The problem how to analyze unanchored structures subjected to seismic excitation arises very often in nuclear power plants. An unanchored body subjected to base excitation may remain at rest, rock, slide or slide and rock simultaneously. The body can even in some cases loose contact with the base. The ocnditions for the determination of the individual body movement modes are formulated. The equations of motion for each mode are numerically solved under the assumption of undeformable body and under the assumption of the vertically flexible foundation. The freestanding body is modeled as the lumped mass with 3 or 6 degrees of freedom (normally only the translational DOF are assumed). This simplified method is compared to the detailed FEM calculation that appears to be more realistic. On the base of performed analysis the applicability and convenience of the simplified method is discussed.
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