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Lead-rubber seismic isolation bearings (LRB) have been installed in a number of essential and critical structures, like hospitals, universities and bridges, located in earthquake-prone areas. The purpose of using LRB is providing the structure with period lengthening and the capacity of dissipating a considerable amount of earthquake energy to mitigate the effects of strong ground motions. Therefore, studying the damage mechanics of this kind of devices is fundamental to understand and accurately describe their thermomechanical behavior, so that seismically isolated structures can be designed more safely. Traditionally, up to this point, the hysteretic behavior of LBR has been modeled using 1) Newtonian mechanics and empirical curve fitting degradation functions, or 2) heat conduction theories and idealized bilinear curves which include degradation effects. The reason for using models that are essentially phenomenological or that contain some adjusted parameters is the fact that universal laws of motion by Newton lack the term to account for degradation and energy loss of a system. In this paper, the Unified Mechanics Theory, which integrates laws of Thermodynamics and Newtonian mechanics at ab-initio level, is used to model the force-displacement response of LRB. When Unified Mechanics Theory is used, there is no need for curve fitting techniques to describe the degradation behavior of the material. Damage or degradation at every point is calculated using entropy generation and the physics based fundamental equations of the material along the Thermodynamics State Index (TSI) axis. A finite element model of a lead-rubber bearing was constructed in ABAQUS, where a user material subroutine UMAT was implemented to define the Unified Mechanics Theory equations and the viscoplastic constitutive model for lead. Finite element analysis results were compared with experimental test data.