Energy Balance Assessment of Base-Isolated Structures

This paper explores the use of energy concepts in the analysis of base-isolated structures subject to severe earthquake ground motions. We formulate the energy balance equations in moving- and fixed-base coordinate frames and provide new physical insight into the time-dependent behavior of individual terms. Conventional wisdom in earthquake engineering circles is that systems with base isolation devices should be economically competitive and designed to: (1) minimize input energy, and (2) maximize the percentage of input energy dissipated by damping and inelastic mechanisms. Through the nonlinear time-history analysis of a base-isolated mass-spring system subject to an ensemble of severe ground motion inputs, we demonstrate that improvements in objective (2) often need to be balanced against increases in input energy. Hence, by itself, objective (1) presents an overly simplified view of desirable behavior.