In this paper, 3D evaluation of performance of seismic rubber isolators with lead core in fixed steel structures at the base and same slenderness ratios under far field earthquakes have been assessed. Thus, three steel structures 6, 9 and 12 stories have been considered with resistant system of intermediate moment frame as 3D. Rubber isolators have mounted on the base level of structures. Models of controlled and not controlled with seismic isolation systems have been under 7 far field

more »

... e records. According to conducted analyses, the performance of this control system with purpose of the most reduction in displacement, drift ratio and maximum stories shear under studied earthquakes has obtained. In last researches, it was usually considered a constant plan for different heights and just assessed height influence which caused rigidity of shorter structures increased and subsequently caused increasing positive performance of isolation at that structure; but in this paper, structures plan change with height to influence of height increasing while same slenderness ratio investigate on seismic isolation performance. Results indicate seismic isolator with goal of controlling maximum values of displacement, drift ratio, acceleration and maximum stories shear among studied structures and considering response in both x and y directions, at structure 9 stories has shown better performance. In this structure, seismic isolation cause reduction more than 27% in displacements, 70% in drift and 31% in roof story acceleration and 31% in base shear of structure.