Effect of Damping Mechanisms on the Response of Seismically Isolated Structures, PEER Report 1998-06


In this report the efficiency of various dissipative mechanisms to protect structures from pulse-type and near-source ground motions is examined. It is first shown that under such motions the concept of equivalent linear damping has limited meaning since the transient response of a structure is more sensitive to the nature of the dissipation mechanism, rather than to the amount of energy dissipated per cycle. Subsequently, physically 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 shaking 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. Therefore, the concept of seismic isolation is beneficial even for motions that contain a long duration pulse which generates most of the unusually large recorded displacements and velocities of near-source events. Dissipation forces of the plastic (friction) type are very efficient in reducing displacement demands although occasionally they are responsible for substantial base shears. It is found that the benefits of hysteretic dissipation are nearly indifferent to the level of the yield displacement of the hysteretic mechanism and that they depend primarily on the level of the plastic (friction) force. Supplemental viscous damping to hysteretic mechanisms further reduces displacements in the superstructure. The study concludes that a combination of relatively low friction and viscous forces is most attractive since base displacements are substantially reduced without appreciably increasing base shear and superstructure accelerations.

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Nicos Makris
Shih-Po Chang
Publication date: 
November 1, 1998
Publication type: 
Technical Report
Makris, N., & Chang, S. (1998). Effect of Damping Mechanisms on the Response of Seismically Isolated Structures, PEER Report 1998-06. Pacific Earthquake Engineering Research Center, University of California, Berkeley, CA.