1. DUAL ISOLATION FOR ENHANCED SEISMIC PROTECTION
Abstract
Base isolated buildings are well known to provide enhanced performance due to minimized
accelerations and decreased interstory drifts. However, the reduced demands are obtained at the
expense of large displacements at the isolation layer. This study investigates an innovative
system, termed ‘dual isolation’, which applies two layers of isolation, one at the base and one
mid-story to resolve this issue. An analytical solution for the equation of motion of the proposed
system is developed based on linear isolation theory. This creates a foundation to assess the
behavior of various types of seismic protection systems and to select the damping, mass and
frequency ratio that leads to an optimal dual isolation design. Time history responses of the dual
isolation system with viscous damping are compared to those of a conventional isolation
counterpart to examine the effectiveness of the system. The system reduces first floor
displacements by 04% on average, while the roof displacement is increased by roughly 15%.
This results in reduced design forces for the structure. In addition, accelerations, especially above
the second isolation layer, are significantly decreased. By reducing story shears and
accelerations, the dual isolation system limits damage to both structural and nonstructural
systems and components, thereby increasing global system performance.
Figure 1-1: Effect of base isolation on spectral acceleration

2. Figure 1-2: Effect of base isolation on displacement demand
Figure 1-3: Dual Isolation Concept

3. Figure 1-4: Dual Isolation, Reduction in Peak Floor Acceleration and maximum
Interstory Drift
Figure 1-5: Dual Isolation, Isolation Hysteresis