Dear Dayuan,
Just to be clear, the blade structural model in FAST v7 and in the ElastoDyn module of FAST v8 uses the user-specified mode shapes as shape functions in geometrically nonlinear beam model. More information on the FAST theory basis is provided in the following forum topic: Coupled blade modes in FAST.
In FAST v7 and in the ElastoDyn module of FAST v8 currently, the blade structural model is discretized using the same discretization defined for blade aerodynamics within AeroDyn (see user inputs BldNodes, RNodes, and DRNodes in the AeroDyn primary input file). The structural discretization is used to compute the generalized stiffness matrix for elastic loads and the generalized mass matrix for inertial loads.
Best regards,