Effect of viscous, viscoplastic and friction damping on the response of seismic isolated structures

In this paper the efficiency of various dissipative mechanisms to protect s tructures from pulse-type and near-source ground motions is examined. Physi cally realizable cycloidal pulses are introduced, and their resemblance to recorded near-source ground motions is illustrated. The study uncovers the coherent component of some near-source acceleration records, and the shakin g potential of these records is examined. It is found that the response of structures with relatively low isolation periods is substantially affected by the high-frequency fluctuations that override the long duration pulse. T herefore, the concept of seismic isolation is beneficial even for motions t hat contain a long duration pulse which generates most of the unusually lar ge recorded displacements and velocities. Dissipation forces of the plastic (friction) type are very efficient in reducing displacement demands althou gh occasionally they are responsible for substantial permanent displacement s. It is found that the benefits by hysteretic dissipation are nearly indif ferent to the level of the yield displacement of the hysteretic mechanism a nd that they depend primarily on the level of the plastic (friction) force. The study concludes that a combination of relatively low friction and visc ous forces is attractive since base displacements are substantially reduced without appreciably increasing base shears and superstructure acceleration s. Copyright (C) 2000 John Wiley & Sons, Ltd.