Using base isolation for tall buildings may reduce the seismic displacement collapse capacity and increase displacement in higher floors, resulting in greater damage, and is therefore not recommended for tall buildings in most cases. So, in an alternative method, use of inter-storey isolation systems in tall buildings has received attention in recent years, especially in Japan, but the conceptual framework of the seismic behavior of these buildings with inter-storey isolation has not been well defined yet. In this research, effects of using inter-storey seismic isolator (ISI) on seismic response of high-rise steel frames including spectral acceleration (SA), story drift, and seismic capacity at different seismic performance levels are investigated and then a method for identifying the ideal level for installing the isolator is presented in the form of a flowchart. For this purpose, two high-rise steel frames of 20 and 25 stories with a dual system of flexural frame and eccentric bracing were designed and modeled and verified using OpenSees software. Also, two types of rubber seismic isolators with lead core and different stiffness and damping were selected, modeled and verified to evaluate the effect of mechanical properties. Since in the design procedure, the column cross-sections change in every five floors, the inter-storey isolation system is defined once for the base level and then for subsequent times only in one of the floors where the column cross-sections change, that is, for all columns of floors 5, 10, 15, and 20, respectively. Modal analysis was used to investigate the effect of the isolator on the structural period and incremental nonlinear dynamic analysis was used to determine the seismic performance levels of the frames. For the incremental nonlinear dynamic analysis, 22 earthquake records were used according to FEMA-P695 guidelines. Seismic behavior levels of continuous use IO, life safety (design) LS, and collapse prevention CP were determined using earthquakes with return periods of 10, 475, and 2475 years, respectively. Maximum relative drift and corresponding spectral acceleration were calculated for the 84th, 50th, and 16th percentiles.
The results show that the optimal floor for installing an inter-storey isolator in the 25-storey and 20-storey frames are at the 20th and 15th floor levels, respectively.