Effect of lower and upper bound values of elastomeric isolators on the floor acceleration responses of seismically isolated buildings

Seismic isolation is based on increasing horizontal flexibility of the structure by extending dominant period of the structure and reducing effective earthquake forces and acceleration reactions by increasing the damping. Current earthquake code specifies that the calculation of horizontal earthquake forces acting on the superstructure should be performed using the upper bound values of the isolation unit parameters, while determining floor accelerations should be conducted using nominal parameters. When the superstructure is designed using the upper bound values of the isolators, the isolator displacement decreases and isolator stiffness increases. Consequently, the acceleration in the superstructure is expected to be higher. Which causes vibration-sensitive equipment in structures intended for continuous use may become inoperable. Therefore, λupper and λintermediate that is determined due to differences are considered in order to investigate the differences in the peak floor acceleration values that may occur in the superstructure compared to nominal conditions. The isolation system design for 3- and 6-story structures with isolation periods of T0 = 2 s and T0 = 3 s are carried out using the formulas of current earthquake code. Three sets of analyses, including nominal, λintermediate, and λupper, are performed using 3DBASIS under 11 near-fault and far-fault earthquake ground motions. The ratios of the peak floor accelerations calculated by λintermediate and λupper values to those calculated by nominal were obtained and the consequences are discussed. It has been revealed that floor accelerations, as superstructure forces, should be calculated using upper limit values.