FAST and masses computation

I’m running FAST simulations of a WTG for which I have a known tower mass of 10000kg. The summary output (.fsm) calculate a tower mass of about 9200kg.
Am I missing something? How FAST calculate total mass starting from the density and element’s length? I suppose the problem is how I assign densities to respective sections.
I know the density is given on the section, what I don’t know is if FAST assume it constant on the next or previous element or if some kind of interpolation occurs.

Thank you in advance.


Hi Stefano,

FAST takes the distributed mass per unit length from the tower input file and interpolates the data to the tower analysis node locations. These new data are then integrated along the tower using rectangular integration in order to calculate the tower mass for the *.fsm file (as well as the internal generalized tower mass matrix).

If the integrated mass does not match what you think it should be, you should be able to improve the integration by (1) specifying the mass per unit length properties closer to the analysis nodes (to improve or eliminate internal interpolation), (2) by increasing the number of tower analysis nodes (the higher the setting of input TwrNodes, the more accurate the solution will be, but the slower FAST will run), and/or (3) using input AdjTwMa to scale the mass density up or down as needed to get the desired integrated mass. The exact location of the tower analysis nodes are described in the description of input TwrGagNd from Table 8 of the FAST User’s Guide.

I hope that helps,

Hi Jason,
thanks for your reply. I think I’ll try to match the “target mass” adjusting AdjTwMa (faster and less invasive?)