city, each with its own expected function and role, such as supporting life, medical care, education, and business. A new way of thinking about design is beginning to be explored. Instead of designing a building in isolation, this approach considers the community as a whole. It takes into account factors such as the acceptable probability and duration of a building's functional downtime during a disaster, the importance of its function, its impact on the region, and the availability of other buildings with alternative functions. To enhance the "social aspect of design," the Kohiyama Laboratory is working to develop design methods that include the evaluation of the post-disaster recovery and reconstruction process for the community.