Methods of Determination of Predominant Vibration Periods for Non-homogeneous Multilayer Ground Sites

The spectrum of periods of any complicated oscillatory process, such as the oscillatory activity of soils during large earthquakes, is its most important feature. During strong earthquakes, the values of the periods of vibration of soils, ground buildings and structures play an important role in analysing the behaviour of ground structures and assuring their safety. Seismic impact is a term used to describe dynamic impacts in which the most significant impact is caused by resonance events, such as the coincidence or closeness of ground vibration and free vibration periods of a ground structure. This chapter is devoted to a thorough examination of methods for determining the exact, simplified, and approximate values of the predominant periods in simple homogeneous to complicated multi-layer subsoils using theoretical and experimental methods. The values of the durations of a variety of real-world subsurface examples are given. The values of the periods (including the periods of the strongest modes of vibration) are estimated theoretically from microseismic vibrations of soils and accelerograms of the soil during a genuine earthquake and compared. According to the findings, during earthquakes, soils vibrate at identical frequencies for similar times. With the values of the periods of soil vibrations, it is feasible to make an informed decision on which buildings and structures to construct in order to ensure their seismic resistance on such soils. The most crucial goal of seismic microzoning and earthquake-resistant construction is to achieve this. It is proposed to use the values of the main prevailing period as an integral characteristic of complex soil conditions for establishing their categories according to seismic properties, because it is shown in the work that its formation is influenced by all physical, mechanical, and geometric characteristics of all layers. The values of the coefficients of soil conditions and spectra of reactions, depending on the categories of soils (periods), as well as their distance from active faults and the magnitude of the earthquake, are examined using data from normative documents from various countries on earthquake-resistant construction. Finally, the book underlines that the first approximation of the seismic impact mechanism is as follows: Ground motions with prominent periods equal to the periods of free oscillations of the near-surface thickness are generated by seismic waves propagating from the seismic focus, which in turn induce fresh vibrations with predominant periods corresponding to the free oscillation periods of the structures. This means that seismic waves are “filtered” twice during an earthquake: first in the near-surface stratum, then in the structure itself.

Author(S) Details

E. Y. Khachiyan
National University of Architecture and Construction of Armenia, Yerevan, Armenia and Institute of Geological Sciences National Academy of Sciences, Armenia.

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