A base isolation unit or a base isolation apparatus is installed between a base and an object to be isolated from vibration for isolating the object from the base when an earthquake occurs. Resultant effects are prevention of transmission of seismic vibration from the base to the object to be isolated, and mitigation of an acceleration applied to the object when an earthquake occurs. Specifically, a base isolation apparatus including a supporting apparatus and a damping apparatus is mounted between the base and the object to be isolated (see Patent Document 1, for example). Patent Document 1 suggests applicability of a spring apparatus to the base isolation apparatus.
Conventional base isolation apparatuses intended to function only when an earthquake occurs may also function when an object to be isolated is subjected to an external force due to wind for example.
In order to inhibit the base isolating operation in conditions other than earthquakes, there is a need for a base isolation apparatus that can fix the base and the object to be isolated with high stiffness in conditions other than earthquakes. If the spring apparatus disclosed in Patent Document 1 for example is applied to the base isolation apparatus, the spring apparatus works as an ordinary spring, and does not have a triggering function causing the base isolation apparatus to work only when an earthquake occurs.
In order to satisfy the aforementioned need, attempts have been made to perform the triggering function using elastic stiffness of a plastic damping apparatus used for base isolation, to use a mechanical element broken when a large earthquake occurs to release connection between the base and the object to be isolated, or to apply a mechanism for activating the base isolating function using a seismic wave observation sensor, to thereby release, only when an earthquake occurs, connection made by fixtures between the base and the object to be isolated (see Patent Document 2, for example).