الفهرس 
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Abstract
In the last few years, many earthquakes strike Egypt. These earthquakes orient engineers to search on safe and economic approach to protect structures from the hazardous effects of earthquakes. The ”Base Isolation System” supplies a new method to control the seismic response of building structure and it is very effective, safe, simple and can be used in very wide range. It can keep the structure working in elastic range in earthquake, so it can be used in different kinds of important building in different seismic zones. It can be used in new buildings, also can be used in existing buildings to improve its earthquake resistant ability. The effect of base isolation technique on the behaviour of structures IS investigated under the excitation of sinusoidal ground motion with varied frequencies. The structural model is asymmetric and analysed in 3D. The superstructure is assumed to be elastic, while the isolator has elasto-plastic behaviour. A parametric study IS conducted to investigate the effect of important parameters of isolated structures on its behaviour. The < parameters studied are the isolator plastic shear capacity, the ratio between the superstructure and base masses, the ratio between the base and superstructure frequencies, the superstructure damping coefficient, and the isolator damping coefficient.