Comportement sous basses températures des composés en élastomère utilisés dans les isolateurs sismiques de ponts

Abstract

Elastomeric seismic base-isolation systems are widely used to protect bridge structures from earthquake damage. Their use allows the introduction of controlled lateral flexibility at the link between the superstructure and the substructure (piers/abutments) and additional damping if needed. These systems may be classified into three types: Elastomeric isolators (RB), high damping elastomeric isolator (HDRB), and lead rubber isolators (LRB). They are typically composed of a succession of elastomer layers and thin steel plates with a lead core for the LRB. Elastomer compounds permitted and used in Canada are based on polyisoprene (natural rubber (NR)) or polychloroprene (synthetic rubber known as neoprene (CR)). A natural rubber-based formulation with additives provides a high damping rubber compound, referred to as HDNR, for an enhanced seismic energy dissipation capacity. However, the behaviour of these elastomers is strongly influenced by low temperatures, such as those encountered in Canada. They cause increases in the stiffness and the hysteresis response of elastomers, thereby causing changes in the mechanical properties of seismic isolators and thus influencing the seismic protection of structures. In this study, the mechanical behaviour under low temperatures of three elastomer formulations made of NR and HDNR already used and/or in development for seismic isolation applications in Canada is experimentally studied. The tests are conducted on reduced scale elastomer samples subjected to shear loading cycles at several levels of deformation (25 to 150%), after exposure to different temperatures (20°C, -8°C and -30 ° C) for different conditioning durations (15 minutes to 14 days). In addition, the effect of low temperatures on the seismic response of isolated bridges with elastomer isolators composed of LRB and HDRB type is numerically studied. To this end, nonlinear dynamic analyses were performed on a bridge structure, modeled by a single degree of freedom, considering the nominal properties (at room temperature, namely 20°C) and modified due to the low temperatures. The results of experimental study show that low damping natural rubber (NR) experiences less instantaneous and overall stiffening due to low temperature than the two high damping rubber compounds (HDNR) studied. However, it is more sensitive to the duration of exposure to very low temperatures (crystallization phenomenon). The results of the numerical analyzes demonstrate that the low temperatures effects on elastomeric isolation bearings can results into considerable increases in the seismic forces transmitted by the isolation system to the substructure of a bridge. The increase of these forces depends strongly on the type and chemical formulation of the elastomer, and is more important for HDNR elastomers type than for NR type.

Date	9 Jul 2019
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Lotfi Guizani (Supervisor)