Modern approaches to the analysis of dynamic stability of natural and technogenic objects with monitoring of pillar type monument as an example
© I.N. Lebedych, S.V. Mostovoy, V.S. Mostovoy
The example of a computer-controlled system applied to monitoring of the large man-made and natural objects is under consideration. As an example here an object is selected of such a size, that its spectral characteristics lay in a frequency range from half up to two Hz. The wave nature of stress propagation through the object is an essential moment in simulation. This object is a pillar-type monument, which was installed at Kyiv Independence square on Ukraine independence honor.
The registration of the monument's oscillation was carried out with the following purposes: to get the monument spectral characteristics; to calculate the logarithmic decrement of attenuation of the object and to analyze the damping ability of the system, which was installed at the object to reduce the oscillations. Three-directional geophone with sensors oriented in three mutually perpendicular axes (one of which is vertical) was used for registration of fluctuations. The spectral characteristics of sensor represent one- modal curve with an extreme point in f=1 Hz. Sensors were located at a horizontal surface, on a mark of 42 meters. They made a part of the interface of the monitoring registration and processing automated system. This system allows correcting the spectral characteristics up to uniform in the chosen range of frequencies. The first part of experiment consisted in registration of monument reaction on natural background seismic oscillations as an input signal. This signal represents a superposition of the large number of external factors from natural micro seismic noise and man made one (including, for example, signals from ground transport). The important moment is that the resultant spectrum of these signals is much wider than own spectrum of the monument response. As a result of the analysis it is possible to note, that in a low-frequency band of a spectrum three modes on frequencies 0,484 Hz, 0,935 Hz and 1,468 Hz with corresponding relative amplitudes 1,00, 0,07 and 0,12 are allocated. The second part of experiment was aimed to calculate the monument characteristics of attenuation (logarithmic decrement) for the basic resonant frequency. The damp pendulum was used for this purpose. This pendulum was used to overbalance the monument; then the fluctuations attenuated by a natural way. The average value estimation of the next amplitudes ratio logarithm in this case was equalled 0,055. Having used the damp device this value was increased up to 0,12. It is necessary to note that the spectrum of a structure is its steady characteristics. This function varies with the change of mechanical parameters of structure and can be used for detection of "age" changes of structure during its lifetime. It is possible to consider that the fixed spectral characteristics of the monument can be further used as the reference values for detection of the beginning of the monument's "age" changes during the structure-monito-ring period.