AbstractFundamental principles from structural dynamics, theory of random processes and perturbation techniques are used to develop a new method for seismic analysis of multiply supported secondary subsystems, such as piping attached to primary structures. The method provides a decoupled analysis of the secondary subsystem wherein the response is given in terms of response spectra associated with the attachment points (‘floors’). In order to account for correlations between modal responses and between support motions, an extension of the conventional floor response spectrum, denotedcrossoscillator, cross‐floor response spectrum, is introduced. Important effects of tuning, interaction, non‐classical damping and spatial coupling, which are inherent characteristics of combined primary–secondary systems, are included through the extended spectra. An efficient method for generation of the extended spectra directly in terms of a ground response spectrum is developed.Numerical comparisons with exact results are used to examine the accuracy of the proposed method and to demonstrate the importance of the characteristics mentioned above. In all cases examined, the proposed method shows excellent agreement with exact results. By accounting for the effect of interaction, the proposed method leads to more realistic and economical design criteria for secondary s
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