Degradation of Dissipative Characteristics of Friction Pendulum Isolators due to Thermal Effect

The purpose of the research is to predict the reliability of friction pendulum devices during their service life. These bearings are characterized by the capability to undergo large displacements despite their compact size. This peculiar property makes this device competitive among other commonly used isolation devices such as lead-rubber bearings. In these supports the dissipation of seismic motion occurs exclusively by the friction produced during sliding of the surfaces while the seismic isolation is obtained by the shifting of the natural period of the superstructure. Over the time, the interest of the scientific community for such devices has focused on the study of the friction coefficient involved during the motion and also on its dependence on certain mechanical variables such as velocity and apparent pressure. Several studies have shown that the friction coefficient in a contact problem between polymer (PTFE) and stainless steel deviates from the Coulomb’s friction law. Furthermore, most recent studies have shown that the coefficient of friction is closely related to the increase of temperature due to the thermal effect. This phenomenon consists in a cyclic degradation of the dissipative capacities of friction pendulum that in the design phase is not considered. The observed reduction of energy dissipated during repetitive cycles is often coupled with peak displacements larger than predicted with potential consequences on the whole structure’s safety. This PhD study is composed by 8 chapter and it start with an introduction of the basic concept in seismic base isolation (Chapter 2) while the main characteristics of friction pendulum devices are introduced are defined in Chapter 3. The basic theory of frictional heating useful to describe the increase of temperature which occurs in polymer-stainless steel surface is introduced in chapter 4. Through an experimental campaign carried out with single pendulum bearings, the dependence of the friction coefficient with the temperature rise has been investigated in chapter 5, in order to propose a phenomenological model able to assess the real performance of the friction pendulum. Specifically, in chapter 5 is described the experimental analysis carried out in Caltrans SRMD Testing Facility of San Diego University of California. A series of friction pendulum have been tested at Caltrans SRMD which is equipped with a shaking table test specifically designed for full-scale tests. During the tests, the table was also equipped with a thermographic camera specially calibrated for the type of material tested (polished stainless steel). Thanks to the use of the camera it has been possible to evaluate the temperature rise during the whole testing time and in the portion of the concave surface affected by the thermal heating. In chapter 6, an analytical comparison has been carried out between the friction coefficient recorded during the test and the temperature rise obtained with the analytical model of degradation of the friction coefficient introduced in chapter 4. Finally in chapter 7 a prediction model that takes into account mechanical variables such as velocity and apparent pressure, and also the degradation of dissipative characteristics of a friction pendulum due to thermal effects, is given. The proposed friction model is suitable for immediate implementation in generalized structural analysis codes and provides an important design tool for a more realistic assessment of the seismic response of structures equipped with Friction Pendulum devices.