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PRACTICAL CONSEQUENCES OF THE SITE NATURAL PERIOD EVALUATION IN NONLINEAR REGIME
Abstract
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 value range corresponding to strong earthquakes. In the range of MGR=7- 7.5, which are considered as destructive magnitudes, with assigned PGA values of approximately ~0.1 to 0.3g and dangerous natural period range between 1.22 up to ~1.65s for INCERC site, the recommendation is to avoid buildings with these resonance characteristics. But for a large majority of sites only seismic recording of low and moderate events are available. The evaluation of the fundamental period for strong earthquakes using only seismic data implies an extrapolation procedure with inherent large errors. The resonant column determination by interpolation of the nonlinear variations in normalized form together with the determination of the normalization value T0 from seismic recording can lead to a better approximation of the natural periods for large PGA values. The paper shows ways of obtaining the site nonlinear natural period from in situ measurements, T0, and from resonant column data, Tn(PGA). With the help of normalized natural period functions Tn = Tn(PGA) we can predict natural period values of sites for strong earthquakes, starting from small and moderate seismic events, which gives us a good possibility in mitigation of seismic risk for future buildings.
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References11
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