Abstract:
Nowadays, many of reinforced concrete building in urban areas allotted parking, shopping
mall areas in some of the stories, which requires large spacing between columns without
infill and exterior walls in the structure. Due to this situation, these buildings will have
low stiffness or the lateral load resisting systems at those stories is quite less than the
stories above or below these stories by 70% percent which is called soft story. For this
reason failure of structure starts at these points of weakness is due to discontinuity in mass,
stiffness and geometry of structure.
The main objective of this study was to evaluate the effect of base isolation in soft storey of
medium-rise moment resisting frame building using Response Spectrum Analysis under
earthquake load. Adama town was selected as a study area since it is located in the rift
valley of Ethiopia which is characterized by a high seismic zone. The study was done on 9-
storey moment resisting building with a regular and vertically irregular building model by
using ES EN, 2015 code. The core methodology of the research was carried out by
modeling and analyzing building with base isolation and without base isolation using
ETABS V.18 software.
According to analyses result the base isolation in the study increase the time period and
storey displacement of the soft storey model due to horizontal flexibility of LRB, both type
of soft storey type I &type II increase lateral displacement averagely by 52%-66% and
also both soft storey time period increased by relative 45% than the fixed base . in this
study the storey acceleration decreased by average 57.85to 64% for type I&II , the other
parameter decreased & basic thing is base shear that may cause damage on structure but
the base isolation reduce the base shear force by average 36.6%-48.6% that occur in fixed
base .
Finally base isolation is effective in reducing the base shear, story acceleration, and
increasing the fundamental period of fixed base structures. But, further study should be
done by considering buildings with different type of vertical irregularity
