Behaviour of Structures Isolated by HDNR Bearings at Design and Service Conditions

Abstract

High Damping Natural Rubber (HDNR) bearings are widely employed for seismic isolation. These bearings are characterized by a remarkable nonlinear behaviour and often by a degrading cyclic response, induced by the addition of filler to enhance its dissipation capacity. This latter phenomenon, denoted as stress-softening or Mullins effect, can significantly influence the nonlinear dynamic response of isolated structures leading to a variability of the seismic response of isolated buildings. Moreover, the behaviour of HDNR bearings may produce an amplification of response, due to higher modes contribution, which can affect the performance of non-structural components and equipment. Models providing an accurate description of the nonlinear behaviour of HDNR bearings with significant stress-softening have been proposed only recently. Thus, the studies in the literature are generally based on simplified models. In this paper, an advanced HDNR model, accounting for variation of stiffness and damping with the strain amplitude and deformation history, is used to analyse the seismic performance of isolated structures at different intensity levels. First, a parametric analysis is carried out on a two-degree of freedom system to study different configurations of practical interest. Subsequently, a multi-degree of freedom system representing a realistic building is analysed to evaluate the response at different floors andthe contribution of higher modes of vibration. The influence of practical aspects, such as the superstructure damping and the friction of the sliders, is also investigated.

Evaluation of optimal FVDs for inter-storey isolation systems based on surrogate performance models

“…However, studies on the supplemental damping in buildings with base isolation (e.g., Kelly 1999;Hall 1999;Alhan et al 2004;Politopoulos 2008;Providakis 2008;Fathi et al 2015) showed that high values of damping, when concentrated only at the isolation level, can be excessive and therefore harmful due to amplification of inter-storey drifts and floor accelerations. Both of these, on the other hand, can be limited by increasing the damping of the superstructure, which is effective in reducing the floor response spectra in correspondence with the second vibration mode of isolated structures (Ragni et al 2020). Although non-linear dampers offer greater dissipation per sinusoidal cycle than linear ones (for the same maximum force and stroke) and limit the damper force transmission at high velocities (Tubaldi et al 2015b), experiments carried out by Wolff et al (2015) proved that these amplifications are smaller in case of linear dampers, especially with highly dissipative isolation devices.…”