A Study on Effectiveness of Viscous Dampers in Isolation Systems

A Study on Effectiveness of Viscous Dampers in Isolation Systems

It is still a challenge to design a seismically isolated structure located at a site with soft soil and/or near a fault. The concern arises from the development of large displacement in the isolation bearings due to a long period and/or pulse-like seismic waves. However, to use very large isolation bearings with much larger characteristic strengths may result into an unsatisfied seismic performance of the isolated structure during small to moderate earthquakes. The current practice to solve this problem is to incorporate viscous dampers into the isolation system. As a consequence, the displacement response of the isolation system may be controlled within an acceptable range and the maximum force transmitted to the superstructure may not be increased significantly compared with the use of isolation bearings with a large characteristic strength. In this study, the formula for determining the maximum force transmitted by the isolation system composed of lead-rubber bearings and viscous dampers was derived and validated. An experimental study was conducted using a shaking table for a rigid mass sitting on the isolation systems composed of a combination of lead-rubber bearings, natural rubber bearings and nonlinear viscous dampers with damping exponents smaller and larger than 1.0. The reason for adopting viscous dampers with the damping exponent larger than 1.0 is that viscous dampers may contribute less damping force resistance while the structure is subjected to small and moderate earthquakes such that the transmission of ground acceleration may be limited. In addition, during a major event, viscous dampers with the damping exponent larger than 1.0 may contribute large damping force such that the maximum displacement of the isolation system may be controllable and the seismic force transmitted to the superstructure may not be significantly enlarged.