Seismic Behavior of LRB and FPS Type Isolators Considering Torsional Effects

Abstract This study aims to investigate seismic behavior of LRB and FPS type base isolated models considering torsional irregularity due to distance between stiffness center of isolators and mass center of superstructure for low- and mid-rise reinforced concrete (RC) frame buildings with no shear walls. Nonlinear behavior of structural members was also considered to be able to observe probable yielding of structural members in superstructure due to the torsional irregularity. 528 different nonlinear time history analyses of 3-dimensional 3, 5, 7 and 9-story models subjected to 11 pairs of earthquake records were performed. The results indicate that FPS type isolators tend higher displacement demands while LRB type isolators are more sensitive to torsional effects. Torsional irregularity coefficient values of LRB models with 20% eccentricity are 47% higher than symmetrical models in terms of averages. Since significant part of the demands is absorbed by the isolator system, the remaining seismic demands for the superstructure is relatively low. Besides, the outcomes underline the careful selection of number of ground motion records in dynamic analysis as mentioned in the literature. While some studies in the literature indicate that torsional effects due to 10% or 20% eccentricity have significant role on building response, they used very limited building models subjected to a few earthquake records. The use of four different RC frame building model with LRB and FPS type isolators subjected to 11 pairs of ground motion records do not strongly support the abovementioned studies in literature. Therefore, more research is needed in this field.