Generation of floor response spectra including oscillator‐structure interaction

A method is presented for generating floor response spectra for aseismic design of equipment attached to primary structures. The method accurately accounts for tuning, interaction and non‐classical damping, which are inherent characteristics of composite oscillator‐structure systems. Modal synthesis and perturbation techniques are used to derive the modal properties of the composite system in terms of the known properties of the structure and the oscillator. Floor spectra are generated directly in terms of these derived properties and the input ground response spectrum using modal combination rules that account for modal correlations and non‐classical damping. The computed spectra, in general, are considerably lower than conventional floor response spectra due to the effect of interaction. They provide more realistic and economical criteria for design of equipment. The method is accurate to the order of perturbation and is computationally efficient, as it avoids time‐history analysis and does not require numerical eigenvalue evaluation of the composite oscillator‐structure system. The results of a parametric study demonstrate the accuracy of the method and illustrate several key features of floor response spectra.