Dear Abhinav,
FAST’s tower model only takes into account fore-aft and side-to-side bending (with no axial, torsional, or shear DOFs) for a straight beam of isotropic material with no mass or elastic offsets. You are using BModes to derive mode shapes for FAST, so, you want the BModes model to be consistent with FAST. I suggest the following values in BModes when deriving tower mode shapes for FAST:
str_tw = 0
tw_iner = 0
flp_iner = very small number (you can’t specify exactly zero for this input in BModes)
edge_iner = very small number
tor_stff = very high number (BModes won’t allow infinite; just set it high enough that it doesn’t impact the BModes results for the modes you care about)
axial_stff = very high number
cg_offst = 0
sc_offst = 0
tc_offst = 0
Best regards,