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Fatigue spectra

Fatigue spectra

Fatigue spectra

(OP)
Hi guys,

I was wondering if there is anyone familiar with wind turbine design?

My question is how could I find the peak tensile stress in the bars in the pile under the maximum overturning moment form the fatigue spectra.

Any help is really appreciated.

Many Thanks,
Leo

RE: Fatigue spectra

do you know the load in the pile from the design case ? what windspeed ?

is your fatigue spectrum in terms of windspeed ?

is it reasonable to scale your design loads ?? (hint, it may not be, depending on your assumptions)

another day in paradise, or is paradise one day closer ?

RE: Fatigue spectra

(OP)
The maximum tension in the piles at peak fatigue moment was 180 kN

RE: Fatigue spectra

it sounds like you've answered your own question ?

ie if you know that, what is the problem scaling for the different fatigue cases ?

another day in paradise, or is paradise one day closer ?

RE: Fatigue spectra

(OP)
No I was not sure how to find the stresses on the bars. I reckon the stress = maximum tension over the reinforcement area which is 1800000/ 8* (3.14* 12^2/4)= 200 MPa

RE: Fatigue spectra

is that the pile area or the area of the re-bar ?

another day in paradise, or is paradise one day closer ?

RE: Fatigue spectra

(OP)
8 rebars size 12 mm

RE: Fatigue spectra

sounds reasonable to me ...

another day in paradise, or is paradise one day closer ?

RE: Fatigue spectra

(OP)
I thought the tensile stress in the bars under the maximum overturning moment form the fatigue spectra should not be more than 70 MPa?

RE: Fatigue spectra

I don't know, but that sounds awfully low (70MPa = 10ksi). But if that is the case, then you need 3 times the amount of rebar !

what's your fty ? your ftu is > 200MPa (30ksi ... should be ! ... sorry 200MPa sounded like a lot !)

how frequent is the peak fatigue load ? how peaky is the spectrum ??

another day in paradise, or is paradise one day closer ?

RE: Fatigue spectra

(OP)


Yield stress 500 MPa

Please find my attached excelsheet calcs

RE: Fatigue spectra

nothing there looks scary ! it looks like 70MPa is stress for infinite life. I think you're ok exceeding it.

Is your design life (20years) meant to be a safe life ? If so, I think your design is marginal (typical safe life factor = 5, so miner's rule damage sum is 0.2, and I think you're a little over that ... maybe safe life factor =4 ?).

another day in paradise, or is paradise one day closer ?

RE: Fatigue spectra

(OP)
I am not sure how you managed to determine the safe life factor? please if you don't mind to explain in details. I am a bit confused fatigue loads damage cycles equal 1,00 E+7, should the allowable stress be 125.82 MPa or shall I use 162.5 MPa because the ΔσRsk for straight and bent bars = 162.5 MPa







RE: Fatigue spectra

use bent bar stresses if the bars are bent !

the previous table looked like a simple cumulative damage table, so I can sum the few numbers to get something like 0.3 by observation. Safe life factor divides into the allowable (sum damage < 1/n) so I figured a safe life factor of 5 wouldn't work, maybe a factor of 3 would.

I don't understand your table "fatigue loads LM and NR", or your term "deltasigmaRsk".

The S/N curve looks reasonable. Your Miners rule calc is good-ish (I don't see why you'd do a fatigue calc for ultimate load), but don't you apply a safe life factor?

this is re-bar, right ? is it installed with preload ??

another day in paradise, or is paradise one day closer ?

RE: Fatigue spectra

(OP)
That table load spectrum is the fatigue loads from normal turbine production. For the structural design of the reinforced concrete components of the foundation, the mean value of the load components have to be taken into consideration. The given fatigue load spectra represents fatigue loads for a design life of 20 years.

deltasigmaRsk is allowable stress for rebar, I am not sure if it is installed with preload, I have to check that.

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