TwrBsMyt & RotThrust

I set an monotower structure with SNL13.2-00 blade in FAST. The hub height is 127.73m above MSL with water depth 30m. I tried to get some info regarding the tower base moment and associated stress.

Firstly I ran this model with all DoFs off and wind speed 72 m/s. Up to now, I did not include any loads from wave and current.

I ran 100s simulation and output the last 10s results. I did not pitch the blade so that it represents one of the most severe situation.

FAST outputs:
TwrBsMyt: 1,084,000 (kNm)
TwrBsMxt: 50,760 (kN
RotThrust: 7,562 (kN)

I tried to correlate these three outputs but the problems are,

  1. RotThrust * (HH above mud line) (1,192,754) is not equal to 1,084,000 (kN*m);
  2. Why is there a TwrBsMxt: 50,760 (kN*m)? I am still feeling confused about the reason for side-side moment. Is it from the non-symmetry?
  3. In FAST, TwrBsMyt is calculated based on what consideration? Which one is better choice for analyzing the bending stress level in tower design?
  4. Is the rotor load too high that the blades DoFs should be on in my tower design analysis?

Thank you!


Dear Lingling Yin,

I have no experience with this particular FAST model, but I’ll try to answer your questions.

  1. I think the answer to this question is sufficiently discussed in my August 18, 2009 post in the forum topic found here:
  2. Unless you pitched the blades to create zero rotor torque, the wind loads on the rotor will generator a torque that will lead to a tower-base moment about the xt axis.
  3. I’m not sure I understand your question. Regardless, FAST output TwrBsMyt (and other tower loads output from FAST) are useful for analyzing the bending stress level in tower design.
  4. This is a difficult question to answer in general, but the case you describe sounds severe enough to warrant the enabling of structural DOFs.

Best regards,

Dear Jason,

That really helps, thank you!