Killer Pulses of Hazardous Earthquake Motions and Transition of Natural Periods of Buildings

Abstract eng:
A theoretical approach of elasto-plastic response with a constant damping factor is proposed because almost all the damping mechanisms of buildings are not viscous but frictional. The skeleton curve of proposed single degree of freedom system is derived from the condition that the dissipative energy during one cycle of the elasto-plastic system is exactly same as that of the visco-elastic system. When the natural period of the elasto-plastic system is selected as the equivalent period at the maximum response, the shapes and amplitudes of the acceleration, velocity, displacement and total dissipative energy response spectra are approximately same as that of visco-elastic response spectra. Although the proposed elasto-plastic response system is non-liner, the spectrum with equivalent period shows linear response with fluctuations in amplitudes of input motions. The transition of equivalent period of the elasto-plastic system indicates significant relation between response amplitudes and damping factors of the system. The transition rate of equivalent period is proportional to the rate of response amplitude and the power of damping factor. This mechanism gives appropriate interpretation on the observed transition of natural periods of buildings. The proposed theory suggests that the natural period of wooden buildings increases about four to five times during severely damaged earthquake motions. This result gives good elucidation that the predominant periods of one second to two second during the 1995 Hyogoken Nanbu earthquake, known as the killer pulses, damaged rather shorter period buildings.

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