The performance of base-isolation devices in mitigating inertia forces due to intense earthquakes strongly depends on the proper calibration of the isolator own frequency, that should be carefully dimensioned taking into account both the dynamical characteristics of the superstructure and the frequency content of the expected disturbance. Generally speaking, the isolator should dissipate energy at frequencies dynamically interacting with the structure and transmit only energy acting in a frequency range that poorly excites the structure.

Anyway one should emphasize that a first isolator is always present in all buildings, consisting of the surface layers of the ground on which the building is founded.

Actually it is important to take into account, in the design stage of the isolator, the interaction effects between the structure itself and the soil characterizing the site, since this behaves like a filter as regards to the incoming seismic excitation, mainly affecting its frequency composition and, definitively, its overall dynamic character.

Under this perspective, the influence of the sub-soil mechanical properties on the performance of base-isolation systems is investigated in the paper. The earthquake disturbance is assumed to be modelled as a coloured weakly stationary stochastic process, whose average spectral density is assumed to be a Kanai-Tajimi curve. The shaping parameters of this spectrum can be related, as well known, to the macro- character of the surface ground layers. The energy transmitted to the superstructure, compared to the energy filtered by the isolator, can be measured by the cross/auto correlation functions of the response degrees of freedom, that are presented in closed-form, allowing to prevent gross errors in the energy evaluation, calculated by integration of the spectral density functions.

On this basis, one approaches the problem of properly designing a seismic isolation device with reference both to the structure dynamic characters and to the ground input expected macro-properties, which can be deducted by the availability of data relevant to the subsoil filtering capacity.

A procedure for optimal design of the isolator is set up in the frequency domain, based on evaluation of energetic quantities, resulting in an effective and flexible approach.

1

Augusti, G., Baratta, A., Casciati, F., "Probabilistic methods in structural engineering", Chapman Hall ed., London, 1984.

2

Baratta, A., "Plastic Deformations in Structures under Random Dynamic Loading", Int. Conf. on Applications of Statistics and Probability in Soil and Structural Engineering, 1983.

3

Baratta, A., Cennamo, C., Voiello, G., "Design of Linear Active Control Algorithms for non-Stationary Unknown Forcing Function", J. on Structural Control. 2, 7-31, 1998. doi:10.1002/stc.4300050202

4

Baratta, A., Corbi, I., "Interaction of Base Isolation Design and Soil Properties" Proc. 7th Intl. Seminar on Seismic Isolation, Passive Energy Dissipation and Active Control of Vibrations of Structures, Assisi, Italy, 2001.

5

Baratta, A., Corbi, I., "Soil-Based Design of Structural Isolation Systems", Proc. 7th Intl. Seminar on Seismic Isolation, Passive Energy Dissipation and Active Control of Vibrations of Structures, Assisi, Italy, 2001.

6

Izumi, M., "Base Isolation and Passive Seismic Response Control", Proc. 9th World Conference on Earthquake Engineering, Tokio, Japan, 1988.

7

Lin, Y.K., "Probabilistic Theory of Structural Dynamics", Mcgraw-Hill Book Company, New York, U.S.A., 1967.

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