AbstractForced vibration tests designed to isolate the effects of soil‐structure interaction are described and the results obtained for the nine‐storey reinforced concrete Millikan Library Building are analysed. It is shown that it is possible to determine experimentally the fixed‐base natural frequencies and modal damping ratios of the superstructure. These values may be significantly different from the resonant frequencies and damping ratios of the complete structure‐foundation‐soil system. It is also shown that forced vibration tests can be used to obtain estimates of the foundation impedance functions.In the case of the Millikan Library it is found that during forced vibration tests the rigid‐body motion associated with translation and rocking of the base accounts for more than 30 per cent of the total response on the roof and that the deformation of the superstructure at the fundamental frequencies of the system is almost entirely due to the inertial forces generated by translation and rocking