Base isolation technology has emerged in the last decades as an innovative seismic design approach aimed at reducing the damage of structures when subjected to earthquake excitations by separating the building from the surrounding ground motions. This paper presents nonlinear seismic analysis of steel structures with 5, 10 and 20 stories designed to meet the requirements of a newly developed code. The structure is either conventional with fixed base or base isolated with different types of elastomeric bearings. A numerical parametric study is carried out to recommend the effective range of isolation bearing parameters represented by the ratio of fundamental period of the isolated structure related to that of corresponding fixed base one. This range is evaluated relying on the performance of both superstructure and elastomeric isolation bearings. The peak responses of the different studied frames are examined to determine the effectiveness of the utilized system.