Loadbearing structures and detailed design - Concrete Structures for Wind Turbines - page 162

structural component fails. The assumption here is that the sequence of the action
effects has no influence on the damage development.
The analysis therefore uses Equation 4.6 to show that the damage D P M to the
structural component as a result of a repeated action effect does not exceed the
damage limit D lim :

¼ X

j

N i
N fi D lim

D P M (4.6)

i ¼ 1

where
N i No. of fatigue cycles of loading block i, for example from Markov matrices
N fi No. of fatigue cycles to failure from the S-N curve for concrete (see Figure 4.30)

The limit value for damage according to [9] is D lim ¼ 1.0 and corresponds to the
stipulations in Model Code 90. Refs. [33] and [66] do not contain details of fatigue
analyses in seawater.
Ref. [71] specifies the limit value for offshore structures depending on the options
available for maintenance and repairs. A damage value of D lim 0.33 must always be
assumed for harsh North Sea conditions. The value specified for splash zones is
D lim ¼ 0.5, the value for areas above these is D lim ¼ 1.0. The damage analysis in [71] is
also based on the linear damage approach of Palmgren-Miner.
Model Code 90 [66] specifies S-N curves for uniaxial compressive loads and also for
tensile or reversed loads; there is no information regarding the fatigue strength of
submerged concrete. Instead, the reader is referred to [71], for instance. The relevant
publications do not cover how multi-axial stress states influence the resultant fatigue
strength.

Fig. 4.30 S-N curves for concrete subjected to a repeated compressive load according to Model
Code 90 [66]