Equivalent Viscous Damping Ratio of Steam Turbine Foundation in Nuclear Power Plants

According to the complex damping theory, the formula to calculate equivalent damping ratio of composite structures is derived from damping stiffness matrix by dynamic analysis. The finite element model of a steam turbine foundation is established. The equivalent viscous damping ratio is calculated from natural frequencies, modes and structural stiffness matrix. Due to the vertical stiffness of spring vibration isolator is larger than lateral stiffness of concrete structure, most of free vibration modes of the foundation are movements of concrete structure. Therefore, the equivalent mode damping ratio is close to the one of concrete structure. The dynamic response is overestimated, if the equivalent mode damping ratio is derived from the average of spring vibration isolator and concrete structural ones.