Effects of rotational component of ground motions on seismic responses of asymmetric base-isolated structures

Abstract

Asymmetric base-isolated structures installed on lead rubber bearings (LRBs) and laminated rubber bearings (LNRs) may suffer severe torsion due to the presence of the eccentricity ratio of the isolation system (eb/r). It is also recognized that the rotational component of ground motions can markedly increase torsional responses in structures, which may further amplify the adverse effect of torsion on asymmetric structures. Therefore, understanding how these rotational motions affect asymmetric base-isolated structures on LRBs and LNRs is crucial. Surprisingly, existing literature overlooks the impact of the structural parameter (eb/r) and the ground motion’s pulse period (Tp) on these effects. To fill this gap, numerical analyses are conducted on typical U-shaped planar asymmetric base-isolated structures. These structures, having 5 and 8 stories, are examined using MATLAB programming. Firstly, the translational and rotational responses of the selected cases under coupled rotational and translational components of ground motions are compared with those under the translational components. The responses are also compared to those of the associated, asymmetric fixed-base structures. Then, the role of eb/r in amplifying the negative effect of rotational excitations on selected cases is examined by varying eb/r from 0 to 0.3. The study compares the responses of selected cases subjected to coupled rotational and translational components of fourteen pulse-type ground motions (with Tp ranging from 0.728 to 8.043 s) to those subjected only to the translational components. The results reveal that seismic isolation effectively mitigates the adverse effects of the rotational component of ground motions on asymmetric structures. While rotational components can increase torsion in asymmetric base-isolated structures, the adverse effects are relatively minor and are not exacerbated by eb/r. However, when facing pulse-type ground motions with substantial Tp values, these structures exhibit at least a 200% increase in maximum interstory rotation and a 28% increase in maximum isolator displacement. These increases significantly exceed those observed for the structure subjected to non-pulse-type ground motions. Consequently, structural engineers should take these findings into account when designing and evaluating asymmetric base-isolated structures installed on LRBs and LNRs, particularly in regions where pulse-type ground motions are typical.