Predominant Period

The paper presents new results from a detailed study, involving an analysis of several deterministic and stochastic indices computed for over 100 tri-axial accelerograms recorded during four strong Vrancea seismic events with moment magnitude, Mw, larger than 6. The study aims to better reveal the spectral characteristics of the analyzed ground motions. Various analytical estimators of the predominant period, such as those based on power spectral density, Fourier amplitude spectrum, response spectra etc. are computed and compared. Additionally, a number of bandwidth measures are used in the assessment. The variability of the analyzed period-type parameters from one event to another is pointed out, as well as, when relevant, their spatial variability. Observations are correlated with available information on local site conditions, geology etc., in order to investigate the influence of different factors on the frequency content of the analyzed motions.

The paper presents results from a recent study in progress, involving an extensive analysis, based on several deterministic and stochastic indices, of the frequency content of ground motions recorded during strong Vrancea seismic events. The study, continuing those initiated by Lungu et al. in the early nineties, aims to better reveal the characteristics of the analyzed ground motions. Over 300 accelerograms, recorded during the strong Vrancea seismic events mentioned above and recently re-digitized, are used in the study. Various analytical estimators of the frequency content, such as those based on Fourier spectra, power spectral density, response spectra and peak ground motion values are evaluated and compared. The results are correlated and validated by using the information provided by various spectral bandwidth measures, as the Vanmarcke and the Cartwright and Longuet-Higgins indices. The capacity of the analyzed estimators to describe the frequency content of the analyzed ground motions is assessed comparatively and the most relevant parameters are being identified.

Building design codes typically specify seismic forces based on a 5-percent damped elastic design spectrum. To consider the case of structures with damping ratios other than 5%, damping correction factors are used. While these factors are determined for elastic systems, several studies have shown that the effects of damping are different for systems with nonlinear behavior. A study concerning the influence of damping on the nonlinear seismic response of building structures is presented. To separate the influence of nonlinear behavior, spectra of the ratio between the seismic strength demands corresponding to nonlinear and linear behavior, respectively, were computed, for damping ratios ranging between 2% and 10% and various constant ductility values. A database of ground motion records from the three strongest earthquakes that affected Romania during the last four decades was used in the study. The records were grouped into two sets, according to their frequency bandwidth. The spectral values computed for each set were processed statistically and results were expressed with reference to the 5%damped spectra. It was found that, for the analyzed damping range, the influence of this parameter on the nonlinear response is weak to moderate, for both considered types of frequency content.