Machine foundation of fossil or nuclear power plants can be classified as either low-tuned or high-tuned according to the vibrational characteristics of the foundation. High-tuned foundations may reduce the required foundation size. This accounts for the preferred use of low-tuned foundations. But even in low-tuned foundtaions, the transmission of the machine vibration to the foundations may render the entire structure short of its intended capacity. For this reason, recent design methods introduce a vibration isolation system placed within the structure. The main purpose of this study is to investigate how the vibration isolation systems affect the response of the structure when the rotating machine is operated.