Abstract

A finite element formulation modelling hyperelastic quasi-incompressible rubber-like materials (elastomers) is developed which takes into account large displacements and large elastic strains as well as inelastic effects. The capacity of laminated rubber-like materials to support high loads in compression and large displacements in shear is the principal reason for their use in devices for seismic base isolation of structures. The energy-dissipation capacity of these devices is increased by using high damping rubber, which is an elastomer incorporating carbon black particles, or having lead-plug insertion. The Ogden strain energy function has been used as a basis for the material model implemented in a total Lagrangian formulation, the strain being decomposed into its deviatory and volumetric parts and the pressure variable being condensed at element level. Mooney-Rivlin and neo-Hooke strain energy functions can also be used by simply changing the parameters of the model. The stress-strain hysteresis, which appears when these devices are subjected to dynamic or quasi-static cyclic loads, has been modelled by frequency dependent viscoelastic and plastic constitutive models. The bearings have been modelled by means of an equivalent single element capable of describing the composite behaviour of the actual isolation system. The proposed model is validated using available experimental results and it is proved to be a powerful tool in dealing with different bearings. Finally, results for a six-storey base isolated building subjected to the El Centro earthquake are given.

Abstract

The objectives of the seismic design of buildings can be achieved by applying techniques of conventional design that consider the that the ground motion is transmitted to the structure through its fixed base. As an alternative to this traditional design, base isolation systems were developed consisting of mechanisms that partially uncouple the structure of the foundation ground to reduce, in this way, the damages of the structure and of its content. Decoupling is carried out by placing support devices between the base of the structure and its foundation that increase the flexibility of the overall structure-base system and that produce, at the same time, an appropriate damping. Despite the large number of studies on different base isolation systems, there are few comparative analyzes between these systems. In this monograph, structures isolated with different systems are compared with structures with conventional design. To this end, criteria for evaluating the behavior of the different systems have been defined. In the numerical studies, models with linear behavior for the structure and nonlinear behavior of the base isolation system have been considered. The type of nonlinearity of the isolation is formulated by means of differential equations, capable of describing both hysteretic and frictional systems.

Abstract

In this monograph, a numerical model based on the finite element method is developed to analyze buildings with base seismic isolation, discretizing the structure-isolator system. In this way, the response of such systems is estimated with greater precision and, thus, the tasks of designing and verifying them is facilitated. The development of the model requires the resolution of the following problems:
• Development of an appropriate constitutive model for elastomers that considers large elastic deformations, incompressibility, visco-elasticity and elasto-plasticity in large strains.
• Incorporation of the constitutive model developed in a general model of dynamic analysis of structures using the finite element method that combines bar elements with plane elements; appropriate constitutive models for the numerical simulation of the structural materials; mixing theory; discretization by layers.
• Application of damage indices that facilitate the evaluation of the global state of the structure after a seismic ground motion.
• Numerical study of the behavior of structures with elastomeric isolation system subjected to earthquakes and their comparison with the corresponding fixed base structures.