Analysis of uplift in structures with friction pendulum isolators: Relevant parameters and mitigation measures

Abstract

Frictional isolators are being used with increasing frequency. However, the efficiency of these devices can be compromised if bearings experience uplift. In this study, two types of base-isolated resistant planes were studied to assess the uplift phenomenon: one with uniform lateral stiffness distribution (Type I), and one with a primary element that concentrates the lateral stiffness (Type II). A broad parametric analysis was conducted aiming to determine the properties of the superstructure that increases the uplift potential. For Type I structures, the uplift and its magnitude increase if the global slenderness of the resistant plane rises or if the isolation period decreases. For Type II structures, the uplift increases if the axial load applied on the primary element decreases. Responses of the superstructure can dramatically increase due to vertical impact after bearing uplift. Hence, in some scenarios, uplifts should be avoided. One alternative is to use uplift-restraining isolators. In both cases, Type I or Type II superstructures, the magnitude of tensile force required to prevent the uplift can exceed the vertical load due to the self-weight of the superstructure leading to design issues. A novel device was assessed as an alternative to attenuate the adverse effects of uplift. This device allows a range of vertical displacements without exhibiting bearing uplift and provides larger energy dissipation in the vertical direction. The new device can avoid the occurrence of uplift or decreasing its magnitude without the need for high-magnitude tensile forces, reducing the maximum responses of the superstructure.