The effects of near-field ground motions with large velocity pulses have motivated passive damping requirements for the protection of seismically isolated structures. Structures in which the first mode damping exceeds 20% or 30% typically do not exhibit classical modes, and simulation via a simple superposition of uncoupled second order equations is not possible. When the damping is produced by viscous or linear visco-elastic devices, we can, however, gain insight into the dynamic behavior of these structures using a convenient first-order formulation and frequency domain methods. When the damping effects are created by nonlinear mechanisms such as yielding or friction, the behavior of the structure is amplitude dependent and analyses are commonly carried out in the time domain. In this paper, frequency domain analysis and earthquake time history analysis are applied to study the influence of isolation damping on higher-mode effects and inter-story drift ratios. Because higher mode effects, plan irregularities, and bi-directional ground motions are all important attributes of the dynamic behavior of these structural systems, a simple comparison of isolation damping mechanisms can not be carried out via simple single or two degree of freedom realizations. In order to incorporate these important details in the study of the dynamic behavior of these structures, a set of 8-story proto-type building models with L-shaped floor plans, different isolation periods, isolation damping characteristics, and levels of isolation stiffnesses are examined. (C) 2004 Elsevier Ltd. All rights reserved.