Seismic isolation systems are increasingly used to mitigate damage to building structures during earthquakes. An isolation system isolates the building from the ground and the foundation by installing an isolation layer between them. During an earthquake, the ground and foundation may shake, but the isolation layer deforms and reduces vibration transmitted to the building. Slow movement during an earthquake is a characteristic of buildings equipped with an isolation system. However, conventional isolation layers are prone to undergo excessive deformation during massive subduction zone or inland earthquakes. An objective of my research is to develop a method for reducing this excessive deformation in isolation layers.
An isolation layer of a seismic isolated building consists of isolators, composed of layers of flexible rubber and steel plates, and dampers. During an earthquake, the vibration of the ground and foundation is transmitted to the isolator. The rubber layers deform and reduce the earthquake force transmitted to the building. However, the isolator only smoothens the vibration and does not stop the movement. Like a swing continues swinging once pushed, the isolator continues shaking. This is where the damper comes in. The damper absorbs the seismic energy and stops the building from moving. These dampers are the main target of my research. Excessive deformation of the isolation layer during a massive earthquake may cause the layer to collide against retaining walls if there is not sufficient clearance between the building and the retaining walls. It is a phenomenon that cannot be overlooked especially in seismic-isolated buildings which are expected to be highly earthquake resistant. It is imperative to reduce the response displacement (deformation of the isolation layer) in newly constructed buildings as well as in those with insufficient clearance around the isolation layers. Either passive dampers may be used to effectively restrict the response displacement of the isolation layer or the damping force could be increased, but these solutions increase the response acceleration during a medium to small size earthquake and lower the performance of the seismic isolation structure.
In our laboratory, we are proposing an on-off damper that can change the damping force depending on the response displacement and response speed (Fig. 1). The on-off damper has a loose hole at the joint. Only when the response displacement exceeds a certain level, the joint collide against the edge of the loose hole and pushes or pulls the cylinder to attenuate the earthquake force. The design is intended to simply modify the shape of the pin support of existing oil dampers and requires only slight modification of existing attenuators. We will investigate the efficacy and the effects of using the proposed attenuator on the responses of the superstructure when it receives a seismic motion that causes the isolation layer to collide with the retaining walls. A parametric analysis involving numerical calculation will be first conducted to determine the optimum experimental model. Structural analysis of a seismic-isolated building featuring an on-off damper requires a new dedicated numerical calculation program to be written because such analysis cannot be achieved by common general-purpose software.
Isolation layers are most widely installed at the undermost layer, i.e., the foundation, of a building. However, this type of base isolation has several disadvantages such as the need to include clearances to the retaining walls, which means a reduction in the area the building can occupy within the lot, and the need to dig deep into the ground. Mid-story isolation systems, which involve installing isolation layers at a level other than the lowest layer, are seeing increased use to avoid these drawbacks. A mid-story isolated building requires a design that accounts for the dynamic properties of the floors below the isolation layer as they have a great impact on the response characteristics of the entire building.
However, attempts to prepare an experimental model of an entire building face various problems such as the large size of the model, high preparation costs, and the model weight exceeding the weight limit of the shake table. We are thus planning real-time hybrid experiments which involve testing a model of the upper part of the building (including the isolation layer) on a shake table and analyzing the behavior of the lower part by using a simulation model (Fig. 2). The interactions of forces between the upper and lower parts will be taken into consideration. The response acceleration at the topmost layer of the lower part will be given as the input acceleration force of the shake table when testing the upper part. At the same time, the damping force of the damper of the isolation layer in the upper part will be input to the simulation model of the lower part. In such a manner, shake table testing and time history response analysis can be performed simultaneously.
I have worked on “SSI (Soil-Structure Interaction)” or interactions between the vibration of a building and the vibration of the ground since I was an undergraduate student. I have performed structural analysis by modeling the superstructure and the substructure including the ground separately and doing separate calculations using the models when given the forces acting between them. This method of structural analysis is useful also for our future real-time hybrid experiments. Development of a damper and shake table testing are a new experience for me, and I’m working hard. Anyone who is interested in creating safety and security for the future is welcome to visit our laboratory.