Testing on Elastomers and Numerical Analyses for the Behavior Prediction of Elastomeric Seismic Isolators and Application of the Isolators in a TMD System

Seismic isolators are increasingly used to decouple the horizontal movement of buildings from ground motion, thus protecting structures from potential earthquake-induced damage. The UNI EN 15129 standard for seismic isolation devices requires numerous time-consuming and costly tests on full-scale isolators for device qualification. From these tests, the primary mechanical properties, shear modulus and damping, that describe the isolator behavior, are derived. This research work focuses on elastomeric seismic isolators, with the main objective of predicting their mechanical properties through laboratory testing on elastomers and finite element method (FEM) simulations. This is to have information about the isolator behavior without performing experimental tests on full-scale isolators to obtain the desired characteristics. Specific laboratory tests are designed and performed for rubber, in order to determine the parameters of the viscoelastic model adopted for this material. In particular, the effect of small repeated oscillations is utilized to accelerate the stress relaxation process, allowing for a relatively rapid pointwise determination of the elastic equilibrium stress response curve. A single small thickness rubber pad specimen is designed to achieve a rapid loading ramp in the relaxation test and to highlight the viscous components of the stress that dissipate quickly. From the experimental tests, the parameters necessary to implement the rubber constitutive model in FEM simulations are derived. Numerous results from laboratory shear cyclic tests on full-scale isolators are collected in this work. Groups of these isolators are selected for FEM modeling and simulation of the same test to make a comparison between the mechanical characteristics obtained form FEM models and real ones. The prediction capacity of these models is good in many cases. In this thesis, also a case study of a five-story reinforced concrete (RC) building seismically protected through a tuned mass damper (TMD) installed at the top is presented. The aim of the study is to investigate the effectiveness of the TMD in changing the structural behavior of medium-rise existing buildings from dissipative to non-dissipative, in order to eliminate reparation or demolition costs resulting from damages caused by strong earthquakes. The TMD mass is made by a RC slab lying on flat surface sliders. Horizontal stiffness and damping of the TMD are provided by lead rubber isolators in a first proposed solution and by low-damping rubber isolators and viscous linear dampers, respectively, in a second one. The improvement in the building's structural behavior attained with the installation of the TMD is assessed by considering the flexural demand over capacity ratios of structural elements and the energy dissipated by the TMD. These results are compared with those of the same building retrofitted with a base isolation system. It is demonstrated that a TMD is a valid solution for the retrofit of medium-rise existing buildings.