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Vertical accelerations in pendulum-type isolation systems: fundamentally necessary yet potentially destructive

Pendulum-type isolation systems have been used extensively to protect buildings and individual pieces of equipment from earthquake-induced ground and floor motions. The protected object is mechanically decoupled from horizontal components of base motions via a rolling (or sliding) interface. Horizontal deflections across the interface result in vertical motion, increasing the gravitational potential energy and generating mass-proportional recentering forces. The equations of motion (in the absence of damping/friction) do not depend on the mass of the isolated object, which eliminates the need to tune stiffness parameters -- a desirable feature in seismic isolation technologies. Another attractive feature is that the slope of the rolling (or sliding) surface constrains the peak horizontal acceleration transmitted to the object. However, in the process of reducing horizontal accelerations, the isolator transmits vertical accelerations that otherwise would not have been present. These vertical accelerations may exceed tolerable limits, defeating the benefits of (horizontal) isolation. For example, experiments on lightly-damped rolling isolation systems (RISs) have measured vertical accelerations four times higher than the horizontal accelerations. In this study, the equation of motion of a single-degree-of-freedom RIS with arbitrary rolling surface profile is developed. Different profiles have previously been studied: spherical, conical, and variable. A parameterized profile function capable of representing a range of shapes (spherical to conical) is presented. One parameter is the radius of curvature in the vicinity of the center of the rolling surface, and the other is the (constant) slope outside the center region. The influence of these parameters on horizontal and vertical peak accelerations is assessed numerically and experimentally. Multiple scale RISs are manufactured and tested for a wide range of disturbance frequencies and intensities. The results of the simulations and experiments show that vertical accelerations are not negligible and should not be ignored in the design of pendulum-type isolation systems.

Author(s):

Philip Scott Harvey    
University of Oklahoma
United States