One of the modern seismic protection technologies widely used in new buildings is the seismic isolation, which is achieved by making a structural disconnection, generally above the foundation or basement level, interposing devices between the substructure and superstructure with low horizontal rigidity devices in order to increase the period of the structure to values around 2-3 seconds, within the low-acceleration design spectrum. The behaviour of the upper structure is that of a rigid body stressed by strongly reduced accelerations and subject to very limited interfloor deformations. The rigid body simply moves without rotation.
As a result, the design approach with seismic isolation has the following advantages over fixed base buildings:
As a result, the cost difference existing between isolated and non isolated becomes so little or even nullifies completely as the number of floors of the building increases or in areas where the seismic force is particularly severe.
For modest buildings, like single or double family houses or even one floor buildings, the extra cost involved into isolating the building (buying of extra devices with the necessary acceptance tests, the realization of a rigid plan, or the architectural modifications needed to allow access to the isolators) are mostly likely compensated by the advantages mentioned before. The considerations mentioned so far are limited to evaluate the advantages only in the construction phase of the building.
An important aspect of the seismic isolation strategy to be considered is the high level of security that they can give and the absence of damages after the seismic event : infact the linear requirement of structure elements and the interfloor displacements of isolated buildings are very modest (or even nullified at all), other than saving the integrity of the whole structure which allows the safeguard of the contents of the structure. It is obvious that such advantages are projected in the long term analysis considering also the fact that after a seismic event, structure and its content aren’t needed to be subjected to intervention or maintenance reducing also social damages as in residential forced to leave their homes in case of civil buildings or the interruption of work in offices buildings, not forgetting the psychological impact that these would have caused.
These long term advantages are more evident when seismic insurance will be compulsory in matter of seismic risk. It is infact predictable and desirable that insurance policies are reduced in isolated structures.
Seismic Base Isolation Devices can be divided into two families:
