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

The main characteristic of any complex oscillatory process, which is the oscillatory process of soils during strong earthquakes, is the spectrum of periods of the oscillatory process. The values of the periods of vibration of soils and ground buildings and structures during strong earthquakes play a major role in studying the behavior of ground structures and ensuring its safety. Seismic impact refers to dynamic impacts in which the greatest impact is due to resonance phenomena, i.e. coincidence or closeness of the periods of ground vibration and free vibrations of a ground structure. The content of this chapter is devoted to a comprehensive study of methods for the theoretical and experimental determination of the exact, simplified and approximate values of the prevailing periods from simple homogeneous to complex multi-layer subsoil. The values of the periods of numerous examples of real subsoil are given. Comparative estimates of the values of the periods (including the periods of the highest modes of vibration) are given, calculated theoretically from microseismic vibrations of soils and from accelerograms of the soil during a real earthquake. The work is of the opinion that during earthquakes the soils vibrate with similar values with such periods. Having the values of the periods of soil vibrations, it is possible to reasonably choose which buildings and structures are advisable to build from the point of view of ensuring their seismic resistance on such soils. This is the most important task of seismic microzoning and earthquake-resistant construction. As an integral characteristic of complex soil conditions for establishing their categories according to seismic properties, it is proposed to take the values of the main prevailing period, since it is shown in the work that its formation is influenced by all physical, mechanical and geometric characteristics of all layers. An analysis of the data of normative documents of different countries on earthquake-resistant construction on 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 is given. In conclusion, the text emphasizes that the process of seismic impact in the first approximation occurs as follows:seismic waves propagating from the seismic focus generate ground motions on Earth’s surface with predominant periods equal to the periods of free oscillations of the near-surface thickness, which in turn, cause new vibrations with predominant periods equal to the free oscillation periods of the structures. This means during an earthquake seismic waves are “filtered” twice: first in the near-surface stratum, and then in the structure itself.