Torsional amplifications in asymmetric base-isolated structures

A parametric study where the torsional response of base-isolated structures when eccentricities are set in the superstructure is presented. Nonlinear dynamic analyses were used to study peak responses for different ratios of the static eccentricities (eses) between the centers of mass and the centers of rigidity in the superstructure due to asymmetries: (a) related to the lateral stiffness of the resisting frame elements, and (b) related to the position of the centers of mass. In order to assess the impact of the relation between the fixed-based period of the superstructure (TsTs) and the effective period for the base-isolated structure (TITI) in torsional amplifications, four fixed-based periods were considered for the symmetric benchmark structure of reference. Unidirectional and bidirectional actions of selected ground motions typical of firm soils of the Mexican Pacific Coast were used. A range of effective periods between 1.5 s and 3.0 s for the base-isolated structures was considered in the present study. Bilinear isolator systems were considered. Peak dynamic responses such as maximum isolator displacements and peak displacement ductility demands were studied and compared with those obtained for symmetric systems of reference for the different ground motions under consideration, assessing the importance of the relative value of eses on those response quantities. In addition, amplifications due to static eccentricities related to the differences in the lateral stiffness of the resisting elements were compared directly with those due to the positions of the centers of mass. Among other relevant issues, it may be concluded from the study that eccentricities related to the positions of the centers of mass in the superstructure lead to higher torsional amplifications for the displacements of the isolation system than eccentricities related to differences in the lateral stiffness of the resisting elements. Also, it can be concluded that torsional amplifications in the isolation system are higher as the TI/TsTI/Ts ratio diminishes, particularly for systems with stiffness eccentricities in the superstructure.