© 2019 Earthquake Protection Systems. All rights reserved.

Design by

Triple Pendulum Isolator

The Triple Pendulum™ isolator offers better seismic performance, lower isolator costs, and lower construction costs as compared to other seismic isolation technology. The properties of each of the isolator's three pendulums are chosen to become sequentially active at different earthquake strengths. As the ground motions become stronger, the isolator displacements increase. At greater displacements, the effective pendulum length and the effective damping increase, resulting in lower seismic forces and isolator displacements. 

The Triple Pendulum™ isolator's inner pendulum consists of an inner slider that slides along two inner concave spherical surfaces. Properties of the inner pendulum are typically chosen to reduce the peak accelerations acting on the isolated structure and its contents, to minimize the participation of higher structure modes, and to reduce structure shear forces that occur during service level earthquakes.

The two slider concaves, sliding along the two main concave surfaces, comprise two more independent pendulum isolators. Properties of the second pendulum are typically chosen to minimize the structure shear forces that occur during the design basis earthquake. This reduces construction costs of the structure. Properties of the third pendulum are typically chosen to minimize isolator displacements that occur during the maximum credible earthquake. This reduces the size and cost of the isolators , and reduces the displacements required for the structure’s seismic gaps.

Triple Pendulum Force Displacement Hysteretic Loop


In the Triple Pendulum™ isolator, the three pendulum mechanisms are sequentially activated as the earthquake motions become stronger. The small displacement, high frequency ground motions are absorbed by the low friction and short period inner pendulum. For the stronger Design Level Earthquakes, both the isolator friction and period increase, resulting in lower isolator displacements and lower structure base shears. For the strongest Maximum Considered Earthquakes, both the isolator friction and lateral stiffness increase, reducing the isolator displacement. When designed for a severe Maximum Considered Earthquake, the plan dimensions of the Triple Pendulum™ isolator are approximately 60% that of the equivalent Single Pendulum isolator.

Triple PendulumIsolator
Center Position
Inner Pendulum Motion
Service Level Earthquake
Lower Pendulum Motion
Design Basis Earthquake
Upper Pendulum Motion
Maximum Considered Earthquake

Principles of Triple Pendulum Seismic Isolation

The period of the Friction Pendulum™ isolator is selected simply by choosing the radius of curvature of the concave surface. It is independent of the mass of the supported structure. The damping is selected by choosing the friction coefficient. Torsion motions of the structure are minimized because the center of stiffness of the isolators automatically coincides with the center of mass of the supported structure. The isolator's period, vertical load capacity, damping, displacement capacity, and tension capacity, can all be selected independently. For the Triple Pendulum™ isolator, three effective radii and three friction coefficients are selected to optimize performance for different strengths and frequencies of earthquake shaking. This allows for maximum design flexibility to accommodate both moderate and extreme motions, including near-fault pulses.

Pendulum Motion
Sliding Pendulum Motion
Triple Pendulum™ Operation