Sufficient Conditions for Decoupled Analysis and Design of In-Building Equipment


A decoupling criterion for the dynamic analysis of equipment can be justified if a piece of equipment does not strongly interact with its supporting structure. A small variation of frequencies of the structure due to installation of the equipment has been considered necessary for this insignificant interaction, but often misunderstood in practice as a sufficient condition toward an accurate determination of seismic responses. A small frequency variation does not assure response errors from an uncoupled analysis within the same tolerance. In this paper, a sufficient condition for decoupling analysis is presented and compared with the available decoupling criteria that are primarily based on the limited frequency change. It is defined as an interactive relation between frequency ratio and mass ratio, and determined by a small variation of the maximum ratio of transfer functions of the equipment with and without interaction over all possible excitation frequencies. Evaluated in this paper are the sufficient conditions for 10%, 20%, and 30% tolerable errors of harmonic responses. The percentage error of the root mean-square displacement of the equipment is then studied when the structureequipment system is subjected to a white noise excitation and their interaction is ignored. The root-mean square displacements agree well to that of Igusa et al (1992) for small mass ratios.

The sufficient conditions introduced here considerably differ from the necessary conditions suggested by Hadjian (1977), indicating that a decoupling analysis acceptable on frequency variation could predict erroneous responses of the equipment under a harmonic excitation. Although the root mean-square response of the equipment is evaluated by the total area under a transfer function and is insensitive to the detailed frequency components, significant discrepancies are observed between variations of the natural frequency and the root mean-square response.

Meeting Name

6th National Conference on Earthquake Engineering : Seismic Design and Mitigation for the Third Millenium (1998: May 31-Jun. 4, Seattle, WA)


Civil, Architectural and Environmental Engineering

Document Type

Article - Conference proceedings

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© 1998 Earthquake Engineering Research Institute, All rights reserved.

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