Étude du comportement thermodynamique d'appuis isolateurs sismiques sous chargements cycliques à basses températures

Abstract

Seismic base isolation allows for separating the motion of an isolated bridge of the ground using seismic isolation bearings. The use of such devices allows increasing the vibration period of the structure. This period shift usually moves away the period of the structure from the dominant periods of earthquakes. However, the seismic performance of an isolated bridge is governed by the behavior of seismic isolation bearings which is directly connected to their hysteresis properties. These hysteretic features are influenced by many factors, including low temperatures, commonly experienced in Canada. The Canadian Highway Bridge Design Code (CSA, 2014) requires to carry out tests at low temperatures on isolation bearings but does not closely regulate these tests. In particular, the impact of test conditions is not precisely known. Moreover, the hysteresis properties and behavior of seismic isolation bearings is generally intimately linked to their thermodynamic behavior with a coupling between the mechanical properties and the effective temperature of bearings during the test. This study's main objective is the development of a numerical modeling of seismic bearings under cyclic loading at low temperatures in order to study and to represent the interaction between their thermodynamics and their hysteresis properties measured by the tests. This study focuses on two types of laminated elastomeric seismic isolation bearings: High damping Rubber Bearings (HRB) and Lead Rubber Bearings (LRB). The hystérésis properties of these two isolators depend on the properties of the rubber, but also on those of the lead of the LRB. The behaviors of these two materials were modeled to obtain representative and effective models suitable for simulations using finite elements. The mechanical and thermal properties and their laws of evolution with temperature are defined and selected on the basis of a literature review. The heat generated in these materials as a result of the energy dissipated during cyclic loading was modeled. A series of simulation has been carried out to validate the models created for materials and heat generation/diffusion. Finally, simulations for both types of isolation bearings, studied by changing the settings, were performed to visualize the impact of low temperatures and some test conditions. The results are compared to available results in the literature and are used to validate the models created for the materials component of the isolators and heat generated within them. Furthermore, the impact of the test temperature on the behavior of the isolators was investigated. For HRB, low temperatures lead to a sharp increase of all mechanical properties of rubber, which implies an increase of both the characteristic strength (Qd) and the postelastic stiffness (Kd). Similarly, for LRB, low temperatures increase Kd and Qd, as well as the elastic limit of lead which, combined with the rubber effects, causes a highly significant increase in the strength characteristic Qd. The simulations confirmed, as expected through literature review, that the heat generated by rubber is low and can be neglected for LRB bearings. However, the lead core experienced important heating, amplified by the low temperatures that increased its elastic limit. This increase in internal temperature of isolators subjected to cyclic loadings is clearly visible on the hysteresis loops of the LRB that exhibited degradation of the response with each cycle. For HRB bearings this degradation with cycling is much less obvious and the responses of successive cycles are almost identical. Finally, our simulations indicate that the temperature of the plates of the test structure have very little influence on the thermodynamics and hysteretic response of isolators LRB and HRB.

Date	12 Sept 2016
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Lotfi Guizani (Supervisor)