Behaviour of Base Isolated Buildings Subjected To Near Field Earthquakes

Abstract eng:
Most of the base-isolated buildings are designed and tested for design basis far-field ground motion. This leaves a scope for investigation of seismic vulnerability of bases isolated buildings under near-field effects. Typically a near-field earthquake is identified as large amplitude, low frequency, short duration, a pulse type ground motion known for its high damage potential and ductility demand due to its whiplash effect. The present study seeks to investigate the effectiveness of base isolation system for seismic hazard mitigation of buildings, especially under near-field effects. For the study response, history analysis is carried out for a ten storey building frame, both fixed base and isolated, subjected to near-field ground motions, and the same structure is also subjected to a set of far-field ground motions. Two levels of earthquakes are used i.e. design level (scaled to have PGA = 0.2g) and extream level (scaled to have PGA = 0.4g). The selected response parameters for the comparative study are peak values of floor displacement, acceleration and base shear. The result of the study highlights that (i) for the two levels of PGA of earthquake considered in the study, the results in base shear, top floor absolute acceleration and maximum storey drift are significant for far-field earthquake and near-field earthquake with directivity effect, (ii) for the near-field earthquake with fling step effect, percentage reduction in the above response quantities is considerably reduced indicating that the base isolation proved to be ineffective for this type of near-field earthquake, (iii) the time history of the top storey displacement for near-field earthquake with fling step effect is distinctively different than those for other earthquakes and (iv) force-deformation loop of the isolator differ widely with earthquakes, for the near-field earthquake with fling step effect. The hysteresis curve is narrow with large displacement especially for the higher level of PGA.

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