V.K. Vostrov, Dr. Sci. (Eng.), Chief Researcher, firstname.lastname@example.org OOO «Kimrsky Association VNIPImorneftegas», Moscow, Russia
Methods for identifying specific and emergency seismic loads and emergency design situations required by the current legislation are proposed. The method suggested earlier concerning the description of movements of the structure with the help of nonlinear common differential equations, and the existence for them of stable limit cycles, which result in self-oscillations initiation, are specified in the article. It is shown that the task of selecting specific (emergency) seismic load reduces itself to the task of the optimal response speed, when the transition of the structure under external specific seismic load from one extreme position to the other extreme position takes place in minimum time. Maximum design earthquake, as was earlier suggested, should correspond to the self-oscillations of the structure (emergence of sharp resonance), which is caused by seismic force that is constant in magnitude and changing the sign to the opposite one at extreme values of structure movement. Unlike this proposal, the maximum design earthquake is caused by seismic force, which is determined by the design emergency harmonic seismic load with specific frequency that is close to, but not coincident with the natural oscillation frequency of the structure in the absence of vibration damping. The magnitude of this proximity is determined using the golden section or its fractional degrees. Out of specific loads the emergency seismic horizontal loads are allocated with the maximum value of seismic magnitude index for the given micro-district as loads that create an emergency situation — the occurrence of mode of beats, which result in structural failure or loss of structures stability, or in violation of norms for assessing the effect of general vibration on maintenance staff or equipment.
An iterative algorithm for determining the specific frequency of harmonic emergency seismic loads is presented based on the comparison of structures maximum movements emerging at beats and self-oscillations. It is required that the maximum movements at beats should be at most close to maximum movements at self-oscillations, and at least as large as on their absolute value. In this case, the maximum value of absolute accelerations at beats exceeds the maximum value of the accelerations at self-oscillations.
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