Hi Dongwon,
The elastic torsion (or twisting) of a blade is certainly important for some wind turbine blades (but not all). However, the current version of FAST does not have blade-torsion degrees of freedom (DOFs).
FAST can output the blade twist – e.g. TipRDzb1 for the twist of blade 1 at the tip, Spn1RDzb1 for the twist of blade 1 at blade gage #1, etc. However, all of these twist outputs are zero-valued from because of the absence of blade-torsion DOFs.
That said, FAST can still output the torque (pitching moment) along the blade – e.g., RootMzb1 for the pitching moment at the root of blade 1, Spn1MLzb1 for the pitching moment of blade at blade gage #1, etc. These torques (pitching moments) include contributions from aerodynamic pitching moments and applied aerodynamic forces, gravity forces, and inertia forces crossed with their position vectors. All of these forces, moments, and distances are vectors that orient with the blade as the blade deflects.
The dynamics of a composite spinning blade with bending, torsion, and large deflection is quite compilicated, which is why it has taken a while for NREL to add blade torsion to FAST. However, NREL is working on introducing blade torsion to FAST as described in my Nov 26, 2012 post in the forum topic found here: Coupled blade modes in FAST.
Best regards,