Dear Francesca,
I’m familiar with the question you raise. The basic issue is that you want to represent the correct mode shape based on the proper boundary conditions, but FAST requires that you input the mode shape of a cantilevered beam. Regardless of FAST’s requirement, it is possible to input the correct mode shape in FAST. Here’s how:
In the correct mode shape, the tower base has an associated translation and rotation, unlike a cantilevered beam, which has no tower-base motion. In FAST, however, the tower-base motion (including tower-base translation and rotation) is represented through the 6 DOFs of the platform. Also, the tower DOFs are defined relative to this motion, so, in effect are cantilevered to the platform motion. If you place a line tangent to the correct mode shape at the tower-base, and project the mode shape onto this tangent line, the mode shape will appear cantilevered to the line. It is this “pseudo-cantilevered” mode shape that you want to specify in FAST. We’ve developed the MS Excel workbook ModeShapePolyFitting.xls, included with FAST, to do this projection and calculate the mode shapes needed by FAST. Applying this procedure, we’ve been able to use FAST to properly model towers with different tower-base boundary conditions, including idealized free-free beams, “coupled springs” and “distributed springs” representations of the foundation, and floating platforms.
If there is little direct coupling between the soil and the turbine (which is typical), then it should be OK to decouple the analysis–i.e., take the tower-base loads from FAST and apply them to the FE soil code. The key thing to keep in mind, however, is that the natural frequencies of the tower as modeled in FAST should be tuned so as to match the proper natural frequencies of the real system (including the influence of the soil). For this, it often OK to include the soil effect in FAST with a simplified model, such as the “apparent fixity”, “coupled springs”, or “distributed springs” representations of the foundation.
I hope that helps.
Best regards,