THE EFFECT OF FORESHOCK AND MAINSHOCK SEQUENCES ON THE INELASTIC PERIOD OF STRUCTURAL SYSTEMS: THE CASE OF THE 2016 KUMAMOTO EARTHQUAKE

Abstract eng:
Earthquake sequences, commonly consisting of a mainshock event and several aftershocks, may seriously jeopardise the structural integrity of structure and infrastructure systems. The accumulated inelastic episodes induce higher levels of structural and non-structural damage and hence, structural systems, subjected to earthquake sequences, are expected of being more vulnerable compared to the single-event seismic excitation. Several studies have already addressed the implications of mainshock-aftershocks events on the structural performance. However, a special type of earthquake sequences, including strong foreshock excitations and a mainshock event, has received, until now, marginal attention at least from a structural engineering perspective. To this end, the current study investigates the inelastic response that a varying set of single degree of freedom (SDOF) systems experience when subjected to the 2016 Kumamoto earthquake sequence, which includes both strong foreshock (Mw =6.2) and mainshock (Mw=7.0) seismic events. The earthquake-induced elongated vibration period of the SDOF systems, expressed via the period shift ratio, Tin/Tel (i.e., the ratio of the inelastic structural period, Tin, to the elastic one, Tel), is currently adopted as an efficient indicator of the inelastic response. Especially, the performance of response history analysis for numerous nonlinear SDOF systems, subjected to 108 foreshocks-mainshocks excitations from the Kumamoto earthquake, enables quantifying the period shift ratio. Its variation is investigated in consideration of the strength reduction factor, R, used herein to represent the inelasticity of the SDOF systems, the adopted hysteretic laws as well as the post-yield stiffness ratios. The analysis results show that this type of earthquake sequences may impose increased inelastic demand to different structural systems amplifying, in turn, the potential for excessive damages during the successive earthquake events.

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