We have seen that though ductile approach strives to tackle the effects of the earthquake, it had various shortcomings as discussed before. Base isolation is a passive control system; meaning thereby that it does not require any external force or energy for its activation. It is necessary to understand why base isolation is needed to enhance performance levels of the structure subjected to seismic excitations. To design structure in such a way, that it may withstand the actual force by fixed base structure elastically, is not feasible in two senses. First, the construction cost of the structure will be highly uneconomical. Second, if the overall strength of the structure is increased by making it more rigid, then it will be at the expense of imparting actual ground forces to the structural contents, thus causing heavy non-structural damage.

Apparently, as the name implies base isolation tries to decouple the structure from the damaging effects of ground motion in the event of an earthquake. Base isolation is not about complete isolation of the structure from the ground, as with magnetic levitation, which may be very rarely practical. Most of the base isolation systems that have been developed over the years provide only ‘partial’ isolation. ‘Partial’ in the sense that much of the force transmitted, and the consequent responsive motions are only reduced by providing flexibility and energy dissipation mechanisms with the addition of base isolation devices to the structure.

Base isolation, as a strategy to protect structure from earthquake, revolves around a few basic elements of understanding:

1. Period-shifting of structure: Base isolator is a more flexible device compared to the flexibility of the structure. Thus, coupling both an isolator and the superstructure together increases the flexibility of the total isolated structural system. In this way, this technique lengthens the structures natural time period away from the predominant frequency of the ground motions, thus evading disastrous responses caused due to resonance.
2. Mode of vibration: The fundamental mode of vibration (first mode shape) is altered from continuous cantilever type structure to an almost rigid superstructure with deformations concentrated at the isolation level.
3. Damping and cutting of load transmission path: A damper or energy dissipater is used to absorb the energy of the force to reduce the relative deflection of the structure with respect to the ground.
4. Minimum rigidity: It provides minimum rigidity to low level service loads such as wind or minor earthquake loads.