## 1 Introduction  
Seismic isolation represents one of the most efficient techniques currently available to protect structures from earthquake excitations [1; 2; 3; 4]. Such a technique is based on the use of seismic isolation bearings which are special devices characterized by flexibility and energy dissipation capacity along their transverse directions and a large stiffness along the axial one [5]. Basically, elastomeric and sliding bearings are commonly used in seismic isolation. The first ones are constituted by alternating layers of rubber and thin reinforcing elements [6], whereas the second ones are made up of a metal slider that moves on a sliding metal surface [7]. Under the effect of a constant axial compressive load, deformation of both types of isolators, along their transverse directions, is characterized by a complex hysteretic behavior. Some examples of their typical restoring force-displacement hysteresis loops are described in [8; 9; 10; 11]. Due to the hysteretic nature of the isolators' behavior, a realistic prediction of the actual response of seismically base-isolated structures can only be achieved by performing nonlinear dynamic analyses. As shown by [PERSON] et al. [12] and [PERSON] et al. [13], the accuracy and computational efficiency of such sophisticated analyses are strongly related to a suitable combination of the time integration method, required to solve the nonlinear equilibrium equations, and the phenomenological models adopted to describe the hysteretic behavior of the seismic devices. Among existing computer programs of structural analysis, 3D-BASIS, ETABS and SAP2000 are typically used in practice to perform nonlinear time history analyses of base-isolated structures. On the contrary, other commercially available computer programs, such as ANSYS and ABAQUS, that are also capable of modeling the behavior of seismic isolators, are rarely used for this purpose since they are not suitable for building analysis. In 3D-BASIS, the nonlinear equilibrium equations are numerically solved by using the [PERSON]'s constant average acceleration method adopted in conjunction with the pseudo-force iterative procedure [14]. As regards the available hysteretic models, such a computer program offers the possibility of using accurate differential models that have been derived by modifying the celebrated Bouc-Wen model [15; 16] to allow for the simulation of the complex response characterizing elastomeric and sliding bearings [9; 14]. In CSi programs, namely ETABS and SAP2000, the nonlinear equilibrium equations can be solved by employing the computationally efficient Fast Nonlinear Analysis method [17], in which stiffness and mass orthogonal load dependent Ritz vectors of the linear elastic base-isolated structure are used to reduce the size of the nonlinear system to be solved. As for 3D-BASIS, in CSi programs the behavior of elastomeric and sliding bearings can be simulated by adopting accurate differential models that represent modified versions of the Bouc-Wen model [15; 16]. The hysteretic models implemented in the above-described computer programs, though capable of accurately simulating the response of seismic isolation devices, are based on parameters lacking a clear mechanical significance, so that their calibration and use in practical applications is troublesome. In addition, sophisticated numerical techniques, such as multi-steps [18] or Runge-Kutta methods [19], are exploited to solve, at each time step of a nonlinear time history analysis, the differential equation defining such models. To overcome such limitations, a class of uniaxial phenomenological models has been formulated in [20; 21]; subsequently, it has been specialized to derive two models which are respectively suitable for elastomeric and sliding bearings [22; 23]. In particular, the one developed for elastomeric bearings, that is of particular interest in this work, exploits just five parameters, directly derivable from the experimental hysteresis loop, and requires a straightforward computer implementation.