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Dynamics of inelastic seismically isolated structures

Seismic isolation is widely implemented as a response modification strategy that enables mitigation of damage in structures subjected to severe ground motion excitations. Behavior of base-isolated structures when the superstructure, intentionally or unintentionally, enters the inelastic response range is not well understood. The goal of this study is to identify the dynamics of such inelastic structures and determine the effect of the fundamental design parameters on their response.
Base-isolated superstructures may respond in the inelastic range in two cases. First, when the applied loads exceed the strength of an existing isolated structure. This can happen due to the construction of an unintentionally weak superstructure or due to the excitation by an extremely strong ground motion. Second, when the base isolated structure is designed to behave inelastically. Such design may result in a lighter base-isolated superstructure, thus offsetting the high installation cost of the isolation system.
The investigation is conducted using a two-degree-of-freedom model of a base-isolated structure. The hysteretic behavior of the base isolation devices and the isolated superstructure is simulated in Matlab and OpenSees using a bilinear inelastic model. Inelastic response history analyses are conducted for a large number of recorded ground motions covering a wide range of ground motion types, magnitudes and distances. The effects of base isolation and superstructure design parameters, such as stiffness and strength, are qualified through similitude analysis and quantified through parametric studies. The base isolation strength values, which indicate the activation of the isolators, are specified. The effect of the post-yielding stiffness of the superstructure on its inelastic behavior is determined. According to these results, design recommendations are proposed for the inelastic design of base-isolated structures.

Author(s):

Anastasios Tsiavos    
ETH Zurich
Switzerland

Kevin Mackie    
University of Central Florida
United States

Bozidar Stojadinovic    
ETH Zurich
Switzerland