Study of sliding bearing and helicalsteel-spring isolation system

A sliding isolation system consisting of Teflon disc bearings and helical steel springs is described. The springs carry no vertical load and provide bilinear restoring force without hysteresis when deformed in shear. The Teflon disc bearings carry the entire weight of the structure. The isolation system is designed so that the mobilized peak frictional force is larger, by at least a factor of two, than the peak restoring force. Under these conditions the system has a low sensitivity to the frequency content of the input. The system has been evaluated in shake-table tests of a six-story, quarter-scale, 52-kip (230-kN) model structure, and is capable of withstanding strong earthquake forces of significant differences in frequency content. Furthermore, the springs are effective in controlling bearing displacements. Also, tests have been carried out without restoring force. In this case excessive permanent displacements are recorded as a result of a small accidental inclination of the sliding bearings. Finally, analytical techniques are presented that provide good prediction of the observed behavior.