A Study of Variable Friction Base Isolation Systems

Thesis (Master's)--University of Washington, 2017-06Variable Friction Systems (VFS) have been recently proposed as possible alternatives to traditional base isolation devices. The main benefit of VFS over other systems lies in their superior energy absorption properties that comes from the greater encompassed area of the hysteresis loops. Greater energy dissipation results in higher damping and, in turn, in lower seismic demand on the system. More specifically, more dissipative base isolation systems offer better seismic protection in that they lower the overall displacement demand and limit the lateral forces and accelerations transmitted to the isolated structure. Recent numerical studies of VFS employing both flat and curved sliding surface systems with variable friction coefficients indicate that VFS are theoretically capable of high seismic performance, and preliminary results suggest that they represent promising solutions. However, the available studies are limited to two kinds of VFS with specific characteristics. As a consequence, a generalized design process for VFS is currently missing. In addition, the design process that was proposed for VFS makes use of a number of empirical equations, particularly for the calculation of the equivalent damping associated to these devices, which have not been sufficiently validated. This thesis presents an extensive discussion on VFS, in the attempt of leading to a better understanding of their seismic behavior and performance. More than 500,000 non-linear time history analyses are conducted to study and characterize the damping properties of VFS, and to investigate their effectiveness at protecting SDOF and MDOF case study structures from the effects earthquakes. The results of the analyses are also used to extend and generalize the current design approach for VFS