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The technique named "base isolation", resorts to place at the bottom of buildings special components, leading to long fundamental periods that would avoid resonance tendency of buildings with its site in the case of strong earthquakes. The nonlinear characteristics of the isolator layer change the linear period-shift estimation and can lead either to dangerous shortening shift in case of hardening nonlinearity or to unnecessary shift enlargement in case of softening nonlinearity. By neglecting this period-shift di...

Maximum damage due to strong earthquakes occurs at the resonance between the natural periods of the structure and its site, given by the coincidence (or just close to) of the two periods, in which case the dynamic amplifications can damage, even destroy, the structure. From the recordings of (strong) Vrancea earthquakes one can see a dependence of the site natural periods and peak ground accelerations (PGA) on magnitude. For safe avoidance of the resonance it is necessary to define for each site a natural period v...