抄録
To reduce building damage due to earthquakes, base-isolation systems have been installed to many buildings in Japan. Rubber bearings efficiently convert the seismic input to large displacement, so clearance is necessary between the building and the retaining wall. Recently, long-period ground motions have been observed, and there are concerns about base-isolated buildings being shaken horizontally more than expected, especially after the 2011 Off the Pacific Coast of Tohoku earthquake. If the displacement is larger than the design, the building may collide with the retaining wall. After the collision, we may have severe acceleration due to the collision impact so it is necessary to control the horizontal displacement of buildings within the designed clearance. In this study, we propose a device to control the displacement of base-isolated buildings as a fault-tolerant system.
In order to control the horizontal displacement, we focus on the law of energy conservation. When something moves, the equation “Ka+Ua=Kb+Ub” holds for two different states, a and b. Here K means kinetic energy and U means potential energy, respectively. From this relationship, we can find that if we can make Ub larger, Kb will become less. If a building is shaking horizontally, the horizontal kinetic energy that the building has will be reduced as the building uplifts and then the shear deformation that the rubber bearings need to absorb will become lessened. To make Ub larger, we use a device with a shape of a prolate spheroid (like a rugby ball) and make it roll when the horizontal displacement becomes larger than the predefined threshold. In order to make the device free from the whole structure during the normal state, we make a small dent on the top of the device and we make a cavity on the bottom to fit with the notch on the surface to put the device on. The device is designed to work as follows:
(a) When the shaking is small and under the expected displacement, the system does not have any effect to the base-isolation system.
(b) In the case of a huge earthquake and the building is subjected to large shaking exceeding the limit of the clearance, the building will touch the device and the device will start rolling.
(c) While the device rolls, it pushes up the building and converts the horizontal displacement to the vertical displacement. Then the weight of the building will work as restoring force, and finally the device returns to the initial position. These movement will work as a fault-tolerant system.
We used steel frames and rubber bearings to model as a base-isolated building. The specimen has two layers with rubber bearings to make it possible to get a large relative displacement and we put our device for the fault-tolerant system to push up the upper frame. We did static and dynamic tests using the shaking-table to check the ability of this system.
By using the shaking-table test data, we found that the second frame was lifted up as designed. Comparing two cases with and without our system, we found that our system worked as a fault-tolerant system and decrease the second-frame's horizontal displacement. We succeeded in decreasing the horizontal displacement with a rolling system and its working efficiency was around 50-60%. When the building started pushing the device, vertical acceleration was observed. Improvements that we have to further investigate would be to decrease the vertical acceleration in the building when it touches the ball and to increase the efficiency of this system. The latter could be achieved by using a mechanism to prevent friction loss between the surfaces of the device and the building.
