RE-CENTERING CAPABILITY EVALUATION OF SEISMIC ISOLATION SYSTEMS BASED ON ENERGY CONCEPTS

摘要

It was not until the most recent years that self-centering capability (sometimes referred to as restoring force) was identified as a fundamental function of an isolation system. This tardy occurrence can perhaps be explained by the fact that, historically, the first seismic isolators were conventional laminated rubber bearings – which are endowed with an optimal self-centering capability. With the introduction in the market of other types of isolators, that are generally not fitted with an intrinsic self-centering capability, the problem of providing this function has re-asserted its vital role. The purpose of the self-centering capability requirement is not so much that of limiting residual displacement at the end of a seismic attack, as instead that of preventing cumulative displacements during the seismic event. This type of defect assumes particular importance in cases involving isolators comprising PTFE sliding elements (sometimes referred to as sliders). Moreover, during the last quarter of the past century energy dissipation has increasingly gained the favor of the design engineers to mitigate the disastrous effects of a seismic attack. However, energy dissipation and self-centering capability are two antithetic functions. Self-centering assumes particular importance in structures located in close proximity to a fault, where earthquakes characterized by highly asymmetric accelerograms are expected (Near Field or Fling effect). Notwithstanding, self-centering capability was never paid sufficient attention by seismic engineering experts, to the point that the formulation of a criterion to quantify it was only acknowledged for the first time in 1991 by the AASHTO Guide Specification for Seismic Isolation Design. Then other criteria were developed, but none of them is based upon solid theoretical fundamentals, but rather make reference to an empirical approach, valid for only one class of devices. In conclusion, present Norms do not furnish an acceptable approach of general validity to evaluate the self-centering capability of seismic isolation systems. This author developed a theoretical approach to this problem, suggesting an energy-based criterion for its quantification. The scope of this paper is precisely that of introducing the newly proposed criterion. To correctly formulate the problem, some elementary cases are examined that serve to illustrate as well as interpret the requirements adopted to date by the Norms on this subject.