A method for generating floor response spectra for aseismic design of equipment attached to primary structure is presented. Floor response spectra are generated by a random vibration approach using a transfer function, which is the relation between response of a oscillator and earthquake acceleration in the frequency domain. The transfer function is composed of the known properties of the primary structure and the oscillator. The method accurately accounts for interaction between the structure and the equipment and non-proportional damping, which are inherent characteristics of the composite oscillator-structure system. In most cases of this study, the computed spectra are considerably lower than those by conventional floor response spectra due to the effect of interaction. They provide more realistic and economical criteria for design of equipment. The method is computationally efficient, because it avoids time-history analysis and does not require the modal properties of the composite oscillator-structure system.
