I am working with a 3 leg truss tower for a 20 kW wind generator. I have developed the mass density and stiffness data for the tower input file. I have reviewed Bmodes manual but I am not sure that it applies here since the tower is not Axisymetric. What approaches are available to create the mode shape data TwFAM1Shi and TwFAM2Shi for this type of tower? If I can use Bmodes are the Mass Moment terms about the Yt axis at the base (origin) or at the center of the section (i.e. at the node for that section)?
BModes does not require an axisymmetric tower and can be applied to the tower you are considering. For a nonaxisymmetric tower, the fore-aft and side-to-side sectional stiffness and inertia can be set differently. This is also possible in FAST, except that FAST assumes that the sectional inertia of the tower is negligible and that sectional principle elastic axes of the tower have the same orientation along the entire length of the tower (in BModes this orientation can change via input str_tw). In BModes, the sectional inertia is defined in the section relative to the sectional principle inertia axes (the orientation of Xg and Yg is defined by input tw_iner as shown in Figure 5 of the BModes User’s Guide). That is, for an isotropic material:
Section fore-aft inertia = flp_iner = rhoIYg = (mass density in kg/m^3)(area moment inertia of cross section about Yg in m^4)
Section side-to-side inertia = edg_iner = rhoIYg = (mass density in kg/m^3)(area moment inertia of cross section about Xg in m^4)
I hope that clarifies things.