# calculating truss tower stiffness

I am trying to figure out how to calculate the stiffness for the FAST tower input file. I understand that it is E*I. At first I thought that I could just do the 3 legs calculation of I, but then it did not seem to make sense that this would give me a real representation of the tower flexibility and mode shapes. Of course the I of the legs is accurate for the locations where there are no braces, but that does not represent all of the tower structure. Do I have to do a FEM of the tower to get I?

Dear Robert,

Because in FAST you cannot currently model all of the individual members of the truss, you can only model an “effective” truss tower currently in FAST.

You are correct that FAST requires as input the distributed bending stiffness (EI in the fore-aft and side-to-side directions, TwFAStif and TwSSStif) at various stations along the tower. While the braces of the truss will certainly impact the area moment of inertia (I) at each section, I suspect that the legs will actually dominate the inertia calculation over the braces because the legs mostly likely have a larger diameter and a further distance from the tower centerline. Thus, you may be able to eliminate the braces from the calculation. This will result in a lower-than-actual stiffness, but you can correct this later with the tower stiffness factors (AdjFASt and AdjSSSt) or stiffness tuners (FAStTunr and SSStTunr) to ensure that the natural frequncies of the tower in FAST much measured test data or the eigenanalysis of an FEA model of the truss tower.

I hope that helps.

Best regards,

Sorry but I don’t believe the simple 3-leg approach will give a close enough number for a meaningful result in subsequent analysis.
I would work out the tower’s stiffness by FEM, however I wouldn’t be daunted by that project, because there are some s/w packages on the internet that make it quite easy.