No.
Flexible: First natural frequency of the system is well below the operating speed of the fan.
Rigid: First natural frequency of the system is well above the operating speed of the fan.
The rigidity of a foundation is a relative quantity. It must be considered in conjunction with the rigidity of the machine-bearing system. The ratio of bearing-housing vibration to foundation vibration is a characteristic quantity for the evaluation of foundation flexibility influences. A foundation may be rigid and of sufficient mass if the vibration amplitude of the foundation (in any direction) near the machine’s feet or base frame is less than 25 % of the maximum amplitude that is measured at the adjacent bearing housing in any direction.
Thus, dynamic characteristics determines support type, foundation material doesn't.
