Dear Motahhare,
The natural frequencies and mode shapes of a spinning rotor blade are influenced by centrifugal stiffening, which depends on the rotor speed, the radial length of the blade, and the blade mass.
If you are using BModes to derive mode shapes for FAST, please see my post dated Mar 18, 2015 in the following forum topic for guidance: NREL 5MW Blade Mode Shape.
Best regards,